May prevent inappropriate targeting of non-secretory polypeptides to the endoplasmic reticulum (ER). May bind to nascent polypeptide chains as they emerge from the ribosome and block their interaction with the signal recognition particle (SRP), which normally targets nascent secretory peptides to the ER. May also reduce the inherent affinity of ribosomes for protein translocation sites in the ER membrane (M sites) (By similarity).