Protein processing in lens intercellular junctions: cleavage of MP70 to MP38

Membrane protein MP70 is a component specifically of lens fiber 16-17 nm junctions. SDS-PAGE analysis of membrane preparations made separately from the sheep lens equatorial region (outer cortex), inner cortex and central region (nucleus) showed abundant MP70 in the young fibers in the outer cortex and drastically reduced levels of MP70 in the older fibers deeper in the lens. MP70 was cleaved to MP38 and thereby lost the epitope to monoclonal anti MP70 (6-4-B2-C6, Kistler et al J Cell Biol 101:28, 1985). This cleavage is fiber age-related and is effected by a calcium-dependent, lens-endogenous protease.