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, ubiquitin^2–4 (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 ε-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 thymocytes^3,4 and the ability of ubiquitin to promote the differentiation of these cells into mature T cells^3,4, its occurrence in chromatin raises the possibility of a role of ubiquitin in cell differentiation and differential gene expression.