Transient involvement of signal recognition particle and its receptor in the microsomal membrane prior to protein translocation.

We have shown that neither the signal recognition particle (SRP) nor the SRP receptor is directly involved in the maintenance of a ribosome-membrane junction for the translocation of secretory proteins. We found that the purified SRP receptor releases the signal-sequence-induced and SRP-mediated elongation arrest of synthesis by displacing SRP from the ribosome. This SRP displacement was not accompanied by binding of the SRP receptor to the ribosome. Using intact microsomal membranes as a source of the SRP receptor, we found that both SRP displacement and binding of the elongation-arrested ribosome to the membrane can occur at 0 degrees C, by a mechanism that is independent of chain elongation.