C-Terminal truncation affects subunit exchange of human alphaA-crystallin with alphaB-crystallin

In human lenses, C-terminal cleavage of alphaA-crystallin at residues 172,168, and 162 have been reported. The effect of C-terminal truncation of alphaA-crystallin on subunit exchange and heterooligomer formation with alphaB-crystallin and homooligomer formation with native alphaA-crystallin is not known. We have conducted fluorescence resonance energy transfer studies which have shown that the rates of subunit exchange of alphaA(1-172 )and alphaA(1-168 )with alphaB-wt were two-fold lower than for alphaA-wt interacting with alphaB-wt. The subunit exchange rate between alphaA(1-162) and alphaB-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 alphaA-wt and the truncated alphaA-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 alphaA-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 alphaA and alphaB subunits.