Beta thymosins are a family of proteins which have in common a sequence of about 40 amino acids similar to the small protein thymosin β₄. They are found almost exclusively in multicellular animals. Thymosin β₄ was originally obtained from the thymus in company with several other small proteins which although named collectively "thymosins" are now known to be structurally and genetically unrelated and present in many different animal tissues.

Monomeric β-thymosins, i.e. those of molecular weight similar to the peptides originally isolated from thymus by Goldstein, are found almost exclusively in cells of multicellular animals. Known exceptions are monomeric thymosins found in a few single-celled organisms, significantly those currently regarded as the closest relatives of multicellular animals: choanoflagellates and filastereans. Although found in very early-diverged animals such as sponges, monomeric thymosins are absent from arthropods and nematodes, which do nevertheless possess "β-thymosin repeat proteins" which are constructed from several end-to-end repeats of β-thymosin sequences.Genomics has shown that tetrapods (land vertebrates) each express three monomeric β-thymosins, which are the animal species' equivalents (orthologues) of human β₄, β₁₀ and β₁₅ thymosins, respectively. The human thymosins are encoded by the genes TMSB4X, TMSB10 and TMSB15A and TMSB15B. (In humans, the proteins encoded by the two TMSB15 genes are identical.) Bony fish in general express orthologues of these same three, plus an additional copy of the β₄ orthologue.

The N-terminal half of β-thymosins bears a strong similarity in amino acid sequence to a very widely distributed sequence module, the WH₂ module. (Wasp Homology Domain 2 - the name is derived from Wiskott-Aldrich syndrome protein). Evidence from X-ray crystallography shows that this part of β-thymosins binds to actin in a near-identical manner to that of WH₂ modules, both adopting as they bind, a conformation which has been referred to as the β-thymosin/WH₂ fold. β-thymosins may therefore have evolved by addition of novel C-terminal sequence to an ancestral WH₂ module. However, sequence similarity searches designed to identify present-day WH2 domains fail to recognise β-thymosins, (and vice versa) and the sequence and functional similarities may result from convergent evolution.

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