Calmodulin-binding proteins in a cloned rat insulinoma cell line

Calmodulin concentrations were measured in isolated hamster islets or in a cloned rat insulin-secreting cell line RIN-m-5F treated with high glucose. There was no change in the cellular calmodulin content of either islets or RIN-m-5F cells despite increases of insulin concentrations in the media. Treatment of cells with the anti-calmodulin drug W13 inhibited insulin-stimulated glucose release, whereas a small effect on insulin accumulation in the media was observed with W12, the dechlorinated, less active analogue of W13. At the lowest dose tested (30 microM) the effect of W13 on insulin accumulation in the media was completely reversible. To further investigate the possible role of calmodulin in insulin secretion, calmodulin-binding proteins in subcellular fractions of the RIN-m-5F cells were identified using a gel overlay technique. Ca2+-dependent binding of 125I-calmodulin was observed to cytosolic proteins with apparent Mr = 125, 110, 56, 52, and 34 k (kilodalton). This binding was completely displaceable with unlabeled calmodulin, whereas only partial displacement was observed when the homologous Ca2+ binding protein of skeletal muscle, troponin C, was used. Four proteins with Mr similar to the histones bind calmodulin in a Ca2+-independent manner. 125I-calmodulin:calmodulin binding protein interactions were inhibited in a dose-dependent manner by the anti-calmodulin drug, W13. Little effect was observed with the analogue W12.(ABSTRACT TRUNCATED AT 250 WORDS)