Abstract
Muscles of invertebrate species contain abundant quantities of soluble, sarcoplasmic, high affinity Ca2+-binding proteins (SCBPs). The SCBPs belong to the calmodulin superfamily and contain four homologous domains (I-IV) which arose by reduplication of a gene for a small ancestral protein. We have determined the amino acid sequence of the SCBP from the sandworm Nereis diversicolor. This protein is the only SCBP which has been crystallized in a form suitable for three-dimensional structure determination by high-resolution x-ray analysis (Babu, Y. S., Cox, J. A., and Cook, W. J. (1987) J. Biol. Chem. 262, 11184-11185). N. diversicolor SCBP is a single polypeptide chain of 174 amino acids, including single residues of glutamine and histidine, 2 tyrosines, and 3 tryptophans. It is devoid of cysteine and has an acetylated amino terminus, a calculated Mr of 19,485, and a net charge of -13 at neutral pH. There was no evidence for heterogeneity in the sequence. Probable Ca2+-binding sites were recognized in domains I, III, and IV. Comparison with other available invertebrate SCBP sequences shows an unusually high degree of variability among these proteins, with only 9 residues common to all species.
Highlights
Musclesof invertebrate species contain abundant these proteins areof considerable interest for structural studquantities of soluble, sarcoplasmic, high affinity Ca2+- ies because of their homology with calmodulin, troponin C, binding proteins (SCBPs)
N . diversicolor SCBP is a single polypeptide chain of 174 amino acids, including single residues of glutamine and histidine, 2 tyrosines, and 3 tryptophans. It is devoid of cysteine and has an acetylated amino terminus, a calculatedM, of 19,485,and a net charge of -13 lins, l974,1976a,1976b) which established that these proteins evolved from a common ancestor
It was suggested that this ancestor would contain two identical halves (1-11 and 111-IV) which arose by gene duplication
Summary
Musclesof invertebrate species contain abundant these proteins areof considerable interest for structural studquantities of soluble, sarcoplasmic, high affinity Ca2+- ies because of their homology with calmodulin, troponin C, binding proteins (SCBPs). Comparison with been repeatedly confirmed and extended toinclude calmoduother available invertebrate SCBP sequences shows an lin and other Ca2+-binding proteiwnshich have evolved from unusually high degree of variability among these pro- a common ancestor to perform diverse functions.
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