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A limited number of transcription factors have been suggested to be regulated directly by Erks within the Ras/mitogen-activated protein kinase signaling pathway. In this paper we demonstrate that ERF, a ubiquitously expressed transcriptional repressor that belongs to the Ets family, is physically associated with and phosphorylated in vitro and in vivo by Erks. This phosphorylation determines the ERF subcellular localization. Upon mitogenic stimulation, ERF is immediately phosphorylated and exported to the cytoplasm. The export is blocked by specific Erk inhibitors and is abolished when residues undergoing phosphorylation are mutated to alanine. Upon growth factor deprivation, ERF is rapidly dephosphorylated and transported back into the nucleus. Phosphorylation-defective ERF mutations suppress Ras-induced tumorigenicity and arrest the cells at the G0/G1 phase of the cell cycle. Our findings strongly suggest that ERF may be important in the control of cellular proliferation during the G0/G1 transition and that it may be one of the effectors in the mammalian Ras signaling pathway.

We developed a simple and accurate method to define the sequence recognition properties of DNA-binding proteins. The method employs polymerase chain reaction (PCR) amplification of sequences selected from a mixture of random oligonucleotides by the gel mobility-shift assay. We used this method to define the sequence requirement of the binding domain of the yeast transcriptional activator GCN4. Using a total of 200 ng of purified protein and four cycles of binding and subsequent amplification, we identified the TGA-(C/G)TCA sequence as the binding consensus of GCN4, which is consistent with the previously reported recognition sequence. In addition, our data indicate that GCN4 can bind with lower affinity to sequences that differ from the optimal sequence in one or even two positions. The most common variation was the C to A at position +2. The majority of the substitutions that still allowed binding were 3' to the central C residue indicating that the two sides of the palindromic recognition sequence are not equivalent.