Affinity selection of chemically modified proteins: role of lysyl residues in the binding of calmodulin to calcineurin.

In affinity selection, calcineurin selects from a population of randomly modified calmodulins those species with which it prefers to interact. The method shows that acetylation of lysines affects calmodulin so as to interfere with its ability to interact with calcineurin. Monoacetylation of any lysine of calmodulin reduces its affinity for calcineurin by 5-10-fold. Multiple acetylations amplify the loss of affinity; none of the modifications are imcompatible with activity. The lack of selectivity of calcineurin against any particular modified lysine indicates that the loss of affinity reflects changes induced by the removal of the charged groups and suggests an important role for electrostatic interactions in the cooperative structural transitions which calmodulin undergoes upon binding its target proteins or calcium. In the presence of calcineurin, a large and specific decrease in the rate of acetylation of Lys-75 and -148 of calmodulin is observed. The reactivity of the same residues is greatly increased in the presence of calcium alone [Giedroc, D. P., Sinha, S. K., Brew, K., & Puett, D. (1985) J. Biol. Chem. 260, 13406-13413]. Lys-75, located in the central helix, and the C-terminal Lys-148 [Babu, Y. S., Sacks, J. S., Greenhouse, T. J., Bugg, C. E., Means, A. R., & Cook, W. J. (1985) Nature (London) 315, 37-40] may act as sensors of the calmodulin allosteric transitions. Their reactivity changes in opposite directions in response to calcium-induced or calcineurin-induced structural changes. The reactivity of other residues such as Lys-21, decreased in the presence of calcineurin but not calcium, is also affected by a conformational change which is induced specifically by calcineurin.