SecB, One Small Chaperone in the Complex Milieu of the Cell

SecB is only one of a plethora of cytosolic chaperones in E. coli whose common property is that they bind nonnative proteins. It plays a crucial role during protein export via the general secretory pathway by modulating the partitioning of precursors between folding or aggregation and delivery to the membrane-bound translocation apparatus. In this latter role SecB demonstrates specific binding to a unique partner, SecA. SecB has the potential to participate in functions outside of export acting as a general nonspecific chaperone to provide buffering capacity of the nonnative state of proteins in the cytosolic pool. We discuss the interactions of SecB with its many binding partners in light of its recently determined structure, emphasizing both kinetic and thermodynamic parameters.