The yeast adenylyl cyclase-associated protein, CAP, was identified as a component of the RAS-activated cyclase complex. CAP consists of two functional domains separated by a proline-rich region. One domain, which localizes to the amino terminus, mediates RAS signaling through adenylyl cyclase, while a domain at the carboxyl terminus is involved in the regulation of cell growth and morphogenesis. Recently, the carboxyl terminus of yeast CAP was shown to sequester actin, but whether this function has been conserved, and is the sole function of this domain, is unclear. Here, we demonstrate that the carboxyl-terminal domains of CAP and CAP homologs have two separate functions. We show that carboxyl-terminals of both yeast CAP and a mammalian CAP homolog, MCH1, bind to actin. We also show that this domain contains a signal for dimerization, allowing both CAP and MCH1 to form homodimers and heterodimers. The properties of actin binding and dimerization are mediated by separate regions on the carboxyl terminus; the last 27 amino acids of CAP being critical for actin binding. Finally, we present evidence that links a segment of the proline-rich region of CAP to its localization in yeast. Together, these results suggest that all three domains of CAP proteins are functional.

We have screened for proteins that interact with v-SNAREs of the late secretory pathway in the yeast Saccharomyces cerevisiae. A novel protein, designated Vsm1, binds tightly to the Snc2 v-SNARE in the two-hybrid system and can be coimmunoprecipitated with Snc1 or Snc2 from solubilized yeast cell extracts. Disruption of the VSM1 gene results in an increase of proteins secreted into the medium but does not affect the processing or secretion of invertase. In contrast, VSM1 overexpression in cells which bear a temperature-sensitive mutation in the Sec9 t-SNARE (sec9-4 cells) results in the accumulation of non-invertase-containing lowdensity secretory vesicles, inhibits cell growth and the secretion of proteins into the medium, and blocks rescue of the temperature-sensitive phenotype by SNC1 overexpression. Yet, VSM1 overexpression does not affect yeast bearing a sec9-7 allele which, in contrast to sec9-4, encodes a t-SNARE protein capable of forming a stable SNARE complex in vitro at restrictive temperatures. On the basis of these results, we propose that Vsm1 is a novel v-SNARE-interacting protein that appears to act as negative regulator of constitutive exocytosis. Moreover, this regulation appears specific to one of two parallel exocytic paths which are operant in yeast cells.SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein [SNAP] receptor) proteins comprise distinct families of membrane-associated receptors that are components of the vesicle docking and fusion machinery in eukaryotes (62; reviewed in references 35 and 54). Evidence from evolutionarily divergent systems, e.g., from yeast to mammals, suggests that the role of SNAREs in protein transport is highly conserved (reviewed in references 7 and 21). SNAREs act throughout the secretory pathway to confer the trafficking of cargo-containing carrier vesicles and may also participate in compartmental organization. SNAREs found on vesicular compartments (v-SNAREs) are proposed to interact with specific cognate receptors (t-SNAREs) present on target compartments to form a prefusion SNARE complex. Thus, v-SNAREt-SNARE (v-t SNARE) assembly confers vesicle docking; one of the first events leading to membrane fusion. Moreover, recent work has suggested that together v-and t-SNAREs may provide the minimal requirements necessary for conferring both membrane association and bilayer fusion, using a liposome-based in vitro assay (70). However, there is still some controversy surrounding the role of SNAREs in the fusion event itself (13, 65).To confer specificity to protein transport, SNARE assembly leading to membrane fusion is expected to occur only on appropriate target membranes. Yet, the v-and t-SNAREs involved in Golgi and post-Golgi trafficking events passage through the early secretory pathway and are likely to reside together on identical endoplasmic reticulum (ER)-and Golgiderived transport vesicles. Therefore, some mechanism(s) should prevent nonproductive SNARE partnering from occurring between opposing membranes early in the pathway. Likewi...

. In this article, a novel SNARE-interacting protein in yeast, Vsm1/Ddi1, was characterized. During more recent studies of this protein, we discovered that the original cloned Saccharomyces cerevisiae open reading frame (YER143w) used for our work, which was generated by PCR amplification, contained two point mutations (G substitutions at bp 925 and 1282 relative to the sequence deposited in the Saccharomyces Genome Database) that resulted in two amino acid substitutions (R309G and Q428E, respectively). Therefore, for our subsequent work with this protein, we have corrected these mutations. Regrettably, restoration of these two altered residues changes some of the results that we previously reported. We originally reported that overproduction of the mutant version exacerbated the growth defect of sec9-4 mutant cells (Fig. 3), whereas we now find that overproduction of the corrected version partially suppresses the temperature-sensitive phenotype of sec9-4 yeast cells. As best as our current analysis has determined, it appears that the results shown in Fig. 1, 2, and 5 to 8 remain valid, although we cannot exclude the possibility that the accumulation of secretory vesicles, shown in Fig. 4, results from the mutations. A fuller description and documentation of these new