Cloning, Expression and Characterization of Bovine aB-crystallin

Abstract

The lens protein, aB-crystallin, plays central role in slowing the formation of pre-cataractous protein aggregates. By binding to newly exposed, hydrophobic regions of damaged proteins, aB-crystallin creates stable, soluble complexes that are resistant to further unfolding and aggregation. Several regions o f aB-crystallin have been identified as possible sites for this chaperone-like binding, three of which are explored further in the present work using assays for both the physical properties and in vitro function of sitedirected mutants of aB-crystallin. A priority in this study was the generation of a bovine aB-crystallin cDNA clone. One of the regions of putative chaperone binding in aB - crystallin, residues 24-32, which was implicated by deuterium exchange experiments, contains several hydrophobic residues. Since hydrophobic residues are thought to play a central role in chaperone binding, the present work investigates a mutant of one of these residues, F28, which was changed to serine. Two charged residues within this same domain, E30, and E34 were each separately mutated to glutamine in order to assess the role of negatively charged residues in chaperone activity. Another two regions implicated as possible binding sites in aB-crystallin, residues 59-68 and residues 92-108, were previously identified using SAED binding studies. In the present study, one hydrophobic residue from each of these domains was mutated to a less hydrophobic residue: 16IS and L94Q. The results show that at 25°C, a serine as position 28 (F28S) causes moderate alterations in secondary structure, tertiary structure and oligomeric assembly. At 58°C, however, this mutant suffers from a disintegration o f oligomeric structure as well as a loss of chaperone function. The data implicate F28 as a critical residue for maintaining the structural integrity of aB-crystallin. The other mutants, with the exception o f L94Q, behaved similar to wild type aB-crystallin whether assayed at 37 ° or 58 °. The L94Q mutant displayed slightly better chaperone activity than wild type in the high temperature (58°) assay. The present work supports the idea that the chaperone-like behavior of aBcrystallin requires intact oligomeric structure and that the activity may not be associated with a discreet binding site but instead a diverse array of residues spread out over the surface of aB-crystallin.