A MULTIFUNCTIONAL DNA-BINDING PROTEIN THAT PROMOTES THE FORMATION OF SERUM RESPONSE FACTOR HOMEODOMAIN COMPLEXES - IDENTITY TO TFII-I

The human homeodomain protein Phox1 interacts functionally with serum response factor (SRF) to impart serum responsive transcriptional activ ity to SRF-binding sites in a HeLa cell cotransfection assay. However, stable ternary complexes composed of SRF, Phox1, and DNA, which presu mably mediate the transcriptional effects of Phox1 in vivo, have not b een observed in vitro. Here, we report the identification, purificatio n, and molecular cloning of a human protein that promotes the formatio n of stable higher-order complexes of SRF and Phox1. We show that this protein, termed SPIN, interacts with SRF and Phox1 in vitro and in vi vo. Furthermore, SPIN binds specifically to multiple sequences in the c-fos promoter and interacts cooperatively with Phox1 to promote serum -inducible transcription of a reporter gene driven by the c-fos serum response element (SRE). SPIN is identical to the initiator-binding pro tein TFII-I. Consistent with this hypothesis, SPIN exhibits modest aff inity for a characterized initiator sequence in vitro. We propose that this multifunctional protein coordinates the formation of an active p romoter complex at the c-fos gene, including the linkage of specific s ignal responsive activator complexes to the general transcription mach inery.