Role of cytosolic factors in the transport of proteins across membranes

In a review article published in 1986 we emphasized that an unfolded conformation is a prerequisite for the transport of precursor proteins across membranes, and that cytosolic factors exist whose function is to maintain what we termed the transport-competent conformation of precursor proteins. Subsequent observations in a number of different in vitro systems related both the competent conformation and the cytosolic factors to the recent observations on the ATP-requirements for protein transport. Here we review the currently available data on such factors, and their ATP-requirements, for prokaryotic as well as eukaryotic organisms. Furthermore, we discuss possible models for their action and relate them to the so-called molecular chaperones. These were originally defined as being involved in the proper folding and assembly of oligomeric protein complexes, but have since been shown in addition to facilitate the transport of proteins across membranes.