ACTIN-SEQUESTERING ABILITY OF THYMOSIN BETA-4, THYMOSIN BETA-4 FRAGMENTS, AND THYMOSIN BETA-4-LIKE PEPTIDES AS ASSESSED BY THE DNASE-I INHIBITION ASSAY

Thymosin beta4 containing 43 amino-acid residues belongs to a family of highly homologous peptides present at high concentrations in various species, cells, and tissues. Safer et al. [J. Biol. Chem. 266, 4029-4032 (1991)] have shown that thymosin beta4 is an actin-sequestering peptide. Because DNase I is inhibited by G-actin and not by F-actin we employed this enzymatic assay to determine the actin sequestering properties of 4 other thymosin beta4-like peptides and fragments of thymosin beta4 generated by enzymatic digestions. Thymosin beta4 sequesters G-actin at a 1 to 1 ratio an thereby inhibits its polymerisation to F-actin in high salt solution. The oxidation of the single methionine residue at position 6 does not abolish its actin-sequestering properties. However neither thymosin beta4(24-43) nor thymosin beta4(13-43) inhibit the polymerisation of G-actin. We conclude from this that some structural features in the amino-acid sequence of thymosin beta4 before position 13 are obligatory for its biological function. Oxidized thymosin beta4 (beta4-sulfoxide) as well as four other thymosin beta4-like peptides were shown to be actin-sequestering peptides like thymosin beta4.