Mso1 Is a Novel Component of the Yeast Exocytic SNARE Complex

Abstract

The yeast exocytic SNARE complex consists of one molecule each of the Sso1/2 target SNAREs, Snc1/2 vesicular SNAREs, and the Sec9 target SNARE, which form a fusion complex that is conserved in evolution. Another protein, Sec1, binds to the SNARE complex to facilitate assembly. We show that Mso1, a Sec1-interacting protein, also binds to the SNARE complex and plays a role in mediating Sec1 functions. Like Sec1, Mso1 bound to SNAREs in cells containing SNARE complexes (i.e. wild-type, sec1-1, and sec18-1 cells), but not in cells in which complex formation is inhibited (i.e. sec4-8 cells). Nevertheless, Mso1 remained associated with Sec1 even in sec4-8 cells, indicating that they act as a pair. Mso1 localized primarily to the plasma membrane of the bud when SNARE complex formation was not impaired but was mostly in the cytoplasm when assembly was prevented. Genetic studies suggest that Mso1 enhances Sec1 function while attenuating Sec4 GTPase function. This dual action may impart temporal regulation between Sec4 turnoff and Sec1-mediated SNARE assembly. Notably, a small region at the C terminus of Mso1 is conserved in the mammalian Munc13/Mint proteins and is necessary for proper membrane localization. Overexpression of Mso1 lacking this domain (Mso1-(1-193)) inhibited the growth of cells bearing an attenuated Sec4 GTPase. These results suggest that Mso1 is a component of the exocytic SNARE complex and a possible ortholog of the Munc13/Mint proteins.The membrane fusion apparatus in eukaryotes relies upon SNAREs 2 to mediate intracellular membrane docking and fusion events. SNAREs are membrane-associated proteins bearing ␣-helical domains (termed SNARE motifs) that can assemble into intermolecular four-helix bundles (1-3). Assembly of the SNARE complex, which is composed of vesicular (v) and target (t) SNAREs (or R-and Q-SNAREs, depending upon the nomenclature followed), bridges apposed bilayers and allows for the extrusion of interposed water molecules, resulting in membrane fusion. Although shown to be sufficient to confer membrane fusion in vitro or under artificial conditions (4 -6), a number of other molecules (termed SNARE regulators) act upstream of the SNAREs and play essential roles in the temporal and spatial control of SNARE assembly in vivo (7-9). At the level of exocytosis, these include the Rab family of small GTPases; the vesicle-tethering exocyst complex; the Sec1/ Munc18 family of SNARE assembly factors, which are conserved from yeast to mammals; and the complexin, Mint, Munc13, and synaptotagmin families of SNARE regulators, which appear to act only upon stimulus-coupled exocytic processes (7-10).In yeast, the exocytic SNARE complex consists of one representative molecule each of the Sso1/2 t-SNAREs (Q-SNARE) (11) and Snc1/2 v-SNAREs (R-SNARE) (12) as well as one molecule of the Sec9 t-SNARE (Q-SNARE) (13). However, all Q-SNARE complexes composed either of a mutated Snc v-SNARE and one molecule each of Sso and Sec9 (14, 15) or of two molecules of Sso and one of Sec9 (16) have also been shown to be ...

Specific SNARE complex binding mode of the Sec1/Munc-18 protein, Sec1p

“…Our ability to reconstitute the specific interaction between Sec1p and the SNARE complex not only supports these in vivo observations, it demonstrates that this interaction does not require other factors, such as additional proteins (30,31) or lipids.…”

mRNAs Encoding Polarity and Exocytosis Factors Are Cotransported with the Cortical Endoplasmic Reticulum to the Incipient Bud in Saccharomyces cerevisiae

“…TUB1 mRNA is also not asymmetrically distributed (Fig. 1B), and all three proteins (Tub1, Snc1, and Mso1) have a symmetric pattern of localization (10,11,41,47). Unlike asymmetrically localized mRNAs, TUB1 and SNC1 mRNA did not associate with She2 (Fig.…”

“…Next, we examined the in vivo localization of MSO1 mRNA, which encodes a Sec1-interacting factor that also localizes to the plasma membrane of both mother and bud (11). Similar to SNC1 mRNA, MSO1 mRNA localized to mother cells (Fig.…”

Regulation of exocytosis by the exocyst subunit Sec6 and the SM protein Sec1

“…Future studies will require reconstitution and testing of assembled exocyst complexes and generation of novel separation-of-function mutants to further characterize these events. In addition, tethering complexes and SM proteins are not the only regulators at these sites; in yeast, the tomosyn homologue Sro7 interacts with Sec9 and the exocyst to form a parallel regulatory pathway (Lehman et al , 1999; Zhang et al , 2005; Grosshans et al , 2006), and a fungal-specific Sec1 cofactor, Mso1, may bridge Sec1 and Sso1 to facilitate SM recruitment and SNARE regulation (Castillo-Flores et al , 2005; Weber et al , 2010; Weber-Boyvat et al , 2011). Their precise role in these processes and their potential cooperation with the exocyst and Sec1 require further study.…”