β-thymosins from marine invertebrates: Primary structure and interaction with actin

Abstract

The β-thymosins are distributed throughout the vertebrate phyla, and all known vertebrate β-thymosins bind actin monomers. To determine whether β-thymosin-like peptides function as actin-binding proteins in invertebrates, we fractionated perchloric acid extracts of the gonads of both the sea urchin, Arbacia punctulata, and the scallop, Argopecten irradians, and screened the fractions for proteins which could be crosslinked to actin. In each case a peptide was isolated which crosslinks to actin from both rabbit skeletal muscle and scallop cross-striated adductor muscle; both peptides were sequenced and each was found to consist of 40 amino acid residues, compared with 41–43 residues for the vertebrate β-thymosins. The sequences of the scallop and sea urchin β-thymosins are 80% identical to each other, 75% identical to residues 1–40 of thymosin β₄, and 72–80% identical to residues 1–40 of other vertebrate β-thymosins. The sea urchin peptide was found to inhibit actin polymerization and nucleotide exchange. The affinity of the sea urchin peptide for rabbit muscle actin is apparently lower than that of thymosin β₄, since about twice the concentration of sea urchin peptide is required to give inhibition of actin polymerization or nucleotide exchange equivalent to thymosin β₄. Cell Motil Cytoskeleton 38:163–171, 1997.