InteractiveFly: GeneBrief

Pi4KIIα: Biological Overview | References

Gene name - Pi4KIIα

Synonyms - PI4KII, PI4KIIalpha, CG2929

Cytological map position - 83B1-83B2

Function - enzyme

Keywords - phosphatidylinositol 4-phosphate, membrane trafficking, rhabdomere biogenesis, salivary glands, mucin-containing glue granules, synapse, vesicles

Symbol - Pi4KIIα

FlyBase ID: FBgn0037339

Genetic map position - chr3R:1383380-1389347

Classification - Phosphoinositide 3-kinase (PI3K)-like family

Cellular location - cytoplasmic

NCBI link: EntrezGene
Pi4KIIalpha orthologs: Biolitmine

Type II phosphatidylinositol 4-kinase (PI4KII) produces the lipid phosphatidylinositol 4-phosphate (PI4P), a key regulator of membrane trafficking (see Metabolic fates of phosphatidic acid). This study generated genetic models of the sole Drosophila melanogaster PI4KII gene. A specific requirement for PI4KII emerged in larval salivary glands. Mucin-like glue, secreted by salivary glands, serves to adhere the pupal case to a solid substrate during metamorphosis. In PI4KII mutants, mucin-containing glue granules fail to reach normal size, with glue protein aberrantly accumulating in enlarged Rab7-positive late endosomes. Presence of PI4KII at the Golgi and on dynamic tubular endosomes indicated two distinct foci for its function. First, consistent with the established role of PI4P in the Golgi, PI4KII is required for sorting of glue granule cargo and the granule-associated SNARE Snap24. Second, PI4KII also has an unforeseen function in late endosomes, where it is required for normal retromer dynamics and for formation of tubular endosomes that are likely to be involved in retrieving Snap24 and Lysosomal enzyme receptor protein (Lerp) from late endosomes to the trans-Golgi network. Genetic analysis of PI4KII in flies thus reveals a novel role for PI4KII in regulating the fidelity of granule protein trafficking in secretory tissues (Burgess, 2012).

Phosphatidylinositol 4-kinases (PI4Ks) synthesize phosphatidylinositol 4-phosphate (PI4P), a crucial regulator of membrane trafficking (reviewed Balla, 2006; D'Angelo, 2008; Graham, 2011). Mammals have two type II and two type III PI4Ks (PI4KIIα, PI4KIIβ, PI4KIIIα, PI4KIIIβ), whereas budding yeast, flies and worms each have a single PI4KII and two type III enzymes. Type II PI4Ks and PI4KIII&brys; regulate intracellular trafficking in yeast and mammalian cells. However, the role of these enzymes in animal development remains largely unknown (Burgess, 2012).

Budding yeast PI4KIIIβ (Pik1p) localizes to the Golgi, where it synthesizes an essential pool of PI4P required for post-Golgi secretion. In mammalian cells, PI4KIIIβ and PI4KIIα localize to the Golgi and are implicated in secretory trafficking. Manipulating PI4KIIIβ by overexpression or dominant-negative constructs affects post-Golgi trafficking, whereas depleting PI4KIIα blocks trans-Golgi network (TGN) recruitment of the clathrin adaptor protein-1 (AP-1) complex and decreases constitutive secretion from the TGN to the plasma membrane. Previous studies have shown that Drosophila melanogaster PI4KIIIβ [Four wheel drive (Fwd)] localizes to the Golgi, where it synthesizes a pool of PI4P required for spermatocyte cytokinesis and male fertility (Brill, 2000; Polevoy, 2009). However, fwd is non-essential, raising the question of whether PI4KII also participates in the synthesis of Golgi PI4P (Burgess, 2012).

In addition to the Golgi, type II PI4Ks also localize to endosomes. Budding yeast PI4KII (Lsb6p) is non-essential and has a non-catalytic role in endosome motility. Mammalian PI4KIIα is palmitoylated and localizes to dynamic endosomal tubules, where it promotes the transport and degradation of EGFR (Minogue, 2006; Barylko, 2009). PI4KIIα requires adaptor protein complex-3 (AP-3) for its endosomal localization, and depleting PI4KIIα from HEK293 cells causes redistribution of AP-3 to the cytoplasm and accumulation of AP-3 cargo proteins and SNAREs on enlarged late endosomes (LEs) (Salazar, 2005; Craige, 2008). Less is known about mammalian PI4KIIβ, which localizes to endosomes and translocates to the plasma membrane in response to activated Rac (Burgess, 2012).

PI4KIIα also associates with a range of secretory organelles, including immature secretory granules, chromaffin granules, glucose transporter 4-containing vesicles and synaptic vesicles. Nonetheless, despite data implicating PI4KIIα in EGFR signaling, neurotransmission and regulated secretion, homozygous mutant mice lacking the catalytic domain of PI4KIIα show no obvious developmental defects, but rather exhibit late onset neurodegeneration (Simons, 2009). The cellular functions of PI4KII enzymes during animal development remain unknown (Burgess, 2012).

Drosophila salivary glands provide an excellent system with which to investigate membrane trafficking pathways. At the mid-third instar larval stage (mid-L3), salivary glands begin producing highly glycosylated glue proteins (cargo) that traffic through the endoplasmic reticulum and Golgi before being incorporated into regulated secretory granules (glue granules) at the TGN. Indeed, previous studies showed that clathrin and AP-1, which colocalize with the adaptor EpsinR (Liquid facets-related) at the TGN, are essential for glue granule formation (Burgess, 2011). Glue granules accumulate in salivary cell cytoplasm, where they undergo growth by accretion and homotypic fusion until a high-titer pulse of ecdysone triggers their release at the end of L3. The secreted mucin-like glue then serves to adhere the pupal case to a solid substrate during metamorphosis (Burgess, 2012).

This study investigated PI4KII function in Drosophila. Flies bearing null mutations in PI4KII are viable, but have strikingly small glue granules and accumulate glue protein in enlarged LEs. Moreover, loss of PI4KII leads to missorting of Snap24, a SNARE implicated in granule fusion. Catalytic activity of PI4KII is required for glue granules and LEs of normal size, as well as for the formation of endosomal tubules. Based on these data, it is proposed that PI4KII is required for the proper sorting and retrieval of secretory granule proteins (Burgess, 2012).

PI4KIIΔ mutants are viable and do not exhibit any gross morphological defects, although they exhibit defects in membrane trafficking during the formation of regulated secretory granules in the larval salivary gland. Mammalian PI4KII has long been suspected to participate in regulated secretion. Nonetheless, Drosophila PI4KII does not localize to the limiting membrane of glue granules and is dispensable for secretion. Instead, PI4KII localizes to the TGN and to endosomes, and PI4KIIΔ mutants exhibit small glue granules and enlarged late endosomes (LEs) that aberrantly accumulate granule cargo proteins, Lerp and SNAREs. The results therefore suggest a role for PI4KII in intracellular trafficking pathways required for normal granule biogenesis (Burgess, 2012).

The data point to a contribution of PI4KII in regulating the fidelity of sorting events at the Golgi or to retrieval of proteins from LEs to the TGN. At the Golgi, failure to properly segregate cargo could lead to mixing of glue proteins with lysosomal hydrolases destined for LEs. Similarly, failure to properly segregate SNARE proteins could result in granule-specific SNAREs being missorted to endosomes. TGN localization of AP-1 and EpsinR appears unaffected in PI4KIIΔ cells. This contrasts with the observation that AP-1 becomes cytoplasmic when PI4KIIα is depleted from HeLa cells, but is consistent with recent studies demonstrating that PI4KIIα is dispensable for AP-1 localization in HEK293 cells (Wang, 2003; Craige, 2008). PI4KII might exert a subtle influence on AP-1 or EpsinR. For example, changes in the levels or distribution of PI4P might influence the kinetics of AP-1 or EpsinR recruitment to membranes or their association with particular subdomains of the TGN. Indeed, partial loss-of-function AP-1 mutants exhibit small granules similar to those found in PI4KIIΔ mutants (Burgess, 2011). Alternatively, PI4KII might affect the Golgi recruitment of other PI4P-binding proteins involved in post-Golgi vesicular trafficking, for example GGA or GOLPH3 (Dippold, 2009; Hirst, 2009; Kametaka, 2010). Nonetheless, the overwhelming majority of Sgs3 appears to traffic normally to small secretory granules, indicating that post-Golgi trafficking of glue cargo proteins is relatively unaffected in PI4KIIΔ mutants. Hence, the data appear more consistent with the idea that PI4KIIΔ mutants are defective in the retrieval of proteins from LEs to the TGN (Burgess, 2012).

PI4KII localizes to an extensive network of dynamic, highly interconnected tubular endosomes. Emergence of these tubular endosomes coincides with the onset of glue granule biogenesis, suggesting that the two processes might be linked. Loss of PI4KII catalytic activity results in the accumulation of enlarged LEs and loss of tubule formation. Moreover, the granule SNARE Snap24 and Lerp accumulate on enlarged endosomes, suggesting a defect in retrograde trafficking of proteins involved in granule maturation and lysosomal trafficking. Indeed, retromer dynamics is greatly attenuated at these aberrant endosomes, further supporting a role for PI4P in retrograde transport (Wood, 2009). However, PI4KII is not generally required for retromer function, as PI4KIIΔ mutants show no obvious defects in Wingless signaling, unlike retromer mutants (Burgess, 2012).

One possibility is that a defect in retrograde trafficking from LEs to the TGN could explain both the small granule phenotype and the accumulation of Sgs3 in LEs in PI4KIIΔ mutants. In wild type, SNAREs could normally be retrieved from growing granules to LEs by the action of AP-1 and clathrin, which are present on immature granules (Burgess, 2011). Retrograde trafficking of SNAREs and Lerp from LEs to the TGN could occur via tubules that require PI4KII and PI4P. Alternatively, retrograde trafficking of these proteins could occur in carriers that are indirectly affected by loss of PI4KII. Any glue cargo proteins (e.g., Sgs3) inadvertently trafficked to LEs during AP-1/clathrin-dependent SNARE recycling would undergo lysosomal degradation. In PI4KIIΔ mutants, the defect in SNARE recycling could block homotypic granule fusion, leading to the small granule phenotype. Similarly, defects in lysosomal hydrolase trafficking caused by a failure to recycle Lerp could reduce lysosomal function and lead to an accumulation of Sgs3 in LEs in PI4KIIΔ mutants (Burgess, 2012).

Although both Fwd and PI4KII localize to the Golgi (Polevoy, 2009; this work), these PI4Ks carry out distinct functions in Drosophila development. Fwd recruits the recycling endosome regulator Rab11 to Golgi membranes and vesicles during spermatocyte cytokinesis (Brill, 2000; Polevoy, 2009), but is dispensable for glue granule formation. By contrast, PI4KII is required during glue granule biogenesis, but is dispensable for spermatocyte cytokinesis. At a cellular level, distinct roles were recently demonstrated for mammalian PI4KIIIβ and PI4KIIα in trafficking of β-glucocerebrosidase, the lysosomal enzyme that is defective in Gaucher disease (Jovic, 2012). Hence, unlike in budding yeast, where the Fwd homolog Pik1p is essential and the only known requirement for the type II PI4K Lsb6p is in endosome motility, PI4KII appears to have evolved a more prominent role in specialized trafficking events that occur in metazoan development (Burgess, 2012).

Rhabdomere biogenesis in Drosophila photoreceptors is acutely sensitive to phosphatidic acid levels

Phosphatidic acid (PA) is postulated to have both structural and signaling functions during membrane dynamics in animal cells. This study shows that before a critical time period during rhabdomere biogenesis in Drosophila photoreceptors, elevated levels of PA disrupt membrane transport to the apical domain. Lipidomic analysis shows that this effect is associated with an increase in the abundance of a single, relatively minor molecular species of PA. These transport defects are dependent on the activation state of Arf1. Transport defects via PA generated by phospholipase D require the activity of type I phosphatidylinositol (PI) 4 phosphate 5 kinase, are phenocopied by knockdown of PI 4 kinase, and are associated with normal endoplasmic reticulum to Golgi transport. It is proposed that PA levels are critical for apical membrane transport events required for rhabdomere biogenesis (Raghu, 2009).

During development, eukaryotic cells undergo morphogenetic changes to suit ongoing physiological needs. Effecting cell shape changes involves complex cell biological processes, including changes in both the cell membrane and the cytoskeletal. An essential element of membrane biogenesis is the need to achieve regulated vesicular transport such that membranes can be delivered to the desired domain of the cell. This process is thought to involve a complex interplay of the physical properties of the lipid constituents in membranes as well as the activities of proteins that can affect membrane curvature. Conceptually, the lipid constituents of the cell membranes could be those with essentially structural roles (such as phosphatidylcholine [PC], phosphatidylethanolamine, phosphatidylserine (PS), and cholesterol) and signaling lipids whose levels change in a regulated manner. These signaling lipids include DAG, its phosphorylated derivative phosphatidic acid (PA), and several phosphorylated species of phosphatidylinositol (PI) (Raghu, 2009).

In the simple eukaryote Saccharomyces cerevisiae that recapitulates most basal transport pathways conserved in higher eukaryotes, genetic analysis has implicated several lipids in regulating membrane traffic. Evidence showing that DAG and PA can affect membrane transport comes from yeast through analysis of SEC14, a gene that encodes a PI/PC transfer protein essential for viability and transport from the Golgi. The sec14 phenotype can be suppressed/bypassed by mutants in several genes that control biosynthesis of PI and PC. However, the ability of such mutants to bypass sec14 has an obligate requirement for SPO14 that encodes phospholipase D (PLD), an enzyme that generates PA from PC. Although Spo14p is not required for vegetative growth, it is required to form the prospore membrane (Rudge et al., 1998) and for PA synthesis during sporulation; loss of Spo14p leads to accumulation of undocked prospore membrane precursors vesicles on the spindle pole body. Thus, in yeast, PA generated by Spo14p activity plays a key role in this membrane trafficking event. Although the analysis of spo14 has implicated PA and its downstream lipid metabolites in membrane transport, to date there is little direct evidence to suggest that PA can function as a regulator of membrane traffic in metazoans. The idea that PA can function in a signaling capacity during membrane transport has been fueled by the observations that (1) in vitro ADP ribosylation factor (Arf) proteins, key mediators of membrane transport, can regulate the activity of PLD, (2) overexpression of PLD in several different cell types affects processes likely to require exocytosis, and (3) overexpression of mammalian PLD1 is reported to promote generation of β-amyloid precursor protein-containing vesicles from the TGN. However, the role of PA in regulating secretion in these settings remains unclear, and currently, there is little evidence linking demonstrable changes in PA levels with the molecular machinery that regulates membrane traffic in vivo (Raghu, 2009).

This study used Drosophila melanogaster photoreceptors as a model system to test the effect of altered PA levels on membrane traffic. It was shown that elevated levels of PA, achieved by manipulation of three genes (CDP diglyceride synthetase, Phospholipase D and retinal degeneration A), disrupt membrane transport to the apical domain of photoreceptors with defects in the endomembrane system (Raghu, 2009).

Work in mammalian cell culture models has suggested that the activity of a PI4K enzyme generating a Golgi localized pool of PI(4)P is important for regulating TGN exit. To test whether the activity of a PI(4)P-generating enzyme might be critical for rhabdomere biogenesis, the effect of down-regulating PI4K activity in developing photoreceptors was tested.The effect of down-regulating two genes that could encode PI4K activity: CG2929 (PI4KIIβ) and CG10260 (PI4KIIIα). This analysis revealed that down-regulation of CG2929 using RNAi phenocopied key aspects of the phenotype of Pld overexpression: (1) down-regulation in the levels of Rh1 protein, (2) formation of small and deformed rhabdomere, and (3) accumulation of abnormal endomembranes within the cell body. These findings suggest that the activity of PI4K is important for membrane transport (Raghu, 2009).

This study elevated PA levels using either cds1 and Pld or rdgA overexpression in each of which the only common and immediate biochemical outcome is the accumulation of PA. Using EM to directly visualize photoreceptor membranes, it was demonstrate that all three genetic manipulations cause defects in endomembrane organization characterized by a reduction in the size of the apical rhabdomere membrane and/or the accumulation of expanded membranous structures in the cell body. These observations, which are consistent with a defect in membrane transport to the apical domain, are highly reminiscent of defects seen in photoreceptors from Drosophila p47 and Rab11 mutants. Importantly, it was also demonstrated that in all three genotypes used to modulate PA levels, the abundance of a single molecular species of PA (16:0/18:2) was elevated without changes in the mass of structural lipids such as PC or of signaling lipids such as PI and DAG. Because this species of PA accounts for <10% of the total PA in photoreceptors, it is hypothesized that it represents a quantitatively minor phospholipid that functions in a signaling capacity to modulate membrane transport. The importance of PA for the described phenotypes is supported by the observation that overexpression of a type II PA phosphatase is able to partially revert the defects in rhabdomere biogenesis and endomembrane structure. Together, these findings provide compelling evidence that PA can affect the transport and organization of endomembranes in metazoan cells (Raghu, 2009).

Interestingly, although cds1, Pld, and rdgA overexpression all caused endomembrane defects in photoreceptors, the ultrastructural features of the abnormal transport intermediates were variable. All three genotypes showed variable degrees of defect in rhabdomere biogenesis. In addition, in the case of cds1, the accumulated endomembranes in the cell body resembled ER-like structures; with Pld overexpression, there were concentric and sheetlike tubular membranes, whereas with rdgA overexpression, in addition to tubular membranes, there were several vesicular intermediates that accumulated. It is likely that these differences reflect the distinct subcellular locations at which PA accumulates in each genotype. In cds1, PA probably accumulates in the ER site at which CDP-DAG synthase activity is normally present; PLD localization is limited to a compartment at the base of the rhabdomeres, and when overexpressed, DGK is distributed in punctate fashion throughout the ER. The generation of a suitable probe to visualize PA levels in a spatial dimension will be required to address this issue (Raghu, 2009).

During development, the precursor cells of the Drosophila eye undergo a substantial increase in size with the concomitant requirement for generating new plasma membrane. During the last 30% of pupal development, photoreceptors show an approximately fourfold increase in plasma membrane surface area, a process that requires a massive surge in polarized membrane transport capacity starting at ~70% pupal development. This study has defined a critical time window ~70% pupal development before which elevation of PA levels by overexpressing Pld results in the endomembrane defects. As this window precedes the onset of rapid membrane transport accompanying rhabdomere biogenesis, it is postulated that PA regulates the activity of a component of the molecular machinery that mediates polarized membrane transport during this period. Conceptually, in this respect the current findings are reminiscent of observations in the yeast Spo14 mutant, in which membrane transport defects are evident only during the generation of the prospore membrane. These findings are the first report of regulation of polarized membrane transport by PA in metazoans (Raghu, 2009).

During this study, it was observed that the effects of elevated PA (through both cds1 and Pld overexpression) were sensitive to the activation state of Arf1. In the cds1 mutant, in which PA is likely to be elevated in the ER, overexpression of the Arf1-GEF garz resulted in significantly less developed apical rhabdomere membrane but was not associated with enhanced accumulation of membranes in the cell body, which is consistent with the known effects of expressing constitutively active Arf1 in cells. In contrast, overexpression of dArf1-GAP resulted in an enhancement of defective rhabdomere biogenesis as well as a massive accumulation of ER membrane-like intermediates in the cell body. This observation suggests that the PA accumulating at the ER in cds1 influences the Arf1 cycle in this setting, resulting in the transport defects described. Previous biochemical analysis has shown that the activity of Arf1-GAP proteins can be regulated by at least three different lipids relevant to this study, namely PC, DAG, and PA. In the lipidomic analysis of cds1 retinae, it was found that the levels of 34:2 DAG and 34:2 PC were no different from wild type, whereas levels of 34:2 PA were elevated. On the basis of these findings, it is likely that the 34:2 PA that accumulates in cds1 photoreceptors causes the transport defects that were described by down-regulating the activity of Arf1 via dArf1-GAP (Raghu, 2009).

The development and maintenance of apical membranes in polarized cells requires both sorting at the TGN with exocytic transport as well as endocytosis. Thus, the phenotypes resulting from PLD overexpression could be a result of (1) altered membrane transport along one of the steps in the secretory pathway from the ER to the developing rhabdomere or (2) the consequence of enhanced endocytosis from the rhabdomere into the cell body (Raghu, 2009).

Experimental evidence presented in this study shows that in photoreceptors overexpressing Pld, the defect in rhabdomere biogenesis was dependent on the levels of active Arf1. In contrast, it was found that (1) altering the activity of Arf6, (2) down-regulation of α-adaptin, and (3) a reduction in the function of dynamin (shi) did not suppress the effects of overexpressing Pld. Collectively, these three observations strongly suggest that excessive clathrin-mediated endocytosis of rhabdomeral plasma membrane does not underlie the endomembrane defects resulting from Pld overexpression. A recent study has suggested a role for Arf1 in regulating a dynamin-independent endocytic pathway in Drosophila cells (Kumari, 2008). The role of this pathway in the effects of Pld overexpression remains unknown (Raghu, 2009).

Arf1 also exerts several effects on distinct steps of the exocytic pathway, including bidirectional transport between the ER and Golgi between Golgi cisternae and the regulation of exit from the late Golgi. In photoreceptors overexpressing Pld, the current analysis suggests that ER to trans-Golgi transport was normal, implying that the observed phenotypes are likely to involve a transport step between the TGN and plasma membrane, although observed phenotypes do not phenocopy exocyst loss of function. In Drosophila photoreceptors, PLD localizes to a restricted subcompartment at the base of the rhabdomeres. Although the molecular identity of this compartment has not been established, its subcellular localization is consistent with the ability of PA produced by PLD to regulate transport between the rhabdomeres and cell body. In TEMs of photoreceptors overexpressing Pld, the endomembranes that were observed in the cell body showed a tubulovesicular morphology extending throughout the cytoplasm. These membranes resemble large pleiomorphic carriers, transport intermediates that derive from the TGN destined for acceptor compartments like the plasma membrane. Furthermore, vesicles containing proteins destined for and normally restricted to the apical rhabdomere membrane (such as Rh1) are found in the cell body of photoreceptors overexpressing Pld. These observations are particularly interesting in the light of previous studies suggesting that PA generated by PLD can regulate the release of vesicles from the Golgi in an Arf1-dependent manner. However, in the absence of a clear identification of the accumulated membranes, the precise definition of the affected transport intermediates that were observed remains elusive (Raghu, 2009).

Arf1 can influence several events at the TGN, including the recruitment and activation of phospholipid-metabolizing enzymes. These include the recruitment and activation of PI4KIIIβ, generating PI(4)P, as well a direct role in activating the type I PIPkin on Golgi membranes in vitro. During this study, it was found that (1) down-regulating the levels of a PI4K expressed during photoreceptor development phenocopies key aspects of that seen with Pld overexpression, and (2) a strong hypomorph of the type I PIPkin (sktl) was able to substantially suppress the effects of Pld overexpression on rhabdomere biogenesis. These observations reflect the importance of tightly regulating type I PIPkin activity by PA for normal transport to the apical domain in polarized cells. They suggest that the regulation of PI(4)P levels is critical for rhabdomere biogenesis. In the context of interpreting the effects of Pld overexpression, it is possible that raised PA levels lead to enhanced activity of type I PIPkin consuming PI(4)P at the TGN, resulting in consequent transport defects to the apical membrane. Although it has not been possible to demonstrate reduced PI(4)P or increased PI(4,5)P2 levels at the Golgi in photoreceptors overexpressing Pld, the observation that overexpression of sktl in developing photoreceptors before the critical time window (but not a kinase-dead version) results in a massive defect in rhabdomere biogenesis underscores the importance of tight regulation of type I PIPkin activity during this process. Thus, a tight regulation of the balance of PI(4)P and PI(4,5)P2 levels through Arf1 activity may underlie the effects of PA in this system (Raghu, 2009).

Given the large number of effectors that can be regulated by PA, in the future, it will be important to identify and understand the functions of those that play a role in the biogenesis of rhabdomeres during photoreceptor development (Raghu, 2009).

Analysis of the catalytic domain of phosphatidylinositol 4-kinase type II

Phosphatidylinositol (PtdIns) 4-kinases catalyze the conversion of PtdIns to PtdIns 4-phosphate, the major precursor of phosphoinositides that regulates a vast array of cellular processes. Based on enzymatic differences, two classes of PtdIns 4-kinase have been distinguished termed Types II and III. Type III kinases, which belong to the phosphatidylinositol (PI) 3/4-kinase family, have been extensively characterized. In contrast, little is known about the Type II enzymes (PI4KIIs), which have been cloned and sequenced very recently. PI4KIIs bear essentially no sequence similarity to other protein or lipid kinases; hence, they represent a novel and distinct branch of the kinase superfamily. This study defined the minimal catalytic domain of a rat PI4KII isoform, PI4KIIalpha, and identified conserved amino acid residues required for catalysis. It was further shown that the catalytic domain by itself determines targeting of the kinase to membrane rafts. To verify that the PI4KII family extends beyond mammalian sources, Drosophila PI4KII and its catalytic domain were expressed and characterized. Depletion of PI4KII from Drosophila cells resulted in a severe reduction of PtdIns 4-kinase activity, suggesting the in vivo importance of this enzyme (Barylko, 2002).


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Biological Overview

date revised: 20 March 2013

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