Identification of novel protein-protein interactions at the cytosolic surface of the Sec63 complex in the yeast ER membrane.

Precursors of secretory proteins are targeted to the membrane of the endoplasmic reticulum by specific protein complexes that recognize their signal sequence. All eukaryotic cells investigated so far have been found to possess the signal recognition particle (SRP) that targets the majority of precursors to the translocation machinery. In Saccharomyces cerevisiae a number of proteins are translocated independently of SRP. These precursors rely on a different signal sequence-binding complex, which includes Sec62p, Sec63p, Sec71p and Sec72p. Identifying interactions between individual components of this tetrameric protein complex is important in the understanding of its function. We demonstrate a specific interaction between the only two essential proteins in this complex, Sec62p and Sec63p. Second, we show evidence of homodimerization of Sec72p molecules and further identify the YLR301w gene product as a novel in vivo interacting partner of Sec72p. Finally, we determine the authentic N-terminus of Sec62p and describe interacting subdomains of both Sec62p and Sec63p.