Dimerization of βB2‐crystallin: The role of the linker peptide and the N‐ and C‐terminal extensions

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Abstract
βB2‐ and γB‐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 βB2‐ and γB‐domains are very similar, γB‐crystallin is always monomeric, whereas βB2‐crystallin associates to homodimers. It has been suggested that the linker or the protruding N‐ and C‐terminal arms of βB2‐crystallin (not present in γB) are a necessary requirement for this association. In order to investigate the role of these segments for dimerization, we constructed two βB2 mutants. In the first mutant, the linker peptide was replaced with the one from γB (βB2γL). In the second mutant, the N‐ and C‐terminal arms of 15‐ and 12‐residues length were deleted (βB2ΔNC). The βB2γL mutant is monomeric, whereas the βB2ΔNC mutant forms dimers and tetramers that cannot be interconverted without denaturation. The spectral properties of the βh2 mutants, as well as their stabilities against denaturants, resemble those of wild‐type βB2‐crystallin, thus indicating that the overall peptide fold of the subunits is not changed significantly. We conclude that the peptide linker in βB2‐crystallin is necessary for dimerization, whereas the N‐ and C‐terminal arms appear to be involved in preventing the formation of higher homo‐oligomers.