Dilemmas of the structural and biochemical organization of lens membranes during differentiation and aging.

This review deals with the biogenesis of MP26 and with the problem of the structural pattern which may be formed by MP26 during differentiation and aging of the lens fibers and as a consequence of the extraction or degradation of other membrane components. The data reported here imply that the MP26 biosynthesis is one of the key steps of cell surface domain formation during terminal differentiation of lens fibers. One striking observation involves the bidimensional long and short range distribution of MP26 copies within the lipid matrix. The protein oligomers may form, in the plane of the membrane, either a geometrical lattice or randomly distributed particle arrays. However, the mechanism controlling the assembly of either one of these patterns and eventually the transition of one into the other, is still unknown. We speculate that the formation of various membrane domains can be depicted as a self-assembly of repeating identical or quasi equivalently related protein subunits, but this process appears to be dependent both on the lipid environment and on interaction of the transmembrane protein oligomers with other membrane or cytoskeletal components, in particular crystalline polypeptides and cytoskeleton constituents. Finally, during the aging process, the post translational modification of MP26 and eventually the variation of the spatial arrangement and composition of the lipids may implicate the predominant lattice arrangement of the lipid and protein phases.