STRUCTURE-FUNCTION STUDIES OF MOLECULAR CHAPERONES

Stanford University, Stanford CA

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Abstract

The capacity of cells to adapt to stresses such as heat shock, chemical insult or nutrient deprivation, to suppress misfolding and aggregations of proteins, and to degrade aberrant polypeptides is essential for viability. Such functions are effected by "molecular chaperones", proteins which exercise specific mechanisms of modulating protein synthesis, folding, renaturation and degradation in cells. The long term goal of this work is to understand the structures and biochemical mechanisms of molecular chaperones. The seventy kilodalton heat shock proteins (Hsp70s) are a near-ubiquitous family of molecular chaperones that suppress aggregation and misfolding of polypeptides and facilitate polypeptide translation and transmembrane translocation. They effect their activity by binding and sequestering hydrophobic segments of polypeptides; peptide binding/release is modulated by ADP/ATP binding. The prevailing model for the mechanism by which ATP controls Hsp70 activity will be tested in vivo using a viability assay developed with E. coli. Further crystallographic and solution small angle x-ray scattering work will be done to determine the molecular mechanism by which ATP binding induces a large conformational change in Hsp70s which results in peptide release. The Clp/Hsp100 chaperones make up a diverse family of proteins whose activities include, but are not restricted to, ATP-dependent proteolysis of specific cellular targets, such as ssrA-tagged "mis-translated" polypeptides in bacteria. A representative chaperone-protease complex, the 850 kDa HslUV complex of Haemophilus influenzae, has been overexpressed, purified, and crystallized in a form suitable for structural studies. The structure of HslV has been solved to 3.9 Angstrom units by molecular replacement. Crystals of HslU diffract to sufficient resolution for de novo structure determination by multiple isomorphous replacement or multiwavelength anomalous dispersion methods. Crystals of HslUV diffract strongly to 6.0 Angstrom units and weakly to 3.5 Angstrom units, which is sufficient to determine the structure by molecular replacement with models of the individual HslU and HslV components. The structures, when completed, will provide a tool for structure-function studies of target specificity and molecular mechanism of substrate recognition, unfolding and degradation by HslUV.