Fig. 6. A combination of SRP and molecular chaperones mediate TA protein integration. The binding of SRP to TA proteins occurs at an early stage of biosynthesis, shortly after the nascent chain is released from the ribosome. If the nascent TA protein is a poor substrate for SRP, it can use the chaperone-mediated pathway. The Hsc70-Hsp40 combination mediates the major ATP-dependent route and their activity can be modulated by co-chaperones such as BAG1. The identity of the membrane component(s) to which the molecular chaperones deliver their substrates remains unclear (High and Abell, 2004).