The 2.2 angstrom crystal structure of Hsp33: A heat shock protein with redox-regulated chaperone activity

Background: One strategy that cells employ to respond to environmental stre sses (temperature, oxidation, and pathogens) is to increase the expression of heat shock proteins necessary to maintain viability. Several heat shock proteins function as molecular chaperones by binding unfolded polypeptides and preventing their irreversible aggregation. Hsp33, a highly conserved ba cterial heat shock protein, is a redox-regulated molecular chaperone that a ppears to protect cells against the lethal effects of oxidative stress. Results: The 2.2 Angstrom crystal structure of a truncated E. coli Hsp33 (r esidues 1-255) reveals a domain-swapped dimer. The core domain of each mono mer (1-178) folds with a central helix that is sandwiched between two beta sheets. The carboxyl-terminal region (179-235), which lacks the intact Zn b inding domain of Hsp33, folds into three helices that pack on the other sub unit. The interface between the two core domains is comprised of conserved residues, including a rare Glu-Glu hydrogen bond across the dyad axis. Two potential polypeptide binding sites that span the dimer are observed: a lon g groove containing pockets of conserved and hydrophobic residues, and an i ntersubunit 10-stranded beta sheet "saddle" with a largely uncharged or hyd rophobic surface. Conclusions: Hsp33 is a dimer in the crystal structure. Solution studies co nfirmed that this dimer reflects the structural changes that occur upon act ivation of Hsp33 as a molecular chaperone. Patterns of conserved residues a nd surface charges suggest that two grooves might be potential binding site s for protein folding intermediates.