Biochemical evidence for adhesion-promoting role of major intrinsic protein isolated from both normal and cataractous human lenses.

In this study, we tested the adhesion-promoting role of major intrinsic protein from both normal human (cadaver) and senile cataractous lenses. Junctional membrane solubilized proteins and pure major intrinsic protein obtained from both type of lenses were reconstituted in neutral phosphatidylcholine liposomes. The interaction of these liposomes with phosphatidylserine vesicles was studied by resonance energy transfer. Our results show that normal human lens junction solubilized proteins and pure major intrinsic protein isolated from them promote adhesion. No quenching effect was observed when major intrinsic protein was omitted in the vesicle reconstitution, no other intrinsic protein of normal human junctional membrane provoked the adhesive effect. In contrast, major intrinsic protein isolated from human senile cataractous lens fails to induce adhesion. The proteolytic cleavages that in vitro originate major intrinsic protein 22,000 Da did not blunt its adhesive capability, suggesting that the proteolytic modifications that major intrinsic protein undergoes in senile cataract were not related with the incompetence of cataractous lens junctions to induce adhesion. Cataractous lens junctional membranes showed protein aggregates. These membranes were treated with sodium hydroxide and reconstituted into liposomes. The sodium hydroxide treatment removed the protein aggregates and restored the adhesive capability. Furthermore, the supernatant obtained after the sodium hydroxide treatment of cataractous junctional membranes, inhibited the adhesive effect of vesicles reconstituted with bovine solubilized proteins. These experiments prove that the failure to induce adhesion of human senile cataractous lens junction proteins is due to the interaction with protein aggregates, which can be removed by sodium hydroxide.