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The amino acid sequences of two polypeptide components of thymosin Fraction 5 termed thymosin alpha1 and polypeptide beta1 have been established. The sequences were determined by automatic Edman degradation of the intact molecules as well as by manual sequence analysis of the enzymatic cleavage products. Thymosin alpha1, an immunologically active polypeptide, is highly acidic with an isoelectric point of 4.2. This molecule is composed of 28 amino acid residues with acetylserine as the NH2 terminus. A chemically synthesized molecule of thymosin alpha1 has been found to be as active as the natural molecule in our bioassay systems. Polypeptide beta1 is a molecule consisting of 74 amino acid residues and has an isoelectric point of 6.7. This peptide is not biologically active in our assay systems, suggesting that it is not involved in thymic hormone action. The sequence of beta1 was found to be identical with ubiquitin and a portion of protein A24, a nuclear chromosomal protein. The relationships among these proteins are discussed.
Low et al. (Thu,) studied this question.
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