Domain interactions and connecting peptides in lens crystallins

beta B2- and gamma B-crystallin from bovine eye-lens are closely related proteins, topologically distinct mainly by virtue of the linker peptide connecting the two domains in each polypeptide chain. In homodimeric beta B2-crystallin, the extended conformation of the connecting peptide has been suggested to force the beta B2-molecule to favor intermolecular domain interactions compared with intramolecular contacts in monomeric gamma B-crystallin. From this one may postulate that the conserved interdomain contacts are essential for the overall stability of crystallins. This was clearly confirmed for gamma B-crystallin, since its isolated C-terminal domain is significantly less stable than in the context of native gamma B. Exchanging the linker peptide of gamma B- for that of beta B2-crystallin yields a monomeric protein with stability characteristics identical to gamma B-crystallin. We conclude that the domain-interface itself rather than the connecting peptide determines the mode of domain association in crystallins, as the linker in the gamma B beta-mutant is evidently twisted to a turn similar to the one in natural gamma B-crystallin.