Calcium-binding lens membrane proteins

Abstract

Calcium-binding membrane-bound proteins are present in the vertebrate eye lens. Among these proteins are a distinct group of immunologically related extrinsic EDTA-extractable proteins (EEP) and calmodulin. The EEP proteins contain calcium-binding sites with a total capacity of 25 mol Ca2+ per mol protein. This high calcium-binding capacity of EEP points to a function of these proteins as intracellular calcium store in the lens. However, EEP undergoes a conformational change upon calcium binding, indicating that these proteins may be involved in the regulation of calcium-dependent cellular processes in the lens. One of these processes is the action of communicating lens fiber junctions, which contain EEP as a main protein component. In addition to EEP, another calcium-binding protein in lens, calmodulin, probably functions as mediator of calcium in the regulation of the structure and function of lens junctions. Like other vertebrate calmodulins, lens calmodulin shows a calcium-dependent mobility shift on SDS-polyacrylamide gels and forms immune complexes with antiserum raised against vertebrate calmodulin. Lens calmodulin binds to the junction proteins MIP (main intrinsic protein, MW 26 Kdalton) and a 17.5 Kdalton polypeptide of lens fiber cells in a calcium-independent manner. Via calmodulin the junctions become calcium-sensitive.

Molecular Basis of pH and Ca2+ Regulation of Aquaporin Water Permeability

“…Since it was known that the COOH-terminal tail of bAQP0 contains a consensus CaM binding site with alternating hydrophobic and charged residues, and that a peptide with this sequence was able to bind CaM in vitro in a Ca 2+ -independent manner ( Peracchia and Girsch, 1989 ), we tested the hypothesis that CaM mediates the calcium-sensitive modulation of water permeability. Though the sequence is somewhat atypical of consensus calmodulin binding sites, there are multiple reports in the literature of calmodulin binding to AQP0 ( van den Eijnden-van Raaij et al, 1985 , 1986 ; Peracchia and Girsch, 1989 ; Louis et al, 1990 ; Girsch and Peracchia, 1991 ; Swamy-Mruthinti, 2001 ), and our present results certainly strengthen the position that calmodulin binds AQP0 and MIPfun in a calcium-independent manner. Expression of a mutant CaM with “crippled” hands completely eliminated calcium sensitivity, but had no effect on normal water permeability or pH modulation.…”

“…In both bAQP0 and MIPfun, the indicated sequence within the COOH terminus tail is atypical for a CaM-binding site, but fits the general requirement of an amphiphilic chain with alternation of positive charges ( Peracchia and Girsch, 1989 ). Moreover, several reports demonstrate that calmodulin binds AQP0 ( van den Eijnden-van Raaij et al, 1985 , 1986 ; Peracchia and Girsch, 1989 ; Louis et al, 1990 ; Girsch and Peracchia, 1991 ; Swamy-Mruthinti, 2001 ).…”

Influence of Age, Diabetes, and Cataract on Calcium, Lipid-Calcium, and Protein-Calcium Relationships in Human Lenses

“…15 Calcium-binding proteins such as calmodulin have K d values in the lower 3-M range. [52][53][54] Although these proteins can be expected to bind to calcium in the lens, they are present at an insufficient concentrations to account for the 100-fold difference between free and bound calcium.…”