Deletion of Specific Conserved Motifs from the N-Terminal Domain of αB-Crystallin Results in the Activation of Chaperone Functions

Abstract

Smaller oligomeric chaperones of α-crystallins (αA- and αB-) have received increasing attention due to their improved therapeutic potential in preventing protein aggregating diseases. Our previous study suggested that deleting 54–61 residues from the N-terminal domain (NTD) of αB-crystallin (αBΔ54–61) decreases the oligomer size and increases the chaperone function. Several studies have also suggested that NTD plays a significant role in protein oligomerization and chaperone function. The current study was undertaken to assess the effect of deleting conserved 21–28 residues from the activated αBΔ54–61 (to get αBΔ21–28, Δ54–61) on the structure–function of recombinant αBΔ21–28, Δ54–61. The αBΔ21–28, Δ54–61 mutant shows an 80% reduction in oligomer size and 3- to 25-fold increases in chaperone activity against model substrates when compared to αB-WT. Additionally, the αB∆21–28, ∆54–61 was found to prevent β-amyloid (Aβ1–42) fibril formation in vitro and suppressed Aβ1–42-induced cytotoxicity in ARPE-19 cells in a more effective manner than seen with αB-WT or αB∆54–61. Cytotoxicity and reactive oxygen species (ROS) detection studies with sodium iodate (SI) showed that the double mutant protein has higher anti-apoptotic and anti-oxidative activities than the wild-type or αB∆54–61 in oxidatively stressed cells. Our study shows that the residues 21–28 and 54–61 in αB-crystallin contribute to the oligomerization and modulate chaperone function. The deletion of conserved 21–28 residues further potentiates the activated αBΔ54–61. We propose that increased substrate affinity, altered subunit structure, and assembly leading to smaller oligomers could be the causative factors for the increased chaperone activity of αBΔ21–28, Δ54–61.