Amino Acid Sequence of α Chain of Sarcoplasmic Calcium Binding Protein Obtained from Shrimp Tail Muscle

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Abstract
The isotypes of sarcoplasmic Ca2+ binding protein (SCP) were purified from shrimp tail muscle. SCP exists in a dimeric form. One sample of shrimp contained only αA chain, whereas another contained αB and β chains, and a heterodimer of αBβ which was not analyzed precisely. The amino acid sequences of the two a chains were determined. The two a chains are composed of 190 and 192 amino acid residues, respectively. The sequences of the two α chains differed in only four amino acids out of 192 residues. The sequences indicate that the α chain has three Ca2+-binding sites which are common to EF-hand type Ca2+-binding protein. In the absence of added Ca2+ and Mg2+, the amounts of bound Ca2+ in αA, αB, and β chains were 3.0, 3.3, and 2.4 mol/22,000 g protein, respectively. Thus, it is suggested that all three isotypes of shrimp SCP have three Ca2+-binding sites which have high affinity to Ca2+. The sequence homology of shrimp SCP with other EF-hand type Ca2+-binding proteins is very low. The protein having the greatest homology with this SCP was cod parvalbumin; the sequence homology is 18%.

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