BINDING OF SECB TO RIBOSOME-BOUND POLYPEPTIDES HAS THE SAME CHARACTERISTICS AS BINDING TO FULL-LENGTH, DENATURED PROTEINS

The interaction of the chaperone SecB with ribosome-bound polypeptides that are in the process of elongation has been studied using an in vi tro protein synthesis system. The binding is characterized by the same properties as those demonstrated for the binding of SecB to full-leng th proteins that are in nonnative conformation: it is readily reversib le and has no specificity for the leader peptide. In addition, it is s hown that the growing polypeptide chains must achieve a critical lengt h to bind tightly enough to allow their isolation in complex with SecB . This explains the longstanding observation that, even when export is cotranslational, it begins late in synthesis. Furthermore, the requir ed length is approximately the same as the length that defines the bin ding frame within denatured, fall-length proteins bound to SecB.