The HMG protein T160 colocalizes with DNA replication foci and is down-regulated during cell differentiation

The high mobility group protein T160, the murine homolog of the human struc ture-specific recognition protein 1, was first supposed to be involved in t he process of V-(D)-J recombination, since it could bind to recombination s ignal sequence probes. We have recently cloned T160 by using an unrelated D NA probe and shown that it binds to either cruciform or linear DNA with no sequence specificity. In this work, we performed a detailed analysis of T16 0 expression and immunolocalization, We show that T160 is a phosphoprotein broadly conserved from yeast to mammals, with a high level of expression in all the cell lines tested and in tissues containing a high degree of proli ferating cells, Indirect immunofluorescence analysis by confocal laser micr oscopy revealed that T160 distribution in the cell nucleus is not uniform, and focus-like staining was observed. Cell cycle studies by BrdU incorporat ion suggest that the appearance of T160 nuclear foci is specific of mid to late S phase, Furthermore, while T160 expression does not change during the cell cycle, it is dramatically down-regulated when cells begin to differen tiate, as highlighted in C2C12 myoblasts and myotubes. The disappearance of T160 nuclear staining in multinucleated myotubes is shown, Taken together, these data suggest that its function may be less specific than V-(D)-J rec ombination and more related to some cellular basic process, such as DNA rep lication or repair. (C) 1999 Academic Press.