14-3-3zeta/leonardo
Slob has been identified as a novel protein that binds to the carboxy-terminal domain of Slowpoke. A yeast two-hybrid screen with Slob as bait identifies the zeta isoform of 14-3-3 as a Slob-binding protein. All three proteins are colocalized presynaptically at Drosophila neuromuscular junctions. 14-3-3 is known to be highly enriched in synaptic boutons at the neuromuscular junction and is present only at much lower levels in the motor axon and muscle (Broadie, 1997). Slob is also enriched in synaptic boutons, although its distribution appears to be less restricted than that of 14-3-3. Both 14-3-3 and dSlo are prominent in synaptic boutons, where they colocalize. Two serine residues in Slob are required for 14-3-3 binding, and the binding is dynamically regulated in Drosophila by calcium/calmodulin-dependent kinase II (CaMKII) phosphorylation of these residues. Slob itself increases the voltage sensitivity of dSlo, whereas 14-3-3 decreases the channel's voltage sensitivity (Zhou, 1999).
What are the molecular details of the profound downregulation of dSlo channel activity by 14-3-3? Members of the family of KCa channels are subject to modulation by a variety of molecular mechanisms, ranging from protein phosphorylation to oxidation/reduction reactions. It is conceivable that simply the binding of 14-3-3 to dSlo via Slob is sufficient to alter the gating of the channel, as appears to be the case for ß subunit interactions with Slowpoke and other potassium channels. Alternatively, 14-3-3 may act as another scaffolding component, to bring one of the protein kinases that it is known to bind into the proximity of the channel. Indeed, because 14-3-3 dimerizes, it might bridge the interactions of several different signaling proteins with the channel. It will be interesting to determine whether the Raf protein kinase, one of the kinases that binds 14-3-3, can phosphorylate and modulate dSlo, because Raf is a key player in the mitogen-activated protein (MAP) kinase pathway that conveys signals from the plasma membrane to the cell nucleus. Activation of this pathway can influence ion channel expression and activity, and potassium channel activity in turn can modulate tyrosine kinase signaling in cells. Thus, the present findings raise the intriguing possibility that a potassium channel regulatory complex is involved in MAP kinase signaling and the regulation of many fundamental cell processes (Zhou, 1999 and references).
The finding that the interaction of Slob with 14-3-3 requires Slob phosphorylation is consistent with studies of other 14-3-3 binding proteins. It is especially intriguing that the binding can be regulated in vivo by changes in the activity of CaMKII; these results suggest that there may be dynamic physiological regulation of dSlo channel activity by 14-3-3 that depends on the phosphorylation state of Slob. In view of the presynaptic colocalization of the three proteins described here, it is interesting that it is the CaMKII phosphorylation of Slob that regulates 14-3-3 binding. CaMKII is also present at a high concentration presynaptically in Drosophila, and thus the same calcium rise that evokes transmitter release might promote phosphorylation of Slob, binding of 14-3-3, and downregulation of dSlo (Zhou, 1999 and references).
14-3-3 eta is required for photoreceptor development in Drosophila, while another isoform, 14-3-3 epsilon, also influences photoreceptor development by regulating Ras-mediated signaling pathways. It is particularly interesting that flies lacking 14-3-3 zeta are severely impaired in an olfactory learning task and exhibit defects in basal synaptic transmission as well as in synaptic plasticity (Broadie, 1997). 14-3-3 zeta is enriched in presynaptic boutons of the neuromuscular junction (Broadie, 1997), consistent with a role in synaptic transmission. This presynaptic localization of 14-3-3 zeta is confirmed and in addition it colocalizes with both dSlo and Slob in presynaptic boutons. Slowpoke channels are also enriched in presynaptic endings in rat brain and frog neuromuscular junction, where they influence transmitter release. Since dSlo current contributes to membrane repolarization and helps to limit transmitter release from Drosophila nerve terminals, and 14-3-3 downregulates dSlo via Slob, it is plausible that there is greater nerve terminal dSlo current in 14-3-3 mutant flies and that this accounts for the decreased synaptic transmission seen in these mutants (Broadie, 1997). The possibility that a modulatory complex associated with a neuronal ion channel may influence synaptic transmission, and ultimately higher brain functions, is an attractive hypothesis for future investigation (Zhou, 1999 and references).
PAR-1 kinases are required to determine the anterior-posterior (A-P) axis in C. elegans and Drosophila, but little is known about their molecular function. Drosophila 14-3-3 proteins, 14-3-3epsilon and 14-3-3zeta/Leonardo (Leo) represent the Drosophila homologs of C. elegans PAR-5. 14-3-3 proteins have been identified as Drosophila PAR-1 interactors; PAR-1 binds a domain of 14-3-3 distinct from the phosphoserine binding pocket. PAR-1 kinases phosphorylate proteins to generate 14-3-3 binding sites and may therefore directly deliver 14-3-3 to these targets. 14-3-3 mutants display phenotypes identical to par-1 mutants in oocyte determination and the polarization of the A-P axis. Together, these results indicate that PAR-1's function is mediated by the binding of 14-3-3 to its substrates. The C. elegans 14-3-3 protein, PAR-5, is also required for A-P polarization, suggesting that this is a conserved mechanism by which PAR-1 establishes cellular asymmetries (Benton, 2002).
PAR-1 contains three conserved domains: centrally-located kinase and ubiquitin-associated (UBA) domains, and a C-terminal domain of unknown function. Since the C-terminal domain is dispensable for PAR-1 function in the germline, a yeast two-hybrid screen was performed using a bait containing the kinase and UBA domains. The largest class of preys, representing over 25% of the recovered clones, corresponded to the two Drosophila 14-3-3 proteins, 14-3-3epsilon and 14-3-3zeta/Leonardo (Leo). These interactors represent the Drosophila homologs of C. elegans PAR-5, and this interaction appears to be conserved, since PAR-5 can bind to a fragment of C. elegans PAR-1 (Benton, 2002).
To confirm this interaction by an independent assay, in vitro-synthesized, labeled full-length Drosophila PAR-1 was incubated with bacterially expressed maltose binding protein (MBP)-tagged 14-3-3 proteins bound to amylose beads. Beads containing MBP:14-3-3 fusion proteins, but not MBP alone, efficiently precipitate PAR-1, indicating that this interaction is direct (Benton, 2002).
14-3-3 proteins regulate the activity or subcellular localization of a diverse set of proteins, including several protein kinases, by binding in a phosphorylation-dependent manner to conserved motifs (RSXpSXP or RX1-2pSX2-3pS). Using the yeast two-hybrid system, it was found that 14-3-3 appears to associate with the kinase domain of PAR-1. This contrasts with the interaction of 14-3-3 with other kinases, such as Raf and Wee1, in which 14-3-3 recognizes a phosphoserine-containing motif lying outside the catalytic domain. This interaction with PAR-1 is kinase specific, since 14-3-3 does not bind to the catalytic domains of PKA or aPKC (Benton, 2002).
The region of 14-3-3epsilon that interacts with PAR-1 was determined using the molecular information of three missense alleles of 14-3-3epsilon (Chang, 1997). These alleles were isolated as suppressors of activated Ras or Raf and impair the function of 14-3-3 in Ras/Raf/MAPK signaling. One mutation, E183K, lies within the phosphoserine binding pocket and affects a residue that directly contacts phosphoserine-peptide ligands. The others, F199Y and Y214F, are both located outside this pocket in a hydrophobic region of unknown function. Each of these three mutations were introduced into a 14-3-3epsilon prey clone and their effects on the intermolecular interactions of 14-3-3epsilon were tested (Benton, 2002).
Since 14-3-3 proteins function as dimers, whether these mutations influenced the dimerization property of 14-3-3epsilon was tested. 14-3-3epsilon can form both homodimers and heterodimers with Leo, and none of the three mutations significantly affects these interactions. This is consistent with the location of these mutations in regions distinct from the dimerization interface and indicates that global protein structure and stability are not affected. Interactions were tested of these mutant proteins with a domain of Drosophila Raf that contains a conserved 14-3-3-recognition motif (R740SApSEP745). Raf binds to both Drosophila 14-3-3 isoforms, and the interaction with 14-3-3epsilon is completely abolished by the E183K mutation, but not by the F199Y and Y214F mutations, as expected for an association via the phosphoserine binding pocket (Benton, 2002).
These mutations have opposite effects upon the interaction with PAR-1: E183K does not impair binding, whereas the other mutations either result in a severe (F199Y) or a more modest (Y214F) reduction in the strength of this interaction. These results indicate that 14-3-3epsilon does not bind PAR-1 via its phosphoserine binding pocket, consistent with the lack of canonical binding motifs in PAR-1. The interaction instead appears to be mediated by a novel interface on the external surface of the 14-3-3 molecule. Since the F199 and Y214 residues are conserved in 99% of 14-3-3 sequences, this interface is likely to exist in all isoforms (Benton, 2002).
Since binding of PAR-1 to 14-3-3 should leave the phosphoserine binding pocket vacant, and PAR-1 is a serine/threonine kinase, it was reasoned that PAR-1 might be involved in regulating the phosphorylation-dependent interactions of 14-3-3 with other proteins. Whether PAR-1 can phosphorylate proteins to generate the phosphoserine epitope recognized by 14-3-3 was tested. Using either immunoprecipitated or bacterially expressed PAR-1, efficient phosphorylation was observed of the 14-3-3-interacting portion of Raf. Phosphorylation of Ser743, or the equivalent residue in Raf homologs, is essential for 14-3-3 binding and for Raf function in vivo. This residue was mutated to alanine and this was found to completely abolish the phosphorylation by PAR-1, indicating that PAR-1 specifically phosphorylates Raf to generate a 14-3-3 binding site. This activity of PAR-1 does not require the presence of 14-3-3, and addition of 14-3-3 to this assay does not detectably affect Raf phosphorylation. A mammalian PAR-1 homolog, C-TAK1, is able to phosphorylate proteins such as KSR and Cdc25C within 14-3-3 binding sites, suggesting that this specificity is a conserved property of this kinase family (Benton, 2002).
To test whether 14-3-3 proteins are involved in par-1-dependent processes in vivo, loss-of-function mutations in 14-3-3epsilon and leo were analyzed. Surprisingly, flies homozygous for a protein null allele of 14-3-3epsilon (14-3-3epsilonj2B10) are viable. However, females lay very few eggs, which fail to hatch. Most egg chambers from these females lack differentiated oocytes, as revealed by DNA staining, which distinguishes the oocyte karyosome from the 15 polyploid nurse cells. An identical phenotype is observed in ovaries from flies containing this allele over a deficiency, indicating that the phenotype is specific for this locus. To determine where 14-3-3epsilon is required, clones of this allele were generated in either the germline or somatic follicle cells. Defects in oocyte differentiation were observed only in germline clones; thus, like PAR-1, 14-3-3epsilon is required in the germline for oocyte differentiation (Benton, 2002).
Oocyte determination depends on the MT-dependent transport of specific factors, such as Orb and the germ cell centrosomes, to one cell in the cyst. These factors initially concentrate at the anterior of this cell but subsequently translocate around the nucleus and concentrate along the posterior cortex. This second step appears to require the establishment of a diffuse MTOC along the posterior of the cell and is essential for its stable determination as the oocyte. The formation of this MTOC can be visualized using an antibody to Minispindles (MSPS), a MAP that localizes to sites of MT nucleation. In wild-type egg chambers, MSPS accumulates along the posterior cortex. This accumulation is undetectable in 14-3-3epsilon mutants, indicating that the MTOC has failed to form. Orb and the centrosomes therefore do not undergo the anterior-to-posterior movement and eventually diffuse away as this cell exits meiosis and adopts a nurse cell fate. These phenotypes are indistinguishable from those of par-1 null mutant cysts, indicating that 14-3-3epsilon and PAR-1 function together in this specific step of oocyte determination (Benton, 2002).
In contrast to 14-3-3epsilon mutants, germline clones of a strong lethal allele of leo (leoP1188) display no defects in this process. Since the 14-3-3epsilon phenotype is incompletely penetrant, whether 14-3-3 proteins have partially redundant functions in the germline was tested. Removal of one copy of leo in 14-3-3epsilon mutant clones results in a fully penetrant defect in oocyte determination. Furthermore, removal of one copy of 14-3-3epsilon in leo mutant cysts uncovers an important contribution of leo in this process, since 84% of these cysts display defects in Orb localization. Thus, although 14-3-3epsilon has the predominant function in oocyte determination, Leo can partially compensate in its absence (Benton, 2002).
Polyclonal antibodies against 14-3-3epsilon and Leo were used to examine their localization in the germline. These antibodies are specific, since they do not stain tissue mutant for the corresponding isoform. 14-3-3epsilon is highly expressed in the dividing germline cells in the germarium, and colocalizes with PAR-1 on the fusome, a membranous structure that branches into each germ cell during the early germ cell divisions. The asymmetric partitioning of the fusome during these divisions results in one cell always inheriting more fusome material, which may provide an initial cue to specify this cell as the oocyte. The colocalization of PAR-1 and 14-3-3epsilon on the fusome may therefore represent a mechanism to concentrate these proteins in the future oocyte (Benton, 2002).
At later stages, 14-3-3epsilon colocalizes with PAR-1 at the ring canals, which interconnect the germline cells in each cyst. 14-3-3epsilon can be detected in the cytoplasm and around the cortex of the oocyte but, unlike PAR-1, does not accumulate at the posterior pole. Leo is also expressed in the germline and displays a similar localization to ring canals, but is expressed at very low levels in the germarium (Benton, 2002).
While the fusome and ring canals may represent sites of physical and functional association of PAR-1 with 14-3-3, its localization to these sites is not affected in 14-3-3 mutants, indicating that 14-3-3 binding does not simply act to target PAR-1 to these subcellular destinations. Mutations in the Drosophila PAR-3 homolog Baz cause similar phenotypes in oocyte determination as par-1 and 14-3-3 mutants. However, Baz concentrates at distinct sites in the germarium, in circles around each ring canal that also contain components of adherens junctions, and this localization is not detectably affected in 14-3-3 mutants (Benton, 2002).
The high early cytoplasmic concentration of 14-3-3epsilon has prevented an conclusive determination of whether the fusome localization of 14-3-3epsilon is PAR-1 dependent. However, 14-3-3epsilon is detectable at ring canals in cysts homozygous for a par-1 null allele (Benton, 2002).
To determine if 14-3-3 proteins function with PAR-1 in the repolarization of the oocyte to define the A-P axis, the distribution of osk mRNA and Stau was examined in late-stage egg chambers recovered from homozygous and hemizygous 14-3-3epsilonj2B10 females, and in germline clones of this allele. These mutants display a partially penetrant phenotype, in which osk mRNA and Stau accumulate in dots in the middle of the oocyte. Twenty seven percent of egg chambers display both ectopic and posterior accumulation of osk mRNA and Stau protein, and four percent contain only mislocalized dots. These defects are very similar to those of hypomorphic par-1 mutants and can be strongly enhanced by removal of one copy of par-1 (Benton, 2002).
Although most bcd mRNA localizes normally to the anterior cortex in 14-3-3epsilon mutants, a small proportion is mislocalized along the lateral cortex, and occasionally at the posterior. In contrast to previous observations, such defects in bcd mRNA distribution are also observed in par-1 mutants. These are more pronounced at stages 8-9 than at stage 10, which might reflect a partial recovery in bcd mRNA localization to the anterior between these stages or the diffusion of the mRNA away from the lateral and posterior cortices due to a failure in anchoring. Other mutants that affect the localization of bcd and osk mRNAs, such as gurken, also disrupt the migration of the oocyte nucleus to the dorsal-anterior corner. As in par-1 mutants, however, oocyte nucleus migration appears to be unaffected in 14-3-3epsilonj2B10 mutants (Benton, 2002).
Oocytes that are homozygous for leoP1188 do not display polarity defects. However, strong dominant genetic interactions are observed between 14-3-3 mutants. Thus, these isoforms also function partially redundantly in this process (Benton, 2002).
To gain insights into the basis for the defects in mRNA localization, the organization of the MT cytoskeleton was examined, using a MT plus end marker, Kin:ß-gal. This marker accumulates at the posterior pole in wild-type oocytes, suggesting that the majority of MT plus ends are focused on this site. In contrast, Kin:ß-gal concentrates in the center of 14-3-3epsilon mutant oocytes, indicating that MT plus ends are focused incorrectly, and providing an explanation for the defects in osk mRNA/Stau distribution. The organization of oocyte MTs was directly analyzed using both a FITC-conjugated anti-alpha-tubulin antibody and a Tau:GFP reporter of MT distribution in living egg chambers . In contrast to the wild-type anterior-to-posterior gradient of MTs, 14-3-3epsilon mutants show a uniform distribution of MTs around the oocyte cortex, with the lowest density of MTs in the center. These defects in MT organization are indistinguishable from those of par-1 mutants (Benton, 2002).
The combination of phenotypes in osk and bcd mRNA localization and MT organization is, thus far, unique to par-1 and 14-3-3 mutants, and strongly suggests that they function together in the polarization of the A-P axis (Benton, 2002).
To determine the importance of the 14-3-3 protein interaction domains in vivo, the phenotypes of the 14-3-3epsilon missense alleles, 14-3-3epsilonF199Y and 14-3-3epsilonE183K, were characterized. Neither mutation significantly affects the level or localization of the protein, as assessed by immunostainings. 14-3-3epsilonE183K displays penetrant defects in both oocyte determination and polarization. The penetrance of the latter is almost three times that observed with the protein null allele, indicating that the E183K mutant protein functions as a dominant negative, presumably through the formation of nonfunctional heterodimers with Leo. Thus, the interaction of 14-3-3 dimers with phosphorylated targets is critical for its function in the germline (Benton, 2002).
14-3-3epsilonF199Y mutant egg chambers do not exhibit significant defects in oocyte determination or polarization, consistent with previous reports that this allele only displays phenotypes under genetically sensitized conditions (Chang, 1997). In the absence of leo, however, this allele has a dominant phenotype, with 11% of leoP1188;14-3-3epsilonF199Y/+ egg chambers displaying defects in oocyte determination. Thus, the PAR-1 interaction interface is also important for 14-3-3 function (Benton, 2002).
Thus loss-of-function mutations in 14-3-3 cause phenotypes identical to par-1 mutants in both the initial polarization of the oocyte and the repolarization that defines the A-P axis. These results indicate that 14-3-3 functions as an essential cofactor for PAR-1 in the generation of polarity (Benton, 2002).
Given the diverse roles of 14-3-3 proteins, it is very surprising that the only essential requirement for 14-3-3epsilon is in PAR-1-dependent polarization events in the Drosophila germline. A similar dedication of 14-3-3 function may exist in C. elegans, where animals homozygous for hypomorphic mutations in the 14-3-3 isoform encoded by par-5 are viable but give rise to progeny with highly penetrant defects in the polarization of the A-P axis. Indeed, the discovery that 14-3-3 is required for the initial polarization of the oocyte in the germarium reveals a remarkable homology between the generation of the first A-P asymmetries in flies and worms. Mutations in 14-3-3epsilon give a very specific defect in oocyte determination, in which the oocyte is initially specified correctly but fails to establish a posterior MTOC and to translocate oocyte-specific factors from the anterior to the posterior cortex. This phenotype is identical to that of par-1 null mutants, and the colocalization of PAR-1 and 14-3-3 on the fusome supports the idea that they function together in this process (Benton, 2002).
The Baz/PAR-6/aPKC complex is also required for this step of oocyte determination but localizes to a distinct site in the germarium. Furthermore, it has recently been shown that mutants in the Drosophila homolog of PAR-4 display this phenotype (S. Martin and D.S.J., unpublished data reported in Benton, 2002). Thus, this early polarization of the oocyte requires the Drosophila homologs of five of the six par genes that mediate the A-P polarization of the C. elegans zygote. The final gene, par-2, has no obvious homologs in other organisms and may perform some function that is unique to C. elegans (Benton, 2002).
Although the full complement of PAR proteins is necessary for the initial polarization of the Drosophila oocyte in the germarium, the Baz/PAR-6/aPKC complex does not appear to be required for the repolarization of the oocyte at stage 7. In baz and par-6 null germline clones, a few egg chambers escape the block in oocyte determination, and these complete oogenesis normally, displaying no defects in the localization of Stau to the posterior. Thus, the PAR-1/14-3-3 complex can function to polarize the oocyte independently of these other PAR proteins. PAR-1 also is required for the apical-basal polarity of the follicular epithelium, and localizes to the basolateral domain in these cells. It is interesting to note that 14-3-3epsilon concentrates basolaterally in follicle cells, raising the possibility that it functions with PAR-1 in this process as well. The PAR-1/14-3-3 complex may therefore represent a conserved polarity 'cassette' that plays an analogous role to the Baz/PAR-6/aPKC complex. This requirement is not universal, however, because PAR-1 does not appear to be necessary for the apical-basal polarization of the neuroblasts (J. Kaltschmidt and R. B., unpublished data reported in Benton, 2002), which depends upon Baz, PAR-6, and aPKC. Thus, the two PAR protein complexes may comprise distinct modules that can function either together or separately to generate polarity in different contexts (Benton, 2002 and references therein).
While the common requirement for the PAR proteins strongly suggests that the mechanisms that generate the first A-P asymmetries are conserved between flies and worms, the regulatory relationships between these proteins are not conserved. The hierarchy of PAR protein function in C. elegans has been inferred from the effects of mutants in each par gene on the localization of the other PAR proteins. This analysis places PAR-5 at the top of the hierarchy because it is required for the anterior localization of the PAR-3/PAR-6/PKC-3 complex and the posterior localization of PAR-2 and PAR-1, whereas PAR-1 lies at the bottom because par-1 mutants have little effect on the asymmetric localization of other PAR proteins. In contrast, in Drosophila, Baz and PAR-1 are localized normally in 14-3-3 mutants. Furthermore, although the localization of members of the PAR-3/PAR-6/PKC-3 complex are codependent in the C. elegans zygote, this is not the case in the Drosophila oocyte, nor are they required for the localization of PAR-1 to the fusome. The different positions of PAR-5 and PAR-1 in the C. elegans hierarchy indicate that PAR-5 functions independently of PAR-1 in the localization of the other PAR proteins, but this early requirement makes it difficult to assess whether it is also necessary at other stages in the pathway. The results in Drosophila and the observation that C. elegans PAR-5 and PAR-1 interact in yeast raise the possibility that PAR-5 also functions downstream of PAR-1 (Benton, 2002).
Although the results indicate that PAR-1 and 14-3-3 function together to polarize the oocyte at two stages of oogenesis, the mechanisms by which they generate these polarities are unknown. The repolarization of the oocyte at stage 7 principally affects the organization of the MTs. The original posterior MTOC is disassembled, and the MTs are reorganized to form an A-P gradient, in which most MTs appear to be nucleated from the anterior cortex, with their plus ends extending toward the posterior pole. In 14-3-3 and par-1 mutants the MTs are evenly distributed around the cortex, and a MT plus end marker and osk mRNA/Stau localize to the center of the oocyte. These observations led to the proposal that PAR-1 functions to recruit the plus ends to the posterior. This study shows that par-1 and 14-3-3 mutants also display mislocalization of bcd mRNA around the cortex. Since this mRNA is believed to be transported to the minus ends of MTs, this suggests that MTs are abnormally nucleated from all regions of the oocyte. Thus, PAR-1 and 14-3-3 may also contribute to the generation of the MT gradient by specifically inhibiting MT nucleation along the posterior and lateral cortices. The role of PAR-1 and 14-3-3 in the initial polarization of the oocyte in the germarium is also likely to involve MTs since their loss results in a failure in the formation of an MTOC at the posterior of the cell. The mechanisms that control the formation of this MTOC are not known, however, and it is unclear whether PAR-1 and 14-3-3 function in the same way to polarize the oocyte at both stages (Benton, 2002).
A model for 14-3-3 function with PAR-1 is presented. 14-3-3 proteins regulate the activity of numerous cellular proteins in a phosphorylation-dependent manner by binding as dimers to phosphoserine/threonine-containing motifs. In many cases, this regulation involves sequestration of the target protein in the cytoplasm. For example, 14-3-3 binding to the proapoptotic factor Bad blocks its translocation to mitochondria. 14-3-3 can also directly regulate the activity of its targets: the association of 14-3-3 with serotonin N-acetyltransferase, for example, enhances its ability to bind substrates. The interaction of 14-3-3 with PAR-1 differs from these canonical 14-3-3/target interactions in several respects: (1) the binding of 14-3-3 does not appear to regulate PAR-1 activity, since 14-3-3 mutants have no effect on PAR-1 localization or stability in vivo, or on kinase activity in vitro; (2) the PAR-1 kinase domain lacks both of the well-defined 14-3-3 binding motifs, and interacts with a novel hydrophobic region that is distinct from the phosphoserine binding pocket, which should therefore still be available to bind to other proteins. Thus, 14-3-3 may act as a cofactor for PAR-1 by binding to proteins that are phosphorylated by the kinase. In support of this, it has been demonstrated that PAR-1 can specifically phosphorylate a 14-3-3 binding site in Raf (Benton, 2002).
These observations suggest a model in which PAR-1 has a dual role in regulating 14-3-3/target interactions, first by generating the 14-3-3 binding phosphoepitope, and second by directly delivering 14-3-3 to these sites. Once 14-3-3 is bound to target proteins, its continued association with PAR-1 would maintain the kinase in close proximity to its substrate, which might ensure the stable maintenance of the phosphorylated state (Benton, 2002).
In addition to their role in establishing cell polarity, PAR-1 kinases have been implicated in a diverse range of other cellular processes. The closest mammalian homolog of PAR-1, C-TAK1, was initially purified as an activity that phosphorylates Cdc25C on Ser216. The in vivo significance of this regulation is unknown, but phosphorylation of this site by a distinct kinase, Chk1, induces 14-3-3 binding, and this inhibits Cdc25C as part of the DNA damage checkpoint. C-TAK1 also phosphorylates KSR to promote 14-3-3 binding, which sequesters KSR in the cytoplasm and inhibits EGF signaling. These biochemical activities of C-TAK1 are consistent with the data in Drosophila showing that PAR-1 phosphorylates a 14-3-3 binding site in Raf, and that 14-3-3 mutants give identical phenotypes to par-1 mutants in the germline. The ability to phosphorylate 14-3-3 binding sites may be a general property of PAR-1 kinases, which accounts for the diversity of their functions (Benton, 2002).
Consistent with this, other PAR-1 substrates have been shown to associate with 14-3-3 or contain conserved potential 14-3-3 recognition motifs. The vertebrate PAR-1 homologs, MARK1 and MARK2, were identified as kinases that phosphorylate Tau to inhibit its MT binding ability. 14-3-3 interacts with the MT binding domain of Tau and appears to compete with tubulin for Tau binding. MARK kinase regulation of Tau may therefore be mediated through 14-3-3, which physically blocks the association of Tau with MTs. PAR-1 also phosphorylates the Wingless pathway component Dishevelled. This phosphorylation has been mapped to a 30 amino acid region of the protein, which contains a putative 14-3-3 recognition motif (amino acids 234-242: RTSSYSS) that is essential for its function in planar polarity (Benton, 2002).
The intimate functional relationship between PAR-1 and 14-3-3 raises the possibility that this kinase might be involved in regulating other processes involving 14-3-3 proteins. For example, the observation that PAR-1 phosphorylates Raf to generate a 14-3-3 binding site makes it a candidate for the unidentified kinase that regulates Raf in vivo. In support of this, this study shows that the F199Y and Y214F mutations in 14-3-3epsilon that affect signaling through Raf, impair the interaction of 14-3-3epsilon with PAR-1 (Benton, 2002).
Although many of the activities of PAR-1 kinases may be mediated by inducing 14-3-3 binding, this is probably not the only mechanism by which they act. Drosophila PAR-1 has recently been proposed to have a third function in the germline, in which it phosphorylates, and so stabilizes, OSK protein at the posterior pole of the oocyte to ensure its levels are high enough to specify the germ cells. Unlike PAR-1, 14-3-3 is not detectably enriched at the posterior, suggesting that this function of the kinase might operate via a 14-3-3-independent mechanism. C. elegans PAR-1 may have a similar function in germline specification, through the regulation of P-granule stability, which does not require PAR-5 (Benton, 2002).
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