A novel proliferation-sensitive and cell cycle-specific basic protein, termed progressin (M r,=33 000), has been identified in proliferating human cells of epithelial, fibroblast and lymphoid origin. Progressin is synthesized almost exclusively during the S-phase of transformed human amnion cells (AMA). Increased synthesis of this protein is first detected late in G1, at or near the G1/S transition border, reaches a maximum in mid to late S-phase, and declines thereafter. Contrary to histones, progressin synthesis is not coupled to DNA replication. As expected for an S-phase-specific protein, no detectable synthesis of progressin was observed in non-proliferating human MRC-5 fibroblasts and epidermal basal keratinocytes. Elevated, but variable levels of this protein were observed in proliferating normal fibroblasts and transformed cells of fibroblast, epithelial and lymphoid origin. Taken together the above observations suggest that progressin may be a component of the common pathway leading to DNA replication and cell division.
