Identification of a low‐<i>M</i>_r acidic nuclear protein as prothymosin α

Abstract

We have purified to homogeneity a 15‐kDa perchloric acid (PCA)‐soluble protein from rat thymus nuclei. This highly acidic protein showed a M r , of ca. 30 kDa in acetic acid/urea gels, probably due to oligomer formation. Sequence analysis of internal tryptic and thennolytic peptides revealed that the purified protein is, in fact, prothymosin α, a very hydrophilic polypeptide, which has been previously classified as a thymic or immunomodulating hormone. We found that prothymosin α is a rather abundant nuclear protein in rat thymus; its concentration is comparable to that of a well‐characterized nonhistone protein HMG‐14. The subcellular localization and physicochemical properties of prothymosin α suggest that its function is related to those of other long polyacidic regions containing nuclear proteins.