Heterogeneous nuclear ribonucleoproteins as regulators of gene expression through interactions with the human thymidine kinase promoter

In search for nuclear proteins that interact with the human thymidine kinas e (htk) promoter, we discovered that p37AUF, a hnRNP C-like protein, and hn RNP Al, both members of the heterogeneous ribonucleoprotein family, can bin d with high affinity to an Am sequence motif contained within the cell cycl e regulatory unit (CCRU). We report here that over-expression of p37AUF sti mulates gene expression mediated by the htk promoter in a promoter-sequence specific manner, whereas hnRNP Al suppresses it. Both recombinant p37AUF a nd hnRNP Al can bind the htk CCRU, suggesting that their binding to the DNA target does not require additional cellular components. We further discove red that hnRNP K is a potent suppressor of htk mediated gene activity. Howe ver, its mechanism of action is mediated through protein-protein interactio n, since hnRNP K itself cannot bind the htk CCRU but can competitively inhi bit the binding of other hnRNPs. The binding site for the hnRNPs on the htk CCRU is not required for S-phase induction of the htk promoter. However, i n stable but not transient transfectants, the mutation of the hnRNP binding site results in 5- to 10-fold reduction of htk mediated gene activity in s ynchronized and exponentially growing cells. Collectively, these findings s upport emerging evidence that hnRNPs, in addition to their traditional role in RNA biogenesis, could be regulators of gene expression through direct D NA binding or interaction with other proteins. (C) 2000 Wiley-Liss. Inc.