Human prothymosin α: Amino acid sequence and immunologic properties

Prothymosin α has been purified from human thymus and its amino acid sequence determined, except for a 15 amino acid segment including 10 glutamyl residues near the middle of the molecule. Like prothymosin α from rat thymus [A. A. Haritos, R. Blacher, S. Stein, J. Caldarella, and B. L. Horecker (1985) Proc. Natl. Acad. Sci. USA 82, 343–346] , human prothymosin contains the thymosin α 1 sequence at its NH 2-terminus. It contains a total of 109–110 residues compared to 111–112 for rat prothymosin α, with deletions corresponding to positions Gln 39 and Lys 108 of the rat polypeptide. Human prothymosin α also differs from rat prothymosin α at positions corresponding to residues 87, 92, and 102 of the latter, with substitutions of alanine for proline, alanine for valine, and aspartic acid for glutamic acid, respectively. Human prothymosin is significantly less active than rat prothymosin in protecting mice against infection with Candida albicans and in stimulating release in vivo of migration inhibitory factor. Thus, the differences in amino acid sequences, present mainly the COOH-terminal half of the polypeptides, may determine species specificity in boilogical properties.