βA3/Al-crystallin association: role of the N-terminal arm

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Abstract
The β– and γ–crystallins of the lens form a protein superfamily, the βγ37–crystallins and have highly conserved twodomain core structures. Whereas γ–crystallins exist as monomers, the β–crystallins associate into large aggregates. The N–terminal extensions to the core domains of β–crystallins are postulated to be essential for their aggregation characteristics. To test the hypothesis, we compared the aggregation properties of a recombinant mouse βA3/Al–crystallin without its N–terminal extension (rβA3tr) to a normal recombinant mouse βA3β/Al–crystallin (rβA3). The identity of the baculovirus system–expressed recombinant crystallins was confirmed by gel ekctrophoresis, immunoHots and N–terminal sequence analysis. Circular dkhroism measurements indicate that the recombinant crystallins have mostly β–sheet conformation, similar to normal β–crystallins. The normal rβA3 migrates on gel filtration chromatography as a homodimer, whereas the rβA3tr migrates mostly as a monomer. After relocating the recombinant crystallins with mouse lens soluble extract, rβA3 migrated with the dimeric βL2 fractions and to a lesser extent with tetrameric βLl fractions. The reassodated rβA3tr migrated with the trailing edge of the βL2 fractions (40 kDa). These results suggest that the N–terminal arm of βA3/A1– crystallin facilitates dimer formation and is necessary for higher–order associations

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