Sss1p, an essential component of the heterotrimeric Sec61 complex in the endoplasmic reticulum (ER) #, 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 (TMS) at or near the carboxyl-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 analyzed 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, diminished ribosome binding to Sec61 complexes, reduced 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 data indicate that by stabilizing multiprotein membrane complexes, the hydrophobic core of a tail-anchor sequence can be more than a simple membrane anchor.