C-terminal truncation affects subunit exchange of human alpha A-crystallin with alpha B-crystallin

In human lenses, C-terminal cleavage of alpha A-crystallin at residues 172,168, and 162 have been reported. The effect of C-terminal truncation of alpha A-crystallin on subunit exchange and heterooligomer formation with alpha B-crystallin and homooligomer formation with native alpha A-crystallin is not known. We have conducted fluorescence resonance energy transfer studies which have shown that the rates of subunit exchange of alpha A(1-172) and alpha A(1-168) with alpha B-wt were two-fold lower than for alpha A-wt interacting with alpha B-wt. The subunit exchange rate between alpha A(1-162) and alpha B-wt was six-fold lower. These data suggest that cleavage of the C-terminal residues could significantly affect heterooligomerization. On the other hand, the subunit exchange rates between alpha A-wt and the truncated alpha A-crystallins were either unchanged or only slightly decreased, which suggest that homooligomerization may not be significantly influenced by C-terminal truncation. The main conclusion from this study is that cleavage of C-terminal residues of alpha A-crystallin including the nine residues of the flexible tail is expected to significantly affect the formation of heteroaggregates. Reconstitution experiments showed that the presence of an intact C-terminus is essential for the formation of fully integrated heteroaggregates with equal proportion of alpha A and alpha B subunits.