Evidence for specific subunit distribution and interactions in the quaternary structure of α-crystallin

Abstract

The quaternary structure of α-crystallin is dynamic, a property which has thwarted crystallographic efforts towards structural characterization. In this study, we have used collision-induced dissociation mass spectrometry to examine the architecture of the polydisperse assemblies of χ-crystallin. For total χ-crystallin isolated directly from fetal calf lens using size-based chromatography, the χB-crystallin subunit was found to be preferentially dissociated from the oligomers, despite being significantly less abundant overall than the χA-crystallin subunits. Furthermore, upon mixing molar equivalents of purified χA-and χB-crystallin, the levels of their dissociation were found to decrease and increase, respectively, with time. Interestingly though, dissociation of subunits from the χA-and χB-crystallin homo-oligomers was comparable, indicating that strength of the χA:χA, and χB:χB subunit interactions are similar. Taken together, these data suggest that the differences in the number of subunit contacts in the mixed assemblies give rise to the disproportionate dissociation of χB-crystallin subunits. Limited proteolysis mass spectrometry was also used to examine changes in protease accessibility during subunit exchange. The C-terminus of χA-crystallin was more susceptible to proteolytic attack in homo-oligomers than that of χB-crystallin. As subunit exchange proceeded, proteolysis of the χA-crystallin C-terminus increased, indicating that in the hetero-oligomeric form this tertiary motif is more exposed to solvent. These data were used to propose a refined arrangement for the interactions of the χ-crystallin domains and C-terminal extensions of subunits within the χ-crystallin assembly. In particular, we propose that the palindromic IPI motif of χB-crystallin gives rise to two orientations of the C-terminus. © 2010 Wiley-Liss, Inc.