Formation of beta A3/beta B2-crystallin mixed complexes: involvement of N-and C-terminal extensions

The sequence extensions of the beta-crystallin subunits have been suggested td play sin important role in the oligomerization of these eye lens protei ns. This, in turn, may contribute to maintaining lens transparency and prop er light refraction. In homo-dimers of the beta A3- and beta B2-crystallin subunits, these extensions have been shown by H-1-NMR spectroscopy to be so lvent-exposed and highly flexible. In this study, we show that beta A3- and beta B2-crystallins spontaneously form mixed beta A3/beta B2-crystallin co mplexes, which, from analytical ultracentrifugation experiments, are dimeri c at low concentrations (<1 mg ml(-1)) and tetrameric at higher protein con centrations. H-1-NMR spectroscopy reveals that in the beta A3/beta B2-cryst allin tetramer, the N-terminal extensions of beta AS-crystallin remain wate r-exposed and flexible, whereas both N- and C-terminal extensions of beta B 2-crystallin lose their flexibility. We conclude that both extensions of be ta B2-crystallin are involved in protein-protein interactions in the beta A 3/beta B2-crystallin hetero-tetramer. The extensions may stabilize and perh aps promote the formation of this mixed complex. (C) 1999 Elsevier Science B.V. All rights reserved.