Proteins destined for posttranslational import to the ER are synthesized on free ribosomes and maintained in an unfolded conformation by cytosolic chaperones. Their signal sequences are recognized by the Sec62/63 complex, which is associated with the Sec61 translocation channel in the ER membrane. The Sec63 protein is also associated with a chaperone protein (BiP), which acts as a molecular ratchet to drive protein translocation into the ER.
Cytosolic chaperones are required to maintain the polypeptide chains in an unfolded conformation so they can enter the Sec61 channel, and another chaperone within the ER (called BiP) is required to pull the polypeptide chain through the channel and into the ER. It appears that the binding of polypeptide chains to BiP is needed to drive the posttranslational translocation of proteins into the ER, whereas the cotranslational translocation of growing polypeptide chains is driven directly by the process of protein synthesis.