Hsp70 chaperone systems: diversity of cellular functions and mechanism of action.

Hsp70 chaperone systems play an essential role in the life cycle of many proteins not only in an hostile environment but also under normal growth conditions. In the course of evolution the diversification of functions was accompanied by an amplification of components of the Hsp70 system. Here strategies are reviewed how different Hsp70 systems work independently or cooperate with each other in a functional network to perform their housekeeping tasks even under stress conditions. We further discuss how co-chaperones which act as targeting factors regulate the cycle of substrate binding and release upon which the Hsp70 chaperone activity depends.