Post-translational translocation of polypeptides across the mammalian endoplasmic reticulum membrane is size and ribosome dependent.

The translation and translocation of two yeast glycoproteins, invertase and carboxypeptidase Y, were studied in a heterologous cell-free translation system from reticulocytes supplemented with dog pancreas microsomes. Using in vitro synthesized mRNA transcripts, encoding complete or truncated invertase forms, the influence of polypeptide size and ribosome dependence was studied. It was found that C-terminal truncated fragments of 25 kDa, i.e. a size larger than the average size of a domain structure, are translocated and processed post-translationally with a similar efficiency to the cotranslational events. Post-translational import decreases with increasing peptide chain, mature polypeptide (60 kDa) being no longer translocated. Post-translational competence is only maintained as long as the peptide remains associated with ribosomes. Translocation of invertase depends on the presence of the leader peptide and requires energy independent of protein synthesis. Size dependence of post-translational import could also be demonstrated for carboxypeptidase Y.