Association Properties and Unfolding of a [beta][gamma]-Crystallin Domain of a Vibrio-Specific Protein

The [beta][gamma]-crystallin superfamily possesses a large number of versatile members, of which only a few members other than lens [beta][gamma]-crystallins have been studied. Understanding the non-crystallin functions as well as origin of crystallin-like properties of such proteins is possible by exploring novel members from diverse sources. We describe a novel [beta][gamma]-crystallin domain with S-type (Spherulin 3a type) Greek key motifs in protein vibrillin from a pathogenic bacterium Vibrio cholerae. This domain is a part of a large Vibrio-specific protein prevalent in Vibrio species (found in at least fourteen different strains sequenced so far). The domain contains two canonical N/D-N/D-X-X-S/T-S Ca.sup.2+ -binding motifs, and bind Ca.sup.2+ . Unlike spherulin 3a and other microbial homologues studied so far, [beta][gamma]-crystallin domain of vibrillin self-associates forming oligomers of various sizes including dimers. The fractionated dimers readily form octamers in concentration-dependent manner, suggesting an association between these two major forms. The domain associates/dissociates forming dimers at the cost of monomeric populations in the presence of Ca.sup.2+ . No such effect of Ca.sup.2+ has been observed in oligomeric species. The equilibrium unfolding of both forms follows a similar pattern, with the formation of an unfolding intermediate at sub-molar concentrations of denaturant. These properties exhibited by this [beta][gamma]-crystallin domain are not shown by any other domain studied so far, demonstrating the diversity in domain properties.