Prokaryotic and eukaryotic cells respond to heat shock and certain other environmental abuses by synthesizing a small set of heat shock proteins (hsps). The two most abundant hsps (hsp70 and hsp90) are highly conserved among organisms as diverse as E. coli, Drosophila, and man. Proteins closely related to them are present in unstressed cells, and the hsp70-like proteins, at least, are important for normal growth (Craig and Jacobsen, MCB 5, 3517-3524, 1985).

The functions of the hsps have long been a matter for speculation; it is widely assumed that these proteins protect cells from the effects of stress, but the mechanisms remain unclear. Recent advances in two areas, however, may give important clues to their functions. First, in vitro studies have identified several ATP-dependent reactions that involve hsp70-like proteins. Second, two proteins that are related to hsp70 and hsp90, and whose synthesis is regulated by glucose starvation (the grps), have been localized to the lumen of the endoplasmic reticulum. This limits the possible functions of these proteins to the few metabolic processes that take place in that cellular compartment. I argue here that members of the hsp70 family, and possibly the hsp90 family, are involved in the assembly and disassembly of proteins and protein-containing structures.