Heat shock of Escherichia coli increases binding of dnaK (the hsp70 homolog) to polypeptides by promoting its phosphorylation

The "molecular chaperone", dnaK, is induced in Escherichia coli upon heat shock and promotes ATP-dependent refolding or degradation of damaged proteins. When cells were grown at 25°C and disrupted, a small fraction of the dnaK bound to affinity columns containing unfolded polypeptides (e.g., a fusion protein named CRAG or casein) and could be dissociated by ATP-Mg²⁺. After shifting cells to 42°C for 30 min, up to 5-fold more dnaK bound to these columns than after growth at 25°C. This enhanced binding capacity was reversed after shifting cells back to 25°C. It resulted from a covalent modification, which decreases dnaK's electrophoretic mobility and isoelectric point. This modification appears to be phosphorylation; after treatment with phosphatases, the ATP-eluted dnaK resembled the predominant form in electrophoretic and binding properties. In addition, after incubating cells with [³²P]orthophosphate at 42°C, the ³²P-labeled dnaK bound quantitatively to the CRAG column, unlike the nonlabeled protein. Thus, the phosphorylated dnaK is a special form of the chaperone with enhanced affinity for unfolded proteins. Its accumulation at high temperatures may account for dnaK's function as the "cellular thermometer.".