Glycation decreases calmodulin binding to lens transmembrane protein, MIP.

Channels of the major intrinsic protein (MIP) of the lens transport water, thus playing an important role in lens fiber cell homeostasis. Calmodulin (CAM) interacts with MIP and possibly regulates MIP channel permeability. Protein glycation has been implicated in lens opacification. We previously identified sites of glycation of MIP, which are in close proximity to the putative CAM binding site. This study is aimed to show the effect of in vitro and in vivo glycation on CAM binding to MIP. Our results show that MIP and MP20 are the major CAM binding proteins of the lens membrane. In vitro incubation of lens membranes with 1 M glucose decreased CAM binding by 38% (P<0.001). Similarly, there was a progressive decrease in CAM binding to diabetic lens membranes compared to age-matched controls (up to 30% decrease, P<0.01). Mutation of K228 and K238 as well as a triple K mutation (K228N, K238N, K259N) of MIP resulted in a decrease in CAM binding. Thus, post-translational protein modifications of MIP influence CAM binding. Since CAM is the ubiquitous Ca(2+) receptor, decreases in CAM binding to the target protein will affect the Ca(2+)-mediated cellular processes leading to lens opacification in diabetic and aging lenses.