Protein–protein interactions among human lens acidic and basic β-crystallins

Human lens β-crystallin contains four acidic (βA1→βA4) and three basic (βB1→βB3) subunits. They oligomerize in the lens, but it is uncertain which subunits are involved in the oligomerization. We used a two-hybrid system to detect protein–protein interactions systematically. Proteins were also expressed for some physicochemical studies. The results indicate that all acidic–basic pairs (βA–βB) except βA4-βBs pairs show strong hetero-molecular interactions. For acidic or basic pairs, only two pairs (βA1–βA1 and βA3–βA3) show strong self-association. βA2 and βA4 show very weak self-association, which arises from their low solubility. Confocal fluorescence microscopy shows enormous protein aggregates in βA2- or βA4-crystallin transfected cells. However, coexpression with βB2-crystallin decreased both the number and size of aggregates. Circular dichroism indicates subtle differences in conformation among β-crystallins that may have contributed to the differences in interactions.