Deletion of 54FLRAPSWF61 Residues Decreases the Oligomeric Size and Enhances the Chaperone Function of αB-Crystallin

alpha B-Crystallin is a member of the small heat shock protein family and is known to have chaperone activity. Using a peptide scan approach, we previously determined that regions 42-57, 60-71, and 88-123 in alpha B-crystallin interact with alpha A-crystallin during heterooligomer formation. To further characterize the significance of the N-terminal domain of alpha B-crystallin, we prepared a deletion mutant that lacks residues (14)FLRAPSWF(61) (alpha B Delta 54-61) and found that the absence of residues 54-61 in alpha B-crystallin significantly decreased the homooligomeric mass of alpha B-crystallin. The average oligomeric mass of wild-type alpha B-crystallin and of alpha B Delta 54-61, calculated using multiangle light scattering, was 624 and 382 kDa, respectively. The mutant subunits aggregate to form smaller, less-compact oligomers with a 4-fold increase in subunit exchange rate. Deletion of the 54-61 region resulted in a 50% decrease in intrinsic tryptophan fluorescence. The alpha B Delta 54-61 mutant showed a 2-fold increase in 1,1'-bi(4-anilino)naphthalene-5,5'-disulfonic acid (bis-ANS) binding as compared to the wild-type protein, suggesting increased hydrophobicity of the mutant protein. Accompanyng the evidence of increased hydrophobicity in the deletion mutant was a 10-fold increase in antiaggregation activity. Homooligomers of 6H alpha A (750 kDa) readily exchanged subunits with alpha B Delta 54-61 homooligomers at 37 degrees C, forming heterooligomers with an intermediate mass of 625 kDa. Our data suggest that residues (14)FLRAPSWF(61) contribute to the higher order assembly of alpha B-crystallin oligomers. Residues (54)FLRAPSWF(61) in alpha B-crystallin are not essential for target protein binding during chaperone action, but this region apparently has a role in the chaperone activity of native alpha B-crystallin.