Interaction of Serum Response Factor (SRF) with the Elk-1 B Box Inhibits RhoA-Actin Signaling to SRF and Potentiates Transcriptional Activation by Elk-1

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Abstract
Serum response factor (SRF) is a transcription factor which regulates many immediate-early genes. Rho GTPases regulate SRF activity through changes in actin dynamics, but some SRF target genes, such as c-fos, are insensitive to this pathway. At the c-fos promoter, SRF recruits members of the ternary complex factor (TCF) family of Ets domain proteins through interactions with the TCF B-box region. Analysis of c-fos promoter mutations demonstrates that the TCF and ATF/AP1 sites adjoining the SRF binding site inhibit activation of the promoter by RhoA-actin signaling. The presence of the TCF binding site is sufficient for inhibition, and experiments with an altered-specificity Elk-1 derivative demonstrate that inhibition can be mediated by the Elk-1 TCF. Using Elk-1 fusion proteins that can bind DNA autonomously, we show that inhibition of RhoA-actin signaling requires physical interaction between the Elk-1 B box and SRF. These results account for the insensitivity of c-fos to RhoA-actin signaling. Interaction of the B box with SRF also potentiates transcriptional activation by the Elk-1 C-terminal activation domain. Combinatorial interactions between SRF and TCF proteins are thus likely to play an important role in determining the relative sensitivity of SRF target genes to Ras- and Rho-controlled signal transduction pathways.