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The sarcoplasmic calcium-binding proteins (SCP) from crayfish, lobster and shrimp myogen have been purified to homogeneity. These proteins exist as dimers and dissociate in the presence of sodium dodecyl sulfate or urea in subunits of 22000 molecular weight. During the last step of purification (DEAE-cellulose chromatography), SCP emerges in three peaks in the ratio of 14: 1.5: 1 for crayfish, of 7:2: 1 for lobster and of 3: 2: 1 for shrimp. Gel electrophoresis and isoclcctrofocusing experiments, under native and denaturing conditions, indicate that among the three SCP isotypes there are only two diflerent polypeptide chains, LX and p, which appear in the form of three dimers: ~2 , L X~ and /j2. The x and fi subunits differ slightly in polypeptide chain composition as found by amino acid analyses of thc crayfish and lobster SCPs, and also by comparison of tryptic peptides for crayfish SCPs. The polymorphism observed in crustacean SCPs, which is increased by their ability to form dimers, contrasts with the situation prevailing among other invertebrate SCPs and vertebrate parvalbumins where only monomeric isotypcs are found. Equilibrium binding studies show that all three SCP isotypes from both crayfish and lobster display the same metal-binding properties. They have in their dimeric form six Ca2+-binding sites : two calcium-specific sites, two Ca/Mg sites that interact with positive cooperativity and two Ca/Mg sites that interact with negative cooperativity. Interactions between the two subunits of SCP seem to result in cooperative binding of Ca", which in turn may control more efficiently Ca2+ fluxes in crustacean muscle.

The sarcoplasmic calcium-binding protein from crayfish muscle has been purified to homogeneity. The protein has a molecular weight of 44000, as determined by sedimentation equilibrium and Sephadex chromatography. It dissociates in the presence of sodium dodecyl sulfate, 8 M urea, or, after succinylation, into two subunits of 22000 molecular weight. The protein is free of carbohydrate and phosphorus but contains 4 g-atoms of calcium/44000 at a free calcium concentration of 0.1 muM. Approximately 45% of the polypeptide backbone appears to be alpha-helical. The amino acid composition reveals a high proportion of alanine and acidic amino acids, a normal content of aromatic amino acids, and the absence of histidine. The isoelectric point, as determined by isoelectric focusing, is 5.1. The protein contains a free threonyl NH2 terminal. Two thiols react rapidly in the native protein, six in the calcium-free form. Immunochemically, there is no difference between the protein from tail, claw, and heart muscle. In these three crayfish tissues, the concentrations of calcium-binding protein, as determined by rocket immunoelectrophoresis, are markedly different: 2.73 g/kg in tail, 0.72 in claw, and 0.073 in heart muscle. A functional analogy with the parvalbumins of vertebrates can be postulated.

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