LENS MAJOR INTRINSIC PROTEIN (MIP) PROMOTES ADHESION WHEN RECONSTITUTED INTO LARGE UNILAMELLAR LIPOSOMES

The vertebrate lens behaves like a syncytium, and it is formed mainly by cells called lens fibers. Between the fibers are extensive networks of membrane junctions. The major intrinsic protein (MIP) constitutes about 50-60% of the intrinsic membrane proteins found in lens fiber ju nctions. The role of MIP is unknown. Nevertheless, it has been propose d that it is the protein responsible for the adhesion between the plas matic membranes of the lens fibers. The aim of our studies was to test the adhesion-promoting role of MIP. We reconstituted MIP into large u nilamellar vesicles (LUV) of phosphatidylcholine (PC) and studied the vesicle aggregation between MIP-reconstituted LUV (PC-MIP) and phospha tidylserine (PS) vesicles. The aggregation process was monitored using methods based on resonance energy transfer (RET) and turbidity measur ements. Neither RET nor an increase in turbidity occurred in any combi nation except in the presence of both MIP and PS. The liposomes thus a ggregate through protein-lipid interactions. These results show that M IP promotes adhesion with negatively charged membranes, indicating tha t the adhesion is electrostatic in nature. Aggregation was fastest at pH 6.0. The aggregation effect was abolished with pronase treatment. P reincubation of PC-MIP vesicles with anti-MIP polyclonal serum also in hibited the aggregation. These studies are the first experimental evid ence supporting the hypothesis of an adhesive role for MIP.