THE MAMMALIAN CUT HOMEODOMAIN PROTEIN FUNCTIONS AS A CELL-CYCLE-DEPENDENT TRANSCRIPTIONAL REPRESSOR WHICH DOWN-MODULATES P21(WAF1 CIP1/SDI1) IN S-PHASE/

Cut is a homeodomain transcription factor which has the unusual proper ty of containing several DNA-binding domains: three regions called Cut repeats and the Cut homeodomain. Genetic studies in Drosophila melano gaster indicate that cut plays important roles in the determination an d maintenance of cell-type specificity. In the present study, we show that mammalian Cut proteins may yet play another biological role, spec ifically in proliferating cells. We found that the binding of Cut to a consensus binding site varies during the cell cycle. Binding was virt ually undetectable in Go and early G(1), but became very strong as cel ls reached S phase. This was shown to result both from an increase in Cut expression and dephosphorylation of the Cut homeodomain by the Cdc 25A phosphatase, We also show that the increase in Cut activity coinci des with a decrease in p21(WAF1/CIP1/SDI1) mRNAs. In co-transfection e xperiments, Cut proteins repressed p21(WAF1/CIP1/SDI1) gene expression through binding to a sequence that overlaps the TATA box. Moreover, p 21(WAF1/CIP1/SD1) expression was repressed equally well by either Cdc2 5A or Cut. Altogether, these results suggest a model by which Cdc25A a ctivates the Cut repressor which then downregulates transcription of p 21(WAF1/CIP1/SDI1) in S phase. Thus, in addition to their role during cellular differentiation, Cut proteins also serve as cell-cycle-depend ent transcriptional factors in proliferating cells.