Protein Science, Vol 3, Issue 9 1392-1400, Copyright © 1994 by Cold Spring Harbor Laboratory Press

ARTICLE

Dimerization of {beta}B2-crystallin: The role of the linker peptide and the N- and C-terminal extensions

S. TRINKL, R. GLOCKSHUBER and R. JAENICKE
Institut fur Biophysik und Physikalische Biochemie, Universitat Regensburg, D-93040 Regensburg, Germany

{beta}B2- and {gamma}B-crystallins of vertebrate eye lens are 2-domain proteins in which each domain consists of 2 Greek key motifs connected by a linker peptide. Although the folding topologies of {beta}B2- and {gamma}B-domains are very similar, {gamma}B-crystallin is always monomeric, whereas {beta}B2-crystallin associates to homodimers. It has been suggested that the linker or the protruding N- and C-terminal arms of {beta}B2-crystallin (not present in {gamma}B) are a necessary requirement for this association. In order to investigate the role of these segments for dimerization, we constructed two {beta}B2 mutants. In the first mutant, the linker peptide was replaced with the one from {gamma}B ({beta}B2{gamma}L). In the second mutant, the N- and C-terminal arms of 15- and 12-residues length were deleted ({beta}B2{Delta}NC). The {beta}B2{gamma}L mutant is monomeric, whereas the {beta}B2{Delta}NC mutant forms dimers and tetramers that cannot be interconverted without denaturation. The spectral properties of the {beta}B2 mutants, as well as their stabilities against denaturants, resemble those of wild-type {beta}B2-crystallin, thus indicating that the overall peptide fold of the subunits is not changed significantly. We conclude that the peptide linker in {beta}B2-crystallin is necessary for dimerization, whereas the N- and C-terminal arms appear to be involved in preventing the formation of higher homo-oligomers.
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