Main intrinsic polypeptide and water movement--recent developments and future prospects

Age-related lens opacity is the major cause of loss of vision affecting more than half of the world's blind. The development of cataract is associated with changes in the structure of the lens. The lens consists largely of closely packed fibre cells forming a transparent syncytium. The main intrinsic polypeptide (MIP) of the lens fibre cells forms 40% of the fibre membrane, and forms a system of membrane channels. The precise structure and function of these channels remains obscure, although their role as large water-filled channels has been suggested in maintenance of structural integrity as well as their role in transport of nutrients. MIP is a member of a large family of channels of common ancestry. Some of the most recent studies of structure and function of this family have been carried out on CHIP (a water channel found in red blood cells) which is a member of this family closely related to MIP. Both CHIP and MIP m-RNA's have been expressed in oocyte membranes, and water permeabilities per channel have been measured. Surprisingly, CHIP and MIP channels have high and low water permeabilities, respectively. This is discussed in terms of recent high resolution structure determinations, the lens fibre MIP function, and future studies.