Human thymidine kinase CCAAT-binding protein is NF-Y, whose A subunit expression is serum-dependent in human IMR-90 diploid fibroblasts.

The nuclear protein interacting with the distal CCAAT box of human thymidine kinase (TK) gene promoter has been suggested to be a specific TK-CCAAT-binding protein, which is responsible for the serum-dependence of TK transactivation in normal human IMR-90 fibroblasts. By biochemical characterization, TK-CCAAT-binding protein was found to be distinct from other known CCAAT-binding proteins (Pang, J. H., and Chen, K. Y. (1993) J. Biol. Chem. 268, 2909-2916). In this study, we identify NF-Y, which is composed of Ya and Yb subunits, to be responsible for the TK-CCAAT binding activity in the crude nuclear extract from HL-60 cells. The interaction of NF-Y with the distal CCAAT box of the TK promoter in the crude extract appeared to be more heat-sensitive than that in the DNA affinity chromatography purified fraction. We have further established that the serum dependence of TK-CCAAT binding activity in normal IMR-90 fibroblasts is due to the decrement of NF-Ya, but not NF-Yb expression following serum-deprivation, and that such serum dependence is absent in HL-60 cells.