Abstract

Cut is a homeodomain transcription factor which has the unusual property of containing several DNA-binding domains: three regions called Cut repeats and the Cut homeodomain. Genetic studies in Drosophila melanogaster indicate that cut plays important roles in the determination and maintenance of cell-type specificity. In the present study, we show that mammalian Cut proteins may yet play another biological role, specifically in proliferating cells. We found that the binding of Cut to a consensus binding site varies during the cell cycle. Binding was virtually undetectable in G₀ and early G₁, but became very strong as cells reached S phase. This was shown to result both from an increase in Cut expression and dephosphorylation of the Cut homeodomain by the Cdc25A phosphatase. We also show that the increase in Cut activity coincides with a decrease in p21^{WAF1/CIP1/SDI1} mRNAs. In co-transfection experiments, Cut proteins repressed p21^{WAF1/CIP1/SDI1} gene expression through binding to a sequence that overlaps the TATA box. Moreover, p21^{WAF1/CIP1/SDI1} expression was repressed equally well by either Cdc25A or Cut. Altogether, these results suggest a model by which Cdc25A activates the Cut repressor which then downregulates transcription of p21^{WAF1/CIP1/SDI1} in S phase. Thus, in addition to their role during cellular differentiation, Cut proteins also serve as cell-cycle-dependent transcriptional factors in proliferating cells.