Full-length prepro-alpha-factor can be translocated across the mammalian microsomal membrane only if translation has not terminated

Abstract

We have previously shown that fully synthesized prepro-alpha-factor (pp alpha F), the precursor for the yeast pheromone alpha-factor, can be translocated posttranslationally across yeast rough microsomal (RM) membranes from a soluble, ribosome-free pool. We show here that this is not the case for translocation of pp alpha F across mammalian RM. Rather we found that a small amount of translocation of full-length pp alpha F is observed, but is solely due to polypeptide chains that were still ribosome bound and covalently attached to tRNA, i.e., not terminated. In addition, both signal recognition particle (SRP) and SRP receptor are required, i.e., the same targeting machinery that is normally responsible for the coupling between protein synthesis and translocation. Thus, the molecular requirements for targeting are distinct from posttranslational translocation across yeast RM. As termination is generally regarded as part of translation, the translocation of full-length pp alpha F across mammalian RM does not occur "posttranslationally," albeit independent of elongation. Most other proteins for which posttranslational translocation across mammalian RM was previously claimed fall into the same category in that ribosome attachment as peptidyl-tRNA is required. To clearly separate these two distinct processes, we suggest that the term posttranslational be reserved for those processes that occur in the complete absence of the translational machinery. We propose the term "ribosome-coupled translocation" for the events described here.