Toothpicks, serendipity and the emergence of the Escherichia coli DnaK (Hsp70) and GroEL (Hsp60) chaperone machines.

THE purpose of this essay is to retrace some of the early steps that I and a few (then) young geneticists took in the late 1960s and early 1970s to define the Escherichia coli functions used by phage to properly execute their developmental cycle. Eventually, this led to the discovery and functional understanding of the so-called DnaK (Hsp70) and GroEL (Hsp60) molecular chaperone machines, universally conserved among the biological kingdoms (Lindquist and Craig 1988; Georgopoulos and Welch 1993). Now we know that these and other molecular chaperone machines are involved in a multitude of biological processes, including protection of nascent polypeptide chains from premature aggregation, disaggregation of protein aggregates, polypeptide transport across biological membranes, and proteolysis (Bukau and Horwich 1998; Hartl and Hayer-Hartl 2002; Craig et al. 2006). Because protein denaturation and aggregation are enhanced by various environmental stresses, e.g., an increase in temperature, it is not surprising that the DnaK (Hsp70) and GroEL (Hsp60) molecular chaperone machines were also discovered independently as ‘‘heat-shock’’ or ‘‘stress’’ proteins. Because of their very short growth cycles, phage have evolved a variety of strategies to subvert and customize host functions for their own use. Phage likely have a greater need than their hosts for quick and abundant chaperone power to carry out their developmental cycle in a timely fashion. If they fail to complete their cycle before the host lyses, infectious phage progeny will not be released into the medium, risking their extinction. This differential need for chaperone power likely explains why many of the bacterial mutations found to block phage development were eventually shown to be in genes encoding the GroEL and DnaK chaperone machines.