The function of the beta3 interactive domain in the small heat shock protein and molecular chaperone, human alphaB crystallin.

Knowledge of the interactive domains on the surface of small heat shock proteins (sHSPs) is necessary for understanding the assembly of complexes and the activity as molecular chaperones. The primary sequences of 26 sHSP molecular chaperones were aligned and compared. In the interactive beta3 sequence, 73DRFSVNLDVKHFS85 of human alphaB crystallin, Ser-76, Asn-78, Lys-82, and His-83 were identified as nonconserved residues on the exposed surface of the alpha crystallin core domain. Site-directed mutagenesis produced the mutant alphaB crystallins: S76E, N78G, K82Q, and H83F. Domain swapping with homologous beta3 sequences, 32EKFEVGLDVQFFT44 from Caenorhabditis elegans sHSP12.2 or 69DKFVIFLDVKHFS81 from alphaA crystallin, resulted in the mutant alphaB crystallins, CE1 and alphaA1, respectively. Decreased chaperone activity was observed with the point mutants N78G, K82Q, and H83F and with the mutant, CE1, in aggregation assays using betaL crystallin, alcohol dehydrogenase (ADH), or citrate synthase (CS). The S76E mutant had minimal effect on chaperone activity, and domain swapping with alphaA crystallin had no effect on chaperone activity. The mutations that resulted in altered chaperone activity, produced minimal modification to the secondary, tertiary, and quaternary structure of human alphaB crystallin as determined by ultraviolet circular dichroism spectroscopy, chymotrypsin proteolysis, and size exclusion chromatography. Chaperone activity was influenced by the amount of unfolding of the target proteins and independent of complex size. The results characterized the importance of the exposed side chains of Glu-78, Lys-82, and His-83 in the interactive beta3 sequence of the alpha crystallin core domain in alphaB crystallin for chaperone function.