Effect of mutations in the β5-β7 loop on the structure and properties of human small heat shock protein HSP22 (HspB8, H11)

Abstract

The human genome encodes ten different small heat shock proteins, each of which contains the so-called α-crystallin domain consisting of 80-100 residues and located in the C-terminal part of the molecule. The α-crystallin domain consists of six or seven β-strands connected by different size loops and combined in two β-sheets. Mutations in the loop connecting the β5 and β7 strands and conservative residues of β7 in αA-, αB-crystallin and HSP27 correlate with the development of different congenital diseases. To understand the role of this part of molecule in the structure and function of small heat shock proteins, we mutated two highly conservative residues (K137 and K141) of human HSP22 and investigated the properties of the K137E and K137,141E mutants. These mutations lead to a decrease in intrinsic Trp fluorescence and the double mutation decreased fluorescence resonance energy transfer from Trp to bis-ANS bound to HSP22. Mutations K137E and especially K137,141E lead to an increase in unordered structure in HSP22 and increased susceptibility to trypsinolysis. Both mutations decreased the probability of dissociation of small oligomers of HSP22, and mutation K137E increased the probability of HSP22 crosslinking. The wild-type HSP22 possessed higher chaperone-like activity than their mutants when insulin or rhodanase were used as the model substrates. Because conservative Lys residues located in the β5-β7 loop and in the β7 strand appear to play an important role in the structure and properties of HSP22, mutations in this part of the small heat shock protein molecule might have a deleterious effect and often correlate with the development of different congenital diseases. © 2007 The Authors.