S phase mouse embryo fibroblasts secrete varying amounts of a 45,000 dalton protein.

Synchronized cultures of mouse embryo fibroblasts upon release from hydroxyurea (HOU) arrest, secreted several proteins of which a polypeptide of molecular weight 45,000 (45K) was barely visible in the conditioned medium of cells that synthesized DNA at peak levels. The quantity of the 45K protein was higher in the medium of HOU arrested cells and the level got progressively reduced as the cells entered into the DNA synthetic phase. Conditioned media containing the 45K protein inhibit DNA synthesis when added to synchronized cultures. These results suggest that the 45K secreted protein may be involved in the autocrine regulation of turning-off of DNA synthesis at the end of S phase.