Stability and function of the Sec61 translocation complex depends on the Sss1p tail-anchor sequence

Sss1p, an essential component of the heterotrimeric Sec61 complex in the ER (endoplasmic reticulum), is a tail-anchored protein whose precise mechanism of action is largely unknown. Tail-anchored proteins are involved in many cellular processes and are characterized by a single transmembrane sequence at or near the C-terminus. The Sec61 complex is the molecular machine through which secretory and membrane proteins translocate into and across the ER membrane. To understand the function of the tail anchor of Sss1p, we introduced mutations into the tail-anchor sequence and analysed the resulting yeast phenotypes. Point mutations in the C-terminal hydrophobic core of the tail anchor of Sss1p were identified that allowed Sss1p assembly into Sec61 complexes, but resulted in diminished growth, defects in co- and post-translational translocation, inefficient ribosome binding to Sec61 complexes, reduction in the stability of both heterotrimeric Sec61 and heptameric Sec complexes and a complete breakdown of ER structure. The underlying defect caused by the mutations involves loss of a stabilizing function of the Sss1p tail-anchor sequence for both the heterotrimeric Sec61 and the heptameric Sec complexes. These results indicate that by stabilizing multiprotein membrane complexes, the hydrophobic core of a tail-anchor sequence can be more than a simple membrane anchor.