Article |
Address correspondence to Mahasin A. Osman, Department of Molecular Medicine, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853-6401. Tel.: (607) 253-3883. Fax: (607) 253-3659. E-mail: mo28{at}cornell.edu
![]() |
Abstract |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key Words: Iqg1; Bud4; Sec3; polarity; cytokinesis
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Once a bud site is selected, polarized secretion is directed to that site (Lew and Reed, 1995). Fusion of the secretory vesicles at the target site requires a protein complex, the exocyst (Finger et al., 1998), composed of the Sec3, -5, -6, -8, -10, and -15 proteins. Some of the exocyst members, such as Sec3p and Sec4p, localize to the bud tip (Bowser et al., 1992; TerBush and Novick, 1995). After nuclear division, the exocyst reorients to the mother bud neck to promote cytokinesis (Finger et al., 1998). Thus, budding and cytokinesis both involve directed secretion. Furthermore, some sec3 mutants exhibit a random budding pattern, suggesting the involvement of the complex in bud-site selection (Finger et al., 1998). Nevertheless, no direct molecular link between the exocyst and the bud site selection proteins has yet been reported.
Sec3p localizes to growth sites independent of the other exocyst components, actin, or septins, indicating that Sec3p works as a landmark for secretion (Finger et al., 1998). The polarized localization of Sec3p has been suggested to require the kinase Cdc28p (Finger et al., 1998), as well as the small GTPases Rho1 (Guo et al., 2001) and Cdc42p (Zhang et al., 2001). It is also believed that the positional signal imposed by the septin and bud site selection proteins is interpreted by Cdc42p and other polarity establishment proteins to polarize the actin cytoskeleton (Johnson and Pringle, 1990) and the secretory pathway (Finger et al., 1998). The Cdc42 effector(s) that mediates these functions and the mechanism by which it is achieved remain important questions. However, one intriguing possibility for the Cdc42 effector is a member of the family of IQGAPs.
We and others isolated the mammalian IQGAPs-1 and -2 as putative target/effectors for Cdc42p (Hart et al., 1996; McCallum et al., 1996; Erickson et al., 1997). The mammalian IQGAPs were localized to cellcell junctions (Hart et al., 1996; Kuroda et al., 1996; Bashour et al., 1997), as well as to Golgi membranes (McCallum et al., 1998), with the latter finding suggesting their possible involvement in protein trafficking events. To better understand the cellular functions of the IQGAP family of proteins, we isolated and characterized the yeast homologue, Iqg1p, and found that the IQG1-null strain produced phenotypes consistent with the involvement of Iqg1p in both polarity and cytokinesis (Osman and Cerione, 1998). Other groups have shown Iqg1 (Cyk1p) to be essential (Epp and Chant, 1997; Lippincott and Li, 1998) and to participate in an actomyosin-based contractile ring function. However, because the actomyosin ring was found to be dispensable for both cytokinesis and yeast cell growth (Bi et al., 1998), Iqg1p must play additional roles in the cell. Here we report that Iqg1p determines the axial budding pattern, interacts with and promotes the localization of the axial markers Bud4p and Cdc12p, and functionally interacts with the secretion marker, Sec3p. Overall, these findings raise the interesting possibility that the Cdc42p target, Iqg1p, serves to interface proteins involved in a key polarity-dependent process (axial budding) with proteins involved in exocytosis/secretion and cytokinesis.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
|
Iqg1p is required for axial budding
To assess whether Iqg1p, like Bud4p, influences the budding pattern, we compared the pattern of the bud scars on the surfaces of haploid and homozygous diploid cells lacking the IQG1 gene with their isogenic wild-type counterparts. Chitin rings were visualized using Calcofluor as described in the Material and methods. Interestingly, the majority of the haploid cells lacking Iqg1p (60%, n = 400) exhibited a bipolar budding pattern (Fig. 2 A, AF) similar to cells lacking BUD4 (Fig. 2 A, top right). This budding pattern was not observed in the isogenic wild-type cells which showed axial budding (Fig. 2 A, top left), but it appears similar to patterns observed in cells defective in BUD3 or BUD4 (Chant and Herskowitz, 1991). Approximately 30% of the iqg1
cells exhibited a semirandom pattern of budding such that the scars were clustered but not located at the opposite poles (Fig. 2 A, C and D). Another subset of these cells (Fig. 2 A, E and F) showed a distribution of bud scars to the opposite poles but these scars were scattered at the pole and not lined up as in the wild-type cells. Homozygous diploid strains lacking IQG1 showed the expected bipolar pattern of diploid budding (unpublished data).
|
Efficient septin localization requires Iqg1p
Bud4p was shown to genetically interact with and require the septin, Cdc12p, for localization (Sanders and Herskowitz, 1996). This prompted us to consider the relationship between Iqg1p and Cdc12p. We first used two-hybrid assays to examine the ability of Iqg1p and Cdc12p to interact. These data showed that Cdc12p interacted with the COOH-terminal region of Iqg1p that contains the RasGAP-like domain (Table II). However, thus far, we have not been able to detect the coimmunoprecipitation of a GFP-tagged Cdc12p on a low-copy plasmid with the HA-tagged Iqg1p on a high-copy plasmid (Fig. 3
A, top, lane 2), suggesting that Iqg1p and Cdc12p may undergo a relatively weak or transient interaction. We detected a band for the endogenous Bud4p after immunoprecipitation of Cdc12p-GFP (Fig. 3 A, bottom, lane 2, compare with lane 5), suggesting that these two proteins are able to interact in cells and may bridge the interaction of Iqg1p with Cdc12p.
|
|
Iqg1p binds and helps localize Sec3p
Sec3p is involved in both bud-site selection and cytokinesis (Haarer et al., 1996; Finger and Novick, 1997). Our previous work implicating Iqg1p in cytokinesis and protein trafficking (Osman and Cerione, 1998), together with the results from this study suggesting that Iqg1p is involved in bud site selection, led us to examine whether there is a functional interaction between Iqg1p and Sec3p. To determine whether Iqg1p associates with Sec3p, we cotransformed the MO3 strain lacking Iqg1p with the high-copy plasmid encoding HA-tagged Iqg1p (which complements the iqg1 phenotype), together with either a SEC3-GFP on low-copy plasmid (Finger et al., 1998) that complements the sec3
strains, or the GFP plasmid. Coimmunoprecipitation experiments showed that HA-Iqg1p associated with Sec3p-GFP (Fig. 4
A, lane 2) but not with GFP alone (lane 4), nor was HA-Iqg1p immunoprecipitated with other control antibodies (e.g.,
-Intersectin or
-Gal4 antibodies; unpublished data).
|
|
|
|
Sec3p and Bud4p cooperate for cytokinesis
Because Iqg1p binds and helps localize both Sec3p and Bud4p, we further examined the relationship between Bud4p and Sec3p in a number of different ways. First, we took a genetic approach and crossed BHY51 (sec3) with SY298 (bud4
). The resultant diploid was sporulated and tetrads yielded four live progenies analyzed by PCR to determine their genotypes. The segregants of a single tetratype tetrad (MO1AD; Table III) were used for further analyses. Some (
10%) of the bud4
single mutant segregants from this cross revealed a phenotype consistent with a cytokinesis defect (Fig. 7
A, top right). Interestingly,
30% of the double mutant bud4
sec3
cells displayed chains of 35 or more elongated cells (Fig. 7 A, bottom right) that were often branched and remained together after sonication, vigorous vortexing or zymolyase treatment. Another 35% of the double mutant cells displayed shorter chains or appeared as two cells which failed to separate. Moreover, at 30°C, the double mutant cells showed substantial lysis, which was not the case for the single mutants.
|
To examine whether Bud4p and Sec3p also cooperate in determining the axial budding pattern, we measured the pattern of bud scars on the cell surface of the bud4 sec3
haploid strain. Approximately 70% of the double mutants displayed bipolar budding pattern (Fig. 7 C, right, chain of cells), similar to that of bud4
single mutants. However, in
60% of those cells, we observed a branching scar that appeared on the surface of the double mutant cells (Fig. 7 C, left, arrows). This scar appears to be the point of a branch of a shorter chain of cells (Fig. 7 A, bottom right, arrow).
Finally, we investigated whether Sec3p-GFP on a low-copy plasmid (Finger et al., 1998) coimmunoprecipitates with endogenous Bud4p. Fig. 7 D shows that a Bud4p band was detected in the immune complexes with Sec3p-GFP (lane 2) but was not detected when immunoprecipitation was performed with control antibodies or with GFP alone (Fig. 7 D, lane 4).
Septal deposition defects are observed in the double mutants
To study the subcellular effects of the double deletion of IQG1 and either BUD4 or SEC3, we examined the cells by thin-section electron microscopy after shifting to the nonpermissive temperature (37°C) for 30 min. As reported previously (Finger and Novick, 1997), Sec3 cells accumulated vesicles all over the mother cell and the bud (Fig. 8
, bottom right). Interestingly, in both of the double-deletion strains (n = 150 each), these vesicles were either no longer visible or in some cases, fused into larger vesicular structures, especially in bud4
sec3
cells (Fig. 8 g, arrows). In contrast to wild-type and single-mutant cells, the double-mutant cells showed septal defects and in some cases, broad necks. In
58% of the iqg1
sec3
cells (n = 100), the septa were either absent (Fig. 8, a and b) or aberrant (panels c and d). In bud4
sec3
cells, the picture was slightly different. We either detected no septa (Fig. 8, e and g) or, in
3% of the cells, we detected septa that remained intact in a broad neck (Fig. 8, f, arrows, and h), especially within the chains of cells. These defects suggest that Iqg1p, Bud4p, and Sec3p all contribute to proper septum deposition and separation.
|
|
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The axial budding pattern in haploid yeast is spatially programmed by Bud3p, Bud4p, Axl1p, and the transmembrane protein Bud10p (for review see Chant, 1999). Using a two-hybrid screen, we have identified Bud4p as an Iqg1p-binding partner (Table I). We further demonstrated that Iqg1p was co-immunoprecipitated with Bud4p (Fig. 1 C) and was required for its localization (Fig. 2 B). We showed that, like Bud4p, Iqg1p specifies axial budding in yeast. Haploid cells lacking IQG1 displayed bipolar budding (Fig. 2 A) similar to bud4 mutants, while the isogenic homozygous diploid cells were unaffected. This axial budding defect (Fig. 2 A) suggests that Iqg1p works together with Bud4p to specify axial bud site selection. However, the fact that Bud4p is localized as a spatial marker in the previous cell cycle, and loses this localization at the neck in iqg1
cells (Fig. 2 B), supports the idea that Iqg1p recruits Bud4p as an axial marker.
Iqg1p also appears to interact with and helps localize the septin Cdc12p (Table II; Fig. 3 B), which is necessary for both cytokinesis and axial budding (for review see Chant, 1996, 1999; Madden and Snyder, 1998). Bud4p may mediate the interaction between Iqg1p and Cdc12p, as we have detected the coimmunoprecipitation of Cdc12p and Bud4p, but have not yet been able to detect an interaction between Cdc12p and Iqg1p in coimmunoprecipitation experiments (Fig. 3 A). However, we have found that Iqg1p, and not Bud4p, affects the efficient localization of the septin Cdc12p to the neck (Fig. 3 B; unpublished data), further supporting the idea that Iqg1p assembles the proteins required for axial budding. In addition, the interaction between Iqg1p, Bud4p, and Cdc12p may involve a cooperation in some aspect of cytokinesis, as both Iqg1p and septins have been previously implicated in this process.
Several lines of evidence presented in this study indicate that Iqg1p and Sec3p work together to influence cell polarity, axial budding and cytokinesis in yeast. We have provided evidence that Iqg1p coimmunoprecipitates with (Fig. 4 A) and is required for the localization of Sec3p (Fig. 4 B). The iqg1sec3
double mutants displayed a lower restrictive temperature and exhibited phenotypes consistent with a polarity defect (Fig. 5) that resulted in random budding (Fig. 6 B). These double mutants also failed to direct growth material to the new bud (Fig. 6 A) and to form correct septa at the mother-daughter junctions (Fig. 8).
Together, these results suggest a role for Sec3p in axial budding. Although earlier studies have suggested that Sec3p is primarily involved in diploid budding (Haarer et al., 1996; Finger and Novick, 1997), our findings are consistent with the idea that targeted secretion is required for budding in both cell types (for review see Finger and Novick, 1998). Our data also suggest that Iqg1p works with Sec3p to maintain cell polarity. Figs. 5 and 6 B show that budding in the iqg1 sec3
double mutant strain was initiated normally but was not maintained such that another bud originated either from the mother, the bud (Fig. 5) and/or at a random location (Fig. 6 B). This may reflect the actions of polarity establishment proteins such as Cdc42p and Bud1p. Iqg1p may mediate the interplay between the bud site selection function of the Bud1 GTPase and the maintenance of polarity by Cdc42p (Fig. 9)
. Three independent pieces of evidence seem to support this view. First, the random budding phenotype of the iqg1
sec3
double mutant cells (Fig. 6 B) is similar to that of bud1
or bud2
mutant cells (Chant and Herskowitz, 1991). Second, analyses of bud1
cdc24
cells revealed that the essential role of the polarity establishment molecules in morphogenesis is to stabilize the axis of polarity, and that in their absence, this axis wanders (Nern and Arkowitz, 2000). Third, a recent study demonstrated direct binding between Sec3p and Cdc42p (Zhang et al., 2001) and showed that Sec3p was mislocalized in cells expressing certain mutations of Cdc42p. Together, these data support the idea that the Cdc42p target Iqg1p coordinates the positional signal for budding and secretion with the signal to maintain polarity and promote cytokinesis.
|
In conclusion, we propose that Iqg1p forms a targeting patch that includes Bud4p, Cdc12p, and Sec3p (Fig. 9). This protein complex may play a primary role in cytokinesis by first determining, and then maintaining, the axis of polarity by choosing a bud site. The complex would recruit the exocyst for targeted secretion and other morphogenetic factors such as Mlc1p and Cmd1p (Table IV) to promote bud growth and eventually lead to septum deposition and separation. In this way, the Iqg1 complex would in effect serve as a checkpoint for cytokinesis by preventing another round of budding until cytokinesis is complete (Fig. 9). In the absence of the Iqg1p-targeting complex, alternative pathways for budding can apparently bypass the checkpoint leading to rounds of budding and chains of cells observed in the double mutants.
![]() |
Materials and methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Coimmunoprecipitation experiments
For coimmunoprecipitation experiments, tagged plasmids were cotransformed into a strain lacking the chromosomal copy of IQG1. To examine the coimmunoprecipitation of Iqg1p and Bud4p, cells were grown in synthetic complete media (cm) lacking tryptophan and leucine after transformation with HA-tagged Iqg1p on a high copy plasmid (pA1; Osman and Cerione, 1998) and Gal4-IBID (amino acids 769880 from Bud4p, Fig. 1 A). Cells harboring HA-Iqg1p (pA1) alone were grown in cm-leucine. For control experiments, cells were transformed with parent plasmids lacking the relevant gene. When examining the interaction between Iqg1p and Cdc12, MO3 cells were transformed with CDC12-GFP on a low copy plasmid (described below) and HA-IQG1, and grown on cm-uracil and leucine. When examining the coimmunoprecipitation of Iqg1p and Sec3p, MO3 cells transformed with SEC3-GFP on a low-copy plasmid (Finger et al., 1998) and HA-IQG1 were grown in cm media lacking uracil and leucine. In all cases, the cell cultures were grown to saturation and the cell pellets washed with and suspended in cold IP-buffer (50 mM Tris, pH 7.5, 1 mM EDTA, 5% glycerol, 0.1% Triton, 1 mM PMSF and fungal protease inhibitor cocktail [Sigma-Aldrich]). Acid-washed glass beads (Sigma-Aldrich) were added to break the cells by vortexing and the cell lysates were collected after centrifugation at 20,000 rpm at 4°C. Two mL of the total cell lysate (scaled up for Sec3p-Iqg1p experiments) was incubated with antibodies (a control antibody was also included) for 3 h with gentle rocking at 4°C. 4 mg of IP buffer-washed protein A Sepharose (Sigma-Aldrich) was added to the immune complexes and incubated overnight at 4°C with gentle rocking. The reaction mix was centrifuged at 10,000 rpm for 30 s and the bead portion was washed four times with ice-cold IP buffer, suspended in 30 µL SDS-loading buffer, boiled at 100°C and separated by 7-10% SDS-PAGE. Western blot analyses were performed using standard methods.
Fluorescence microscopy
To visualize chitin deposition and bud scars on the yeast cell wall, cells were collected at log phase and stained with Calcofluor (Fluorescent Brightener; Sigma-Aldrich) as described (Pringle, 1991). Cells with three or more bud scars were scored for budding pattern. For Cdc12p localization, a CDC12-GFP fusion gene was constructed by using PCR to fuse an enhanced version of the GFP (Cormack et al., 1997) to the carboxyl terminus of Cdc12p. The CDC12-GFP gene was cloned into the low-copy vector YCplac111 and also into the high-copy vector YEplac181 (Gietz and Sugino, 1988). Wild-type (MO5) and iqg1 cells (MO2) carrying the indicated CDC12-GFP plasmid, or a control vector, were grown overnight at 23°C and then adjusted to 106 cells/mL and shifted to 30°C for 6 h. The cells were then examined using an Olympus BH2 fluorescence microscope, photographed with Kodak TMAX400 film, and then the negatives were scanned into digital images under identical conditions. By comparing different exposures, we found that it requires at least a fourfold longer exposure to see similar neck staining in MO2 than in MO5. CDC12-GFP fluorescence was better at 23°C in the MO2 strain, but was still weaker than the wild-type strain.
Strains resulting from single tetratype tetrads of the cross between either SY298 (bud4) or MO3 (iqg1
) with sec3
were transformed with a SEC3-GFP plasmid (Finger et al., 1998) which encodes GFP fused to the COOH terminus of Sec3p. Log phase cells growing in cm-uracil liquid media were processed as described in (Finger et al., 1998) and visualized under 100x using a Zeiss fluorescence microscope. Images were collected using Axiovision software under identical conditions.
Thin-section electron microscopy
Log phase overnight cultures of wild-type (MOB2 or MO1B), sec3 (MOB1), iqg1
(MOB4), bud4
(MO1A), iqg
sec3
(MOB3), and bud4
sec3
(MO1D) strains growing at room temperature were adjusted to 0.3 A600nm units/mL in YEPD and incubated at 37°C for 30 min. Cultures were directly fixed in 0.1 M cacodylate containing 2.5% glutaraldehyde and 2.5% paraformaldehyde for 2 h. Cells were treated with 0.2 mg/mL zymolyase 100T in 0.1 M KPi, pH 7.5. The cell pellets were incubated with ice-cold OsO4 in 0.1 M cacodylate for 1 h, washed twice with water and incubated for 1 h in 1.5 mL of filtered 2% uranyl acetate at room temperature. A series of ethanol concentrations (50, 70, 90, and 100%) were used to dehydrate the cells followed by acetone treatment. The cells were then incubated for a few hours in 50% acetone/50% SPURR (Electron Microscopy Sciences), changed to 100% SPURR, and incubated overnight at room temperature. After changing to SPURR 2x, cells were baked at 80°C for 24 h. This and the rest of the sectioning and processing were carried out at the Cornell Integrated Microscopy Center (Ithaca, NY).
![]() |
Footnotes |
---|
![]() |
Acknowledgments |
---|
Submitted: 16 May 2002
Revised: 16 October 2002
Accepted: 21 October 2002
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Bashour, A.-M., A.T. Fullerton, M.J, Hart, and G.S. Bloom. 1997. IQGAP1, a Rac- and Cdc42-binding protein, directly binds and cross links microfilaments. J. Cell Biol. 137:15551566.
Bi, E., P. Maddox, D.J. Lew, E.D. Salmon, J.N. McMillan, E. Yeh, and J.R. Pringle. 1998. Involvement of an actomyosin ring in Saccharomyces cerevisiae. J. Cell Biol. 142:13011312.
Bowser, R., H. Muller, B. Govidan, and P. Novick. 1992. Sec8p and Sec15p are components of a plasma membrane-associated 19.5 S particle that may function downstream of Sec4p to control exocytosis. J. Cell Biol. 118:10411056.[Abstract]
Chant, J. 1999. Cell Polarity in yeast. Annu. Rev. Cell Dev. Biol. 15:365391.[CrossRef][Medline]
Chant, J., and J.R. Pringle. 1995. Patterns of bud-site selection in the yeast Saccharomyces cerevisiae. J. Cell Biol. 129:751765.[Abstract]
Cormack, B.P., G. Bertram, M. Egerton, N.A. Gow, S. Falkow, and A.J. Brown. 1997. Yeast-enhanced green fluorescent protein (yEGFP) a reporter of gene expression in Candida albicans. Microbiology. 143:303311.[Abstract]
Erickson, J.W., R.A. Cerione, and M.J. Hart. 1997. Identification of an actin cytoskeleton complex that includes IQGAP and the Cdc42 GTPase. J. Biol. Chem. 272:2444324447.
Finger, F., and P. Novick. 1997. Sec3 is involved in secretion and morphogenesis in. Saccharomyces cerevisiae. Mol. Biol. Cell. 8:647662.[Abstract]
Finger, F., and P. Novick. 1998. Spatial regulation of exocyst: lessons from yeast. J. Cell Biol. 142:609612.
Gietz, R.D., and A. Sugino. 1988. New yeast-Escherichia coli shuttle vectors constructed with in vitro mutagenized yeast genes lacking six-base pair restriction sites. Gene. 30:527534.[CrossRef]
Haarer, B.K., A. Corbett, Y. Kweon, A.S. Petzold, P. Silver, and S.S. Brown. 1996. SEC3 Mutations are synthetically lethal with profilin mutations and cause defects in diploid-specific bud selection. Genetics. 144:495510.
Hart, M.J., M.G. Callow, B. Souza, and P. Polakis. 1996. IQGAP1, a calmodulin-binding protein with a RasGAP-related domain, is a potential effector for Cdc42Hs. EMBO J. 15:29973005.[Abstract]
James, P., J. Halladay, and E.A. Craig. 1996. Genomic libraries and a host strain designed for highly efficient two-hybrid selection in yeast. Genetics. 144:14251436.
Johnson, D.I., and J.R. Pringle. 1990. Molecular characterization of CDC42, a Saccharomyces cerevisiae gene involved in the development of cell polarity. J. Cell Biol. 111:143152.[Abstract]
Kuroda, S., M. Fukata, K. Kobayashi, M. Nakafuku, N. Nomura, A. Iwamatsu, and K. Kaibuchi. 1996. Identification of IQGAP as a putative target for the small GTPase, Cdc42 and Rac1. J. Biol. Chem. 271:2336323367.
Lippincott, J., and R. Li. 1998. Sequential assembly of myosin II, an IQGAP-like protein, and filamentous actin to a ring structure involved in budding yeast cytokinesis. J. Cell Biol. 140:355366.
McCallum, S.J., W.J. Wu, and R.A. Cerione. 1996. Identification of a putative effector for Cdc42Hs with high sequence similarity to the RasGAP-related protein IQGAP1 and a Cdc42Hs binding partner IQGAP2. J. Biol. Chem. 271:2173221737.
McCallum, S.J., J.W. Erickson, and R.A. Cerione. 1998. Characterization of the Association of the actin-binding protein IQGAP, and activated Cdc42 with Golgi membrane. J. Biol. Chem. 273:2253722544.
Osman, M., and R.A. Cerione. 1998. Iqg1p, a yeast homologue of the mammalian IQGAPs, mediates Cdc42p effects on the actin cytoskeleton. J. Cell Biol. 142:443455.
Sanders, S.L., and C. Fields. 1995. Bud-site selection is only skin deep. Curr. Biol. 5:12131215.[Medline]
Sanders, S.L., and I. Herskowitz. 1996. The BUD4 protein of yeast, required for axial budding, is localized to the mother/BUD neck in a cell cycle-dependent manner. J. Cell Biol. 134:413427.[Abstract]
Shannon, K.B., and R. Li. 1999. The multiple roles of Cyk1p in the assembly and function of the actomyosin ring in budding yeast. Mol. Biol. Cell. 10:283296.
TerBush, D.R., and P. Novick. 1995. Sec6, Sec8, and Sec15 are components of a multisubunit complex which localizes to small bud tips in Saccharomyces cerevisiae. J. Cell Biol. 130:299312.[Abstract]
Zhang, X., E. Bi, P. Novick, L. Du, K.G. Kozminski, J.H. Lipschutz, and W. Guo. 2001. Cdc42 interacts with the exocyst and regulates polarized secretion. J. Biol. Chem. 276:4674546750.