Structural and functional roles for beta-strand 7 in the alpha-crystallin domain of p26, a polydisperse small heat shock protein from Artemia franciscana

Oviparous development in the extremophile crustacean, Artemia franciscana, generates encysted embryos which enter a profound state of dormancy, termed diapause. Encystment is marked by the synthesis of p26, a polydisperse small heat shock protein thought to protect embryos from stress. In order to elucidate structural/functional relationships within p26 and other polydisperse small heat shock proteins, and to better define the protein's role during diapause, amino acid substitutions R110G, F112R, R114A and Y116D were generated within the p26 alpha-crystallin domain by site-directed mutagenesis. These residues were chosen because they are highly conserved across species boundaries, and molecular modelling indicates that they are part of a key structural interface between dimers. The F112R mutation, which had the greatest impact on oligomerization, placed two charged residues at the p26 dimer-dimer interface, demonstrating the importance of beta-strand 7 in tetramer formation. All mutated versions of p26 were less able than wild-type p26 to confer thermotolerance on transformed bacteria and they exhibited diminished chaperone action in three in vitro assays; however, all variants retained protective activity. This apparent stability of p26 may, by prolonging effective chaperone life in vivo, enhance embryo stress resistance. All substitutions modified p26 intrinsic fluorescence, surface hydrophobicity and secondary structure, and the pronounced changes in variant R114A, as indicated by these physical measurements, correlated with the greatest loss of function. Although mutation R114A had the greatest effect on p26 chaperoning, it had the least on oligomerization. These results demonstrate that in contrast to many other small heat shock proteins, p26 effectiveness as a chaperone is independent of oligomerization. The results also reinforce the idea, occasioned by modelling, that R114 is removed slightly from dimer-dimer interfaces. Moreover, beta-strand 7 is shown to have an important role in oligomerization of p26, a function first proposed for this structural element upon crystallization of wheat Hsp16.9, a small heat shock protein with different quaternary structure.