Beta A3/A1-crystallin association: role of the N-terminal arm

The beta- and gamma-crystallins of the lens form a protein superfamily, the beta gamma-crystallins and have highly conserved two-domain core structures. Whereas gamma-crystallins exist as monomers, the beta-crystallins associate into large aggregates. The N-terminal extensions to the core domains of beta-crystallins are postulated to be essential for their aggregation characteristics. To test this hypothesis, we compared the aggregation properties of a recombinant mouse beta A3/A1-crystallin without its N-terminal extension (r beta A3tr) to a normal recombinant mouse beta A3/A1-crystallin (r beta A3). The identity of the baculo-virus system-expressed recombinant crystallins was confirmed by gel electrophoresis, immunoblots and N-terminal sequence analysis. Circular dichroism measurements indicate that the recombinant crystallins have mostly beta-sheet conformation, similar to normal beta-crystallins. The normal r beta A3 migrates on gel filtration chromatography as a homodimer, whereas the r beta A3tr migrates mostly as a monomer. After relocating the recombinant crystallins with mouse lens soluble extract, r beta A3 migrated with the dimeric beta L2 fractions and to a lesser extent with tetrameric beta L1 fractions. The reassociated r beta A3tr migrated with the trailing edge of the beta L 2 fractions (40 kDa). These results suggest that the N-terminal arm of beta A3/A1-crystallin facilitates dimer formation and is necessary for higher-order associations.