Lens junctions are communicating junctions

Lens fibers are electrically coupled with each other and directly exchange dyes and metabolites. In most cells, this form of communication is mediated by gap junctions. Lens fibers lack typical gap junctions. The lens junctions, although morphologically similar to gap junctions, differ from them structurally, chemically and immunologically. Nevertheless, recent evidence suggests that indeed lens junctions are communicating junctions. The lens junction protein, MIP26, displays structural characteristics similar to other channel proteins. Once incorporated into liposomes it forms channels permeable to molecules as heavy as 1.5 kDa. Like other communicating junctions, lens junctions assume crystalline arrays and uncouple with Ca++. The liposome incorporated channels close with Ca++ and H+ in the presence of calmodulin (CaM). Partial loss of gating competency occurs after proteolytic cleavage of the C-terminal arm of MIP26. The need for a unique type of communicating junction in lens is unclear. A possibility is that this tissue has some special cell-to-cell transport requirements, in terms of size and/or charge of permeants, not shared by coupled cells of other tissues.