Mso1p: A yeast protein that functions in secretion and interacts physically and genetically with Sec1p

Abstract

The yeast Sec1p protein functions in the docking of secretory transport vesicles to the plasma membrane. We previously have cloned two yeast genes encoding syntaxins, SSO1 and SSO2, as suppressors of the temperaturesensitive sec1-1 mutation. We now describe a third suppressor of sec1-1, which we call MSO1. Unlike SSO1 and SSO2, MSO1 is specific for sec1 and does not suppress mutations in any other SEC genes. MSO1 encodes a small hydrophilic protein that is enriched in a microsomal membrane fraction. Cells that lack MSO1 are viable, but they accumulate secretory vesicles in the bud, indicating that the terminal step in secretion is partially impaired. Moreover, loss of MSO1 shows synthetic lethality with mutations in SEC1, SEC2, and SEC4, and other synthetic phenotypes with mutations in several other late-acting SEC genes. We further found that Mso1p interacts with Sec1p both in vitro and in the two-hybrid system. These findings suggest that Mso1p is a component of the secretory vesicle docking complex whose function is closely associated with that of Sec1p.

Molecular Interactions Position Mso1p, a Novel PTB Domain Homologue, in the Interface of the Exocyst Complex and the Exocytic SNARE Machinery in Yeast

“…However, during PSM initiation, the membrane fusion machinery seems to be more stringently regulated compared with constitutive exocytosis in vegetatively grown cells. This is exemplified by the fact that PSM formation depends on the presence of Mso1p, whereas in Mso1p-deleted vegetatively grown cells only a moderate accumulation of vesicles at the sites of membrane growth is visible (Aalto et al, 1997). This essential function of Mso1p in PSM formation seems to be mediated through its interaction with Sec1p as mutant cells expressing a Sec1p binding defective form of Mso1p had a very similar phenotype compared with cells where the whole Mso1p was deleted.…”

“…The T47A mutation reduced, but did not inhibit, Sec1p binding when compared with T43A and T46A mutants (our unpublished data). For all constructs used, similar expression of the encoded mutant protein was verified by Western blotting (our unpublished data).MSO1 deletion in combination with sec1-1, sec1-11, sec2-41, or sec4-8 mutation is lethal (Aalto et al, 1997). This suggests that Mso1p function may be functionally closely associated not only with Sec1p, but also Sec2p and Sec4p.…”

“…The t-SNARE Sso2p was shown previously to interact with Sec1p in the yeast two-hybrid assay (Brummer et al, 2001). The Sec1p interacting protein Mso1p was identified in a multicopy suppressor screen for sec1-1 temperature-sensitive mutant (Aalto et al, 1997). Deletion of MSO1 is not lethal in vegetatively growing cells, but it leads to vesicle accumulation at the site of cell growth, implicating a positive role for Mso1p in exocytosis.…”

“…In pull-downs with Ni-NTA resin, specific binding between His 6 -Sec1p and MBPMso1p(38-84) was observed, whereas no binding was detected in samples containing His 6 -Sec1p and MBP or MBP-Mso1p(38-84) and the empty plasmid control (Figure 3C). Pull-downs performed with amylose resin revealed the same specific interactions between His 6 -Sec1p and MBP-Mso1p(38-84) (our unpublished data).The Sec1p Binding Domain Is Necessary, but Not Sufficient, for Mso1p In Vivo Function MSO1 was identified in a genetic screen for multicopy suppressors of sec1-1 temperature-sensitive mutant (Aalto et al, 1997). We made use of this genetic interaction to test the role of the observed Sec1p binding domain for in vivo Mso1p function.…”

“…Assembly of these so-called prospore membranes occurs at the cytoplasmic side of the spindle pole bodies (SPBs) and requires the function of a meiosisspecific structure of the SPBs, the meiotic plaque (Taxis and Knop, 2004). Genetic interaction of MSO1 deletion with sec1 and sec4 mutations link MSO1 functionally to the vesicle docking and SNARE complex assembly (Aalto et al, 1997).To shed light on Mso1p function and regulation of exocytosis, we have analyzed in detail association of Mso1p with the molecular machinery regulating membrane fusion in yeast cells. Our study positions Mso1p functionally in close association with Sec4p and the exocytotic SNARE complex.…”