Major intrinsic polypeptide (MIP26K) from human lens membrane: Characterization of low-molecular-weight forms in the aging human lens

Antisera made against a C-terminal octapeptide of bovine MIP26K and against an N-terminal nonapeptide from the same protein have been used to determine the location of age-dependent cleavage sites of MIP26K in the human lens. Neither the C-terminal antiserum (anti-MIP26K_256-263) nor the N-terminal antiserum (anti-MIP26K_1-9) binds to the 22000 MW form of MIP26K, suggesting cleavage from both the N- and C-terminus during lens aging. Anti-MIP26K_256-263, but not anti-MIP26K_1-9, binds to 20000 and 15000 MW forms of MIP26K, demonstrating that age-dependent production of these forms occurs by cleavage from the N-terminal side of the molecule. Together, these results show that age-dependent processing of MIP26K in the human lens occurs from both ends of the molecule, with cleavage from the N-terminal end being mainly responsible for production of the lower-molecular-weight 20000 and 15000 MW components.