Free ubiquitin is a non-histone protein of trout testis chromatin.

Abstract

A NON-HISTONE chromosomal protein extracted from trout testis chromatin by 0.35 M salt, protein S, (ref. 1) has now been purified to homogeneity and its primary structure determined (Fig. 3) and shown to be identical with that of the thymus protein, ubiquitin2–4 (originally called thymopoietin) and sequenced by Schlesinger et al.5. Ubiquitin was previously reported in chromatin by Goldknopf et al.6 as a component of protein A-24 in which it is covalently bound by the α-carboxyl group of its C-terminal glycine to the e-amino group of lysine 119 of histone H2A in an isopeptide linkage7. Goldknopf and Busch8 have reported that protein A-24 is a component of the core nucleosomes of rat liver chromatin being present in quantities sufficient for it to be linked to H2A in 10–20% of the total nucleosomes. The present studies indicate clearly that ubiquitin can also occur as a free polypeptide in trout testis chromatin and is readily extracted by 0.35 M NaCl. Moreover, free trout testis ubiquitin (protein S) is rapidly released together with HMG-T (ref. 9) (and small amounts of Hl) during limited micrococcal nuclease digestion of trout testis chromatin and is probably also located in the linker regions of the chromatin. DNase I digestion does not release free ubiquitin preferentially10. In view of the pronounced biological effects of free ubiquitin on membrane adenyl cyclase of immature thymocytes3,4 and the ability of ubiquitin to promote the differentiation of these cells into mature T cells3,4, its occurrence in chromatin raises the possibility of a role of ubiquitin in cell differentiation and differential gene expression.