The study shows that the whole thymosin‑beta‑4 protein can bind to single actin molecules and stop them from forming fibers, but short pieces of the protein that miss the first 12 amino acids cannot do this. Changing the methionine at position 6 to its oxidized form doesn’t stop the protein from working.

Thymosin beta 4 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 beta 4 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 beta 4-like peptides and fragments of thymosin beta 4 generated by enzymatic digestions. Thymosin beta 4 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 beta 4 24-43 nor thymosin beta 4 13-43 inhibit the polymerisation of G-actin. We conclude from this that some structural features in the amino-acid sequence of thymosin beta 4 before position 13 are obligatory for its biological function. Oxidized thymosin beta 4 (beta 4-sulfoxide) as well as four other thymosin beta 4-like peptides were shown to be actin-sequestering peptides like thymosin beta 4.