pH-induced reversible dissociation of tetrameric duck lens delta-crystallin

Animal lenses constitute many soluble proteins, which play a prominent role in eyes' light transparency. delta2-Crystallin, one of the major taxon-specific crystallins in duck lens, is a tetrameric protein consisting of four identical subunits, which contain endogenous argininosuccinate lyase activity. Under a neutral pH environment in this work, the protein was cross-linked with glutaraldehyde as tetrameric and dimeric forms with tetramer as the major form. Under acidic conditions, the protein was time-dependently dissociated into monomers with amino acid residues of pKa values 6.29+/-0.45 and 7.17+/-0.49 being involved in the monomer-monomer interactions and 6.20+/-0.10 and 8.88+/-0.07 in the dimer-dimer interactions. Duck lens delta2-crystallin thus possesses a double dimer structure (alpha2)2 with stronger monomer-monomer interactions than the dimer-dimer interactions. The acidic protein solution's reneutralization caused rapid reassociation of monomers into dimers and tetramers. The tetramer-dimer-monomer dissociation-reassociation thus is a pH-dependent freely interconvertible process.