A Phosphate-Acceptor Protein Related to Parvalbumins in Dogfish Skeletal Muscle

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Abstract
A phosphate-acceptor protein was isolated from the skeletal muscle of the Pacific dogfish (Squalus acanthias) displaying properties extremely similar to those of the parvalbumins, i.e., the low-molecular-weight, soluble, Ca-binding muscle proteins found in fish and amphibians. It has the same characteristic UV spectrum, strong affinity for calcium, and immunological crossreactivity with antibodies against homogeneous dogfish parvalbumin. Although it was isolated in three states of aggregation with molecular weights of about 350,000, 75,000, and 25,000, all species dissociate in Na dodecyl sulfate into subunits of 11,000 and 13,000 molecular weight. Furthermore, whereas no phosphorylation of parvalbumins could be demonstrated under any experimental conditions, the aggregated forms could be readily phosphorylated by a cyclic AMP-independent dogfish protein kinase, but not by phosphorylase kinase. One acid-stable and base-labile phosphate group was introduced per subunit which could be rapidly released by a dogfish protein phosphatase, but only very slowly if at all by phosphorylase phosphatase. It is speculated that this "phosphate-acceptor protein" might represent a physiologically active form of the parvalbumins.