Apparent cooperativity of Ca2+ binding associated with crystallization of Ca2+-binding protein from sarcoplasmic reticulum

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Abstract

Needle-shaped crystals of the Ca2+-binding protein (CBP) isolated from rabbit skeletal muscle sarcoplasmic reticulum were studied with regard to the influence of Ca2+, K+, and H+ on its solubility and cation binding. The solubility of CBP is sharply decreased with concentration of Ca2+, whereas K+ increased it. Aggregation of the CBP and crystal formation is correlated with the binding of Ca2+. The Ca2+ bound to the crystalline CBP is two to three times higher than that of the soluble form. A strong apparent positive cooperative behavior of Ca2+ binding by CBP was observed concomitant with the shift in equilibrium from the soluble to the crystalline form. From the steepest Hill slope we obtained Hill coefficients of 3.3 for soluble CBP and 14 for the transition between soluble and crystalline forms of CBP. A detailed treatment is presented to validate the applicability of Hill plots for the combined binding and crystallization process. Two-thirds of the Ca2+-binding sites were K+ sensitive and one-third were K+ insensitive. An increase in H+ concentration decreased the Ca2+ binding by crystalline CBP without affecting its solubility, with a pK value of 6.2 determined for this process. These results indicate that the equilibrium between the soluble and crystalline forms of CBP is determined by the amount and nature of the bound cations, Ca2+, K+, and H+. They suggest the possibility that a cycle of aggregation and solubilization of CBP attends the uptake and release of Ca2+ in the sarcoplasmic reticulum, respectively.

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    This work was supported in part by grants from the National Institutes of Health (AM 14632) and the Muscular Dystrophy Association, a Biomedical Research Support Grant from the National Institutes of Health administered by Vanderbilt University (to S.F.), Grants-in-Aid for Special Project Research (60123007) from the Ministry of Education, Science and Culture, Japan (to T.O.), grants from the Nato Foundation and the Uehara Memorial Foundation (to M.T.), and by a fellowship from the American Heart Association, Tennessee Affiliate (to A.M.).

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    Present address: Department of Microbiology, Swiss Federal Institut of Technology, CH-8092 Zurich, Switzerland.

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