Main intrinsic polypeptide proteolysis and fiber cell membrane domains.

The main intrinsic polypeptide (MIP) of ocular lens fiber cells, a putative gap junctional polypeptide, has been shown to undergo a physiologic proteolytic reduction in relative molecular weight with age. Electron microscopic studies of isolated lens fiber cell membranes have revealed the existence of two distinct classes of gap junction-like membrane-membrane interactions, which differ from each other in profile thickness by about 4 nm ("thick" and "thin" fiber cell junctions). The authors report that both classes of membrane-membrane interaction are seen in situ as well. Using exogenous and endogenous proteases to mimic the molecular "aging" of the MIP, we have explored the hypothesis that one junctional class is derived from the other as the MIP is proteolytically degraded. From the results presented the authors conclude that neither limited or exhaustive proteolytic degradation of the MIP substantially alters the relative percentages of the three identifiable fiber cell membrane domains (thick junctions, thin junctions, and unit membranes); thick fiber cell junctions, because they possess a gap between the membrane outer leaflets, are, by definition, gap junctions; thin junctions are not artifacts of tannic acid fixation; and unit membranes are capable of associating in a manner that mimics thin junctions.