Growing the intracellular bridges that connect nurse cells with each other and to the developing oocyte is vital for egg development. These ring canals increase from 0.5 microns in diameter at stage 2 to 10 microns in diameter at stage 11. Thin sections cut horizontally as you would cut a bagel, show that there is a layer of circumferentially oriented actin filaments attached to the plasma membrane at the periphery of each canal. By decoration with subfragment 1 of myosin, actin filaments of mixed polarities were found in the ring such as found in the 'contractile ring' formed during cytokinesis. In vertical sections through the canal the actin filaments appear as dense dots. At stage 2 there are 82 actin filaments in the ring; by stage 6 there are 717 and by stage 10 there are 726. Taking into account the diameter, this indicates that there are 170 microns of actin filaments per canal at stage 2, 14,000 microns at stage 9 and approximately 23,000 microns at stage 11 or one inch of actin filament! The density of actin filaments remains unchanged throughout development. What is particularly striking is that by stages 4-5, the ring of actin filaments has achieved its maximum thickness, even though the diameter has not yet increased significantly. Thereafter, the diameter increases. Throughout development, stages 2-11, the canal length also increases. Although the density through a canal remains constant from stage 5 on, the actin filaments appear as a net of interconnected bundles. Further information on this net of bundles comes from studying mutant animals that lack Kelch, a protein located in the ring canal that has homology to the actin binding protein, scruin. In this mutant, the actin filaments form normally but individual bundles that comprise the fibers of the net are not bound tightly together. Some bundles enter into the ring canal lumen but do not completely occlude the lumen. All these observations lay the groundwork for understanding of how a noncontractile ring increases in thickness, diameter, and length during development (Tilney, 1996).
The Drosophila kelch gene produces a single transcript with a UGA stop codon separating two open reading frames (ORF1 and ORF2). From the transcript, 76 kDa ORF1 and 160 kDa full-length (ORF1 + ORF2) proteins are made. The expression of these two proteins is regulated in a tissue-specific manner causing the ratio of full-length to ORF1 protein to vary in different tissues. The only detected defect for kelch mutants is female sterility, and Kelch protein is localized to the ovarian ring canals. kelch mutant ring canals are disorganized and have partly occluded lumens, causing a failure to transport cytoplasm. ORF1 and full-length Kelch proteins co-sediment with ring canals suggesting that both proteins are found in the ring canals. Transgenetic analysis reveals that ORF1 Kelch protein is sufficient to rescue ring canal morphology and fertility. In addition, the UGA stop codon has been mutated to a UAA stop codon and to three sense codons that allow constitutive readthrough. Analysis of these mutants reveals that a full-length Kelch protein can partially compensate for the loss of endogenous Kelch, but the residue included at the stop codon is critical for function. Finally, these studies suggest that the mechanism of stop codon suppression of Kelch is by tRNA suppression (Robinson, 1997b).
The ratio of full-length Kelch to ORF1 kelch proteins is approximately 1:20. Both of these proteins are disrupted by mutations in the kelch locus (Xue, 1993). Characterization of the developmental expression pattern has revealed that, in the adult, most of the kelch protein is found in the ovary. However, in female adult fly carcasses from which the gonads were removed, both proteins are present but at a ratio of approximately 1:1. A similar ratio and level is seen in the male carcasses and little detectable protein is seen in the male gonad. This suggests that suppression of the stop codon is controlled in a tissue-specific manner (Robinson, 1997b).
Kelch was expressed throughout development; however, during metamorphosis, the ratio of full-length to ORF1 is again high compared to the adult ovary. Since kelch mutations affect the ovary, it is suspected that the major source of the increase in full-length protein relative to ORF1 was due to expression in the pupal ovary. If this is the case, the expression should primarily be in female pupae. To test this, third instar larvae were collected and separated into males and females. These were allowed to pupate and synchronized by testing for the formation of air bubbles that form at 4 hours of metamorphosis. Pupae were aged and then protein extracts were prepared. In late third instar larvae, ORF1 and a small amount of full-length protein is observed in both males and females. In early pupae, full-length Kelch increases relative to ORF1 in both males and females. This increase continues into late metamorphosis (LP) in both males and females. Since the ratio of full-length protein to ORF1 is elevated in both males and females, pupal ovary-specific expression is ruled out. To determine which tissues express elevated levels of full-length Kelch, late third instar larvae were dissected and respective tissues were separated. Western analysis of extracts of each tissue type revealed that imaginal discs, the presumptive adult tissue, shows an increase in full-length Kelch relative to ORF1. Larval testis also show a slight increase compared to other tissues. Some full-length kelch is detected in adult ovary, whole larvae extracts, salivary gland and cuticle. Immunofluorescence experiments were performed to determine the subcellular localization of the Kelch proteins in imaginal discs. There is some enrichment of the Kelch proteins subcortically in the epithelia of the imaginal discs (Robinson, 1997b).
Two lines of evidence indicate that both Kelch proteins associate with the ovarian ring canal complex. (1) Both proteins co-sediment with hts-RC protein and ring canals in sucrose density gradients. (2) When only a full-length Kelch protein that contains either an Alanine, Serine or no amino acid (DUGA) at the UGA stop codon was expressed, these proteins associate with the ring canals, indicating that the full-length protein is capable of associating with the ring canal complex. Localization of Nmyc-ORF1 to ring canals, together with the failure of Nmyc-ORF2 to localize to ring canals, suggests that the full-length protein localizes by virtue of the ORF1 component of the protein (Robinson, 1997b).
Transgenes that produce ORF1 Kelch are capable of rescuing all detectable defects from loss of the endogenous kelch locus. Ring canal morphology of a molecular null mutant, kelDE1, containing ORF1 kelch was rescued in 40% of stage 4-7 ring canals and 69% of stage 8-10 ring canals. The wtKcD transgene appears to perform slightly better in this background, suggesting a possible function for the full-length protein in the maintenance of ring canal morphology. In stages 4-7, wtKcD rescues 59% of the ring canals to wild type and, by stages 8- 10, it rescues 87% of the ring canals to wild type. Although these results suggest that wtKcD might work a little better than ORF1, not enough lines of each transgene that express at high enough levels are available to be able to conclusively demonstrate a difference between these two transgenes (Robinson, 1997b).
It has been proposed that ring canal growth involves sliding the ring canal actin filaments with respect to one another in order to increase the circumference of the ring canal (Robinson, 1994; Tilney, 1996). Tilney (1996) demonstrated that the actin filaments in the ring canal are bipolar and extend circumferentially around the ring canal. Additionally, they showed that the maximum number (approximately 720) of actin filaments per cross-sectional area in wild-type egg chambers associate with ring canals by stage 4 when the ring canals are 3-4 mm in diameter. Thus, in order for the ring canals to expand to 10 mm by stage 11, they likely slide with respect to one another while adding additional filaments or while increasing the length of existing actin filaments. If Kelch ORF1 is a dimeric actin cross-linker as hypothesized, then ORF1 may be able to provide additional cross-links to the filaments during growth, improving the morphology of the ring canal (Robinson, 1997b).
ORF1 Kelch is also sufficient for Drosophila development. To test the function of the full-length protein, three mutant transgenes, Alanine, DUGA and Serine, were expressed; these provide expression of full-length kelch proteins. None of the full-length proteins is capable of rescuing female sterility of the molecular null kelDE1 allele. If ORF1 functions as an actin filament cross-linker during ring canal growth, this activity might be blocked by the presence of the ORF2 portion of the protein. In contrast, in the presence of a small amount of ORF1 protein, Alanine, but not Serine or DUGA, is capable of rescuing the fertility. However, Alanines rescue is only partial because even by stages 8-10 only about half of the ring canals had a wild-type morphology. These results are very interesting for two reasons: (1) they suggest that at least some ORF1 protein is required for Kelch function; (2) the dramatic difference in the rescuing activity of Alanine versus DUGA and Serine suggests that the amino acid incorporated in the protein at the junction between ORF1 and ORF2 is important for proper function. Although this residue does not appear to affect the ability of the protein to localize to ring canals, it may confer some important protein folding information. The DUGA protein has a little more activity than the Serine full-length protein, suggesting that the serine residue may be specifically deleterious to the folding of the protein. In contrast, it is possible that Alanine is a more permissive residue that minimizes steric hindrance by the 84 kD ORF2 fused to ORF1. Of course, rescue of ring canal morphology and female sterility might not be the correct assays to measure the function of full-length kelch proteins (Robinson, 1997b).
The levels of full-length kelch compared to ORF1 Kelch are regulated throughout development. The ratio of full-length to ORF1 reaches a maximum approaching 1:1 during metamorphosis. This increase occurs in a largely tissue-specific manner, suggesting that the most efficient stop codon suppression occurs in the imaginal discs. However, the possibility cannot be ruled out that the relative levels of ORF1 compared to full-length Kelch are controlled, at least in part, at the level of protein stability (Robinson, 1997b). The increase in the full-length Kelch compared to ORF1 in the imaginal tissues suggests that full-length kelch might have an important function in these tissues. However, since mutants in the Kelch locus only affect egg chamber development, a function to Kelch in imaginal discs cannot be defined. The conservation of the two-kelch-protein motif across several Drosophila species suggests that both Kelch proteins may be providing an important, perhaps redundant, function during metamorphosis (Robinson, 1997b).
Since stop codon context has been demonstrated to be non-random, the context of the UGA in Kelch is likely to be important. The nucleotide immediately 3' to the Kelch UGA is an A. In Drosophila, UAA(A/G) is the preferred stop codon for highly expressed genes and UGA(A/G) is generally preferred in most eukaryotes, although less so in Drosophila. In mammals, stop codon context has also been shown to influence the decision to incorporate a seleno-cysteine or to terminate. A UGA followed by a C or a U is three times more likely to be suppressed than terminated, while a UGA followed by an A or a G is three times more likely to be terminated than suppressed (Robinson, 1997b).
The UGA A in Kelch might favor termination, which is what is seen in the ovary where the ORF1 product is the predomi-nant form. The UAA codon in the UAA Kelch mutant transgene may not be decoded by a suppressor tRNA, or a stop codon might have been introduced that simply favors termination so that any product of stop codon suppression is below the level of detection. Cell-type specific factors may allow suppression to be favored over termination, perhaps by blocking the UGA to translation release factors (Robinson, 1997b).
The major candidate mechanisms for stop codon suppression in Kelch are incorporation of selenocysteine or another amino acid by a suppressor tRNA, tRNA hopping, or RNA editing. tRNA hopping that preserves the same reading frame has been shown to occur when the takeoff and landing codons are similar. The Kelch UGA is flanked 5' and 3' by in-frame AUG codons, making such a mechanism an intriguing possibility (Robinson, 1997b and references therein).
Selenocysteine-incorporation into Kelch as the mechanism of stop codon suppression is favored, since the stop codon readthrough mechanism for Kelch appears to be UGA-specific and the Kelch transcript has a potential selenocysteine insertion sequence (SECIS), in the 3' UTR. To try to test this, whole pupae were metabolically labelled by feeding isotopic selenium to larvae. Although incorporation into full-length Kelch could not be detected, probably because of the rarity of full-length Kelch, it was possible to see incorporation of isotopic selenium into three major protein species, suggesting that the selenocysteine-incorporation apparatus is intact in Drosophila and making this mechanism a reasonable possibility for full-length Kelch (Robinson, 1997b).
It is concluded that stop codon suppression can be a very efficient mechanism of gene regulation. It can be regulated in a tissue-specific manner, allowing the ratio of two protein products to vary. Since no function can be assigned to full-length Kelch, the significance of this level of gene regulation for Drosophila Kelch remains unclear; however, it is likely to have been conserved through-out the evolution of Drosophila species. The ORF1 product of Kelch is sufficient for Drosophila development in a laboratory setting and is capable of rescuing the defect in ring canal morphology and female sterility that results from disruption of the endogenous kelch locus. However, the degree to which ORF1 rescues ring canal morphology has only been assessed by confocal microscopy. Ultra-structural analysis using electron microscopy might reveal a more subtle function for full-length Kelch. Examination of the function of different mutant full-length Kelch proteins suggests further constraints on the amino acid that may be included at the stop codon. Identification of the amino acid residue that is incorporated in the endogenous full-length Kelch protein will permit these results to be more fully interpreted (Robinson, 1997b).
kelch mutant ring canals are highly disorganized and have additional actin filaments that extend into the canal partially obstructing cytoplasm transport (Robinson, 1994; Tilney, 1996). Although Drosophila kelch ORF1 is a member of a large family of kelch proteins, the biochemical functions for this family have not yet been discerned. However, two domains in Kelch are also found in diverse nonkelch proteins. By comparison to the nonkelch proteins, a simple model for kelch function is that there are three interaction domains: (1) Kelch might bind to ring canal actin filaments through the kelch repeat (KREP); (2) it dimerizes through the BTB domain, thereby crosslinking the actin filaments in the ring canal into the well organized inner rim; (3) since Kelch localizes to the ring canals, it is hypothesized that there is a third interaction of Kelch that allows it to bind to the ring canal actin specifically. A structure-function analysis of Kelch has been performed to test various aspects of this model (Robinson, 1997a).
Because the amino terminal region (NTR) is not present in other kelch family members, whether the NTR is required for Kelch function was tested. The myc:DeltaNORF1 15-2 line, which expresses moderate amounts of protein, is able to rescue kelch mutant sterility, restoring ring canal morphology. This indicates that the NTR domain is not strictly required for Kelch's ring canal organizational activity. However, the highly expressing lines myc:DeltaNORF1 hace a strong dominant-negative effect on ring canal stability, indicating that Kelch has a very important function. There is a dramatic loss of plasma membrane stability in stage 6 and older egg chambers. Although it is possible that DeltaNORF1 disrupts plasma membrane directly, it is believed membrane instability is due to a specific defect in ring canal assembly. First, myc:DeltaNORF1 is specifically localized to ring canals in early stages of oogenesis. It is not obviously localized to the cortical cytoskeleton until the time when the ring canals begin to break down. Some cortical localization of myc:ORF1 is detected that does not result in a breakdown in plasma membrane integrity. Second, myc: DeltaNORF1 shows earlier than normal ring canal localization correlating with myc:DeltaNORF1's dominant-negative effect being due to a defect in ring canal assembly. Since myc:DeltaNORF1 localizes to region 1 ring canals, it is speculated that the protein disrupts initial assembly of the ring canals so that they are destabilized later. Supporting this idea, myc:DeltaNORF1's localization to region 1 ring canals is even earlier than hts-RC and the robust inner rim of actin that normally begins to accumulate by region (Robinson, 1994). These results suggest that kelch function is fairly tightly controlled and is not required, and in fact is not desirable, until after the initial assembly of the ring canal (Robinson, 1997a).
The NTR is apparently required to regulate the localization of kelch to ring canals. Since the ovarian tumor gene promoter in the pCOG vector provides germline expression beginning in the stem cell, one possible explanation for myc:DeltaNORF1's early localization is that it is transcribed earlier than normal. However, myc:ORF1 expressed using the same pCOG vector had a wild-type time course for localization to ring canals. A second possible explanation is that there might be negative regulators of translation in the NTR region allowing myc:DeltaNORF1 to be translated earlier than normal. However, in high expressing lines of other kelch proteins that contain the NTR domain, the expression of unlocalized proteins in the cytosol in region 2 egg chambers has been detected. Therefore, the best explanation for myc:DeltaNORF1's early localization is that the NTR regulates the timing of protein localization. The NTR might accomplish this by interacting with an unknown protein that sequesters Kelch in the cytosol until the time to localize. Alternatively, intramolecular interactions between the NTR and another domain in Kelch could cause the protein to fold in such a way that it cannot bind to ring canals until the protein is activated. There are no known proteins with domains that have a high sequence homology to the NTR. The NTR is rich in asparagines, glutamines, and histidines having two stretches of polyglutamines including one six residues long and a second eight residues long. These stretches might mediate some of the interactions of the NTR. For example in the human protein, huntingtin, which has a long polyglutamine stretch, the polyglutamine tract is involved in protein-protein interactions (Robinson, 1997a).
The BTBIVR region is the minimal unit that localizes to ring canals in an ORF1-dependent manner. Since ORF1-R fails to localize to ring canals that contain only the kelch domains, the BTBIVR region must interact with the amino half of kelch. One model suggests that the BTBIVR region mediates oligomerization. Consistent with this, there are subtle defects in the ring canal inner rims of wild-type egg chambers expressing myc:ORF1-R, perhaps because some nonproductive complexes form. However, the in vivo experiments cannot distinguish between direct and indirect interactions. In vitro, purified recombinant kelch BTB and BTBIVR domains are capable of mediating dimerization. The fact that the BTB domain is not sufficient to bind ring canals in vivo might indicate that kelch BTB-mediated dimerization is a relatively low affinity interaction, and additional residues in the IVR region are required to form a high affinity interaction domain that can promote dimerization in vivo (Robinson, 1997a).
BTB (also called POZ) domain oligomerization has been analyzed in vitro for some of the transcription factors that have this domain. In gel shift assays, the proteins produced a shift expected for dimerization. The amino-terminal 50 amino acids in the bric à brac (bàb) BTB domain are sufficient to mediate dimerization. However, in another zinc finger protein called ZID, the first 69 amino acids of its BTB domain are insufficient to dimerize, indicating that there are differences in the way different BTB domains interact. Furthermore, some BTB domains have different specificities. For example, the Tramtrack (Ttk) BTB domain forms homodimers or heterodimers with the GAGA transcription factor BTB domain, while the ZID BTB domain does not interact with the Ttk BTB domain. The BTB domain is predicted to be largely alpha-helical, and alpha-helical wheel modeling reveals a hydrophobic face that could mediate the dimerization interaction. Two highly conserved, charged residues (corresponding to Drosophila kelch residues: D163 and R171; Xue, 1993) map to the hydrophobic face. Mutations that change the charge of these residues in the bàb BTB domain disrupt in vitro dimerization, while mixed populations with compensatory mutations in trans restore the interaction, suggesting that these residues are involved in electrostatic interactions. Clearly there are differences in the way this interaction domain functions in different proteins. Comparative structural studies are needed to elucidate the basis of BTB domain interaction (Robinson, 1997a).
The KREP domain is both necessary and sufficient for localization to ring canals. From sequence comparisons with alpha-scruin, which has two KREP domains (Way, 1995) that each bind actin (Tilney, 1975; Bullitt, 1988; Owen, 1993; Schmid, 1994), it is speculated that the kelch KREP domain might bind to ring canal actin filaments. Preliminary data indicate that the KREP domain can bind to actin filaments in vitro. Since the KREP domain specifically binds to ring canals, it is proposed that there is at least one additional interaction with an unknown factor that promotes specificity for ring canals. Without the specificity interaction, it is unlikely that KREP would preferentially bind ring canal actin filaments over other actin in the nurse cells since ring canals contain a very small proportion of the actin in the cell. In addition, because myc:DeltaNORF1 can localize to ring canals as early as region 1, the unknown factor is present on ring canals before the addition of hts-RC and the actin filaments that form the inner rim. Since cheerio is required for kelch to localize to ring canals (Robinson, 1997b), it could encode the factor that allows KREP to bind to ring canals (Robinson, 1997).
Another example of a protein in which a kelch repeat domain might mediate two interactions is the Physarum polycephalum actin-fragmin kinase (AFK). In this protein, the 35-kD amino half of the protein is a novel protein kinase domain, and the 35-kD carboxy half consists primarily of six kelch repeats. This protein specifically phosphorylates actin in the actin-fragmin complex (Eichinger, 1996). Perhaps the AFK KREP domain recognizes sites on both actin and fragmin, giving it its specificity so that the kinase domain can phosphorylate its actin substrate (Robinson, 1997).
The data suggest that kelch has at least three activities: ring canal localization, dimerization, and ring canal organization. The ring canal organizational activity has been separated from the ring canal localization and dimerization functions using nonrescuing, full length Kelch proteins. Full length Kelch proteins have been expressed by creating alanine-substituted (alanine full length), serine-substituted (serine full length), and UGA-deleted (DeltaUGA full length), full length Kelch transgenes (Robinson, 1997b). The alanine full length Kelch is partially functional and is able to rescue the hypomorphic kelneo allele but not the molecular null kelDE1 allele. Serine and DeltaUGA full length kelch proteins can not rescue either kelch allele. All three of the mutant full length proteins are able to localize to ring canals, indicating that the ring canal specificity interaction is intact. Each of the full length Kelch proteins interact with myc: ORF1-R allowing myc:ORF1-R to localize to ring canals. This indicates that the full length kelch proteins are capable of dimerizing normally. Consequently, the full length proteins are defective in a third interaction required for ring canal organization. Perhaps the 84-kD ORF2 domain in the full length protein sterically disrupts the proposed ability of the KREP domain to interact with actin, and the degree of steric hindrance depends on the residue substituted at the stop codon (Robinson, 1997a).
It has been speculated that ring canals increase their diameter from 3-4 µm (stage 4) to 10 µm (stage 11) by sliding the actin filaments with respect to one another (Robinson, 1994; Tilney, 1996). The density of actin filaments and the filament number per cross-sectional area are relatively constant (~720) during these stages of ring canal development (Tilney, 1996). This implies that existing actin filaments must increase their length or additional actin filaments are gradually added during expansion to maintain the filament density. During actin filament sliding, cross-linking proteins could break and reform their associations with the actin filaments to facilitate expansion while maintaining ring canal organization. Since the concentration of actin in the ring canal inner rim is so high (millimolar range), low affinity interactions with Kd's in the µM range can easily promote the reversibility of these interactions. Kelch is a good candidate for doing just this, since in kelch mutants the ring canals become disorganized during these stages (Robinson, 1994; Tilney, 1996). The interactions that allow KREP to localize and BTBIVR to dimerize might be relatively high affinity, while the putative actin interaction site might form the hypothesized low affinity interaction site that maintains ring canal organization during growth (Robinson, 1997).
These in vivo studies of kelch function support and extend the model for Kelch activity in at least three ways. (1) The dominant-negative protein that results from the deletion of the NTR is unexpected and points out how important temporal control and order of addition during ring canal assembly are. Identification of proteins that interact with the NTR should provide great insight into the events that regulate the ring canal cytoskeleton assembly. (2) In vivo and in vitro data support a model in which the BTB domain mediates dimerization. However, in vivo, the BTB domain is not sufficient and requires sequences in the IVR region. Certainly, more complex interactions can be envisioned to explain the data, such as having an additional factor that promotes binding between the BTBIVR domains. In addition, since there are differences in the requirements for BTB domain dimerization from different proteins, further mutagenesis and structural studies are needed to elucidate the molecular basis of this interaction. (3) There are two remaining functions that can be assigned to Kelch: ring canal localization and ring canal organization. The results verify that the KREP domain provides ring canal localization and suggest that it might mediate the ring canal organizational function. Discovery of the binding partner that promotes ring canal localization may identify new ring canal components and will facilitate many new studies on how proteins are added to the ring canals. Ring canal organizational activity likely involves a direct or indirect interaction with the ring canal actin filaments. Experiments are in progress to test direct interactions with actin. Comparative functional studies on the KREP domain would be particularly informative since this motif is wide spread and there is considerable sequence divergence in many kelch-related proteins (Robinson, 1997a).
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