Biochemistry and cell biology of the major human heat-shock protein, HSP70

The heat-shock or stress response is a very highly conserved biological response which has been found in every organism examined thus far. Cells exposed to temperatures a few degrees above their normal physiological temperature induce the synthesis of a small set of highly conserved proteins, the heat-shock or stress proteins. All organisms contain major stress-induced proteins of 70 and 90 kilodaltons, as well as low molecular weight heat-shock proteins. These proteins are thought to aid in the repair of stress-induced cellular damage and to protect cells from further environmental stress, but the functions of the individual stress proteins are unknown. We have characterized the biochemical properties and cell biology of the major heat-induced protein, HSP70, in human cells, in an attempt to understand its role in unstressed and heat-shocked cells.;In order to determine what regions of the HSP70 protein are important for interacting with other proteins we constructed a series of deletion mutations within the HSP70 protein coding region. Analysis of the deletion mutants revealed that HSP70 is comprised of two domains. Sequences in the amino terminus of HSP70 are important for the ATP-binding ability of the protein, while sequences in the carboxy terminus are important for association of HSP70 with nucleoli following heat shock.;We have demonstrated that the expression of HSP70 in unstressed cells is cell cycle-regulated. HSP70 is synthesized at the G1/S boundary of the cell cycle, and indirect immunofluorescence with HSP70-specific monoclonal antibodies revealed that HSP70 becomes concentrated in the nucleus of S-phase cells. Using several different HSP70-specific monoclonal antibodies we have demonstrated that the availability of certain HSP70 epitopes varies throughout the cell cycle. This is presumably due to interactions with other cellular proteins, as nondenaturing immunoprecipitations with the HSP70 monoclonal antibodies revealed an array of coprecipitated proteins. The interaction of HSP70 with some of these proteins was cell cycle-specific. We have identified some of the proteins associated with HSP70 as components of heterogeneous nuclear ribonucleoprotein complexes.