Analysis of SSS1P: An Essential Tail-Anchor Protein of the ER Translocon in the Yeast Saccharomyces cerevisiae

Abstract

Sss1p is an essential component, along with Sec61p, of the protein conducting channel (PCC) in the ER of the yeast Saccharomyces cerevisiae. It belongs to a family of proteins termed tail-anchor (TA) proteins. The TA consists of a single hydrophobic sequence at the carboxyl-terminus which anchors the protein to the membrane in a Type II (N_cytoplasm-C_lumen) orientation. TA proteins are targeted to their membranes of function through an uncharacterized SRP-independent, post-translational mechanism. The targeting mechanism and function of Sss1p are not known. In this thesis, results will be presented from targeting and functional studies of Sss1p. Sss1p is predicted to contain an ER targeting signal similar to mammalian VAMP-1A. Disruption of this putative signal caused incomplete mislocalization of Sss1p to the mitochondria which did not affect yeast growth. Mutations in the TA of Sss1p had numerous effects. Yeast expressing these mutants showed diminished growth, a defect in co-and post-translational translocation, inefficient ribosome binding to Sec61 p and the mislocalization of translocon components from light membranes (predominantly ER) to heavy membranes (predominantly mitochondria). It is argued that mutations in the TA of Sss1p disrupt the function of the protein, subsequently leading to the general defects listed above. Two possible functions for Sss1p are proposed: Sss1p is involved in forming the signal sequence binding pocket of the translocon and/or is essential for the integrity of the PCC.