Signal recognition particle (SRP) stabilizes the translocation-competent conformation of pre-secretory proteins

When affinity-purified proOmpA was diluted out of 8 M urea into a sample of yeast microsomes, it was translocated and processed in the absence of any cytosolic factors; an intact membrane and ATP were the only requirements. The translocation competence of proOmpA was lost, however, during a 15-h incubation at 0 degrees C. The competence was retained when trigger factor and a yeast cytosolic extract were present during incubations at 0 degrees C. The same reactions were carried out with affinity-purified prepro-alpha-factor, and the same results were obtained with the exception that trigger factor was not required. When the various cytosolic factors were replaced with SRP, the addition of yeast microsomes after 15 h resulted in the translocation and processing (and glycosylation) of both proOmpA and prepro-alpha-factor. Pancreatic microsomes were also used in this type of assay, and it was found that proOmpA (but not prepro-alpha-factor) could be translocated when diluted out of urea. In this case, as with yeast microsomes, translocation competence was maintained by SRP. These results show that in addition to a recognition and targeting function, SRP can stabilize the translocation-competent conformation of pre-secretory proteins in vitro for translocation across eukaryotic membranes.