Identification of Barx2b, a serum response factor-associated homeodomain protein.

CC(A/T)(6)GG or serum response elements represent a common regulatory motif important for regulating the expression of many smooth muscle-specific genes. They are multifunctional elements that bind serum response factor (SRF) and are important for serum induction of genes, expression of muscle-specific genes, and differentiation of vascular smooth muscle cells. In the current study, a yeast two-hybrid screen was used to identify proteins from mouse intestine that interact with SRF. A novel homeodomain-containing transcription factor, called Barx2b, was identified that specifically interacts with SRF and promotes the DNA binding activity of SRF. Northern blotting, RNase protection analysis, and Western blotting revealed that Barx2b mRNA and protein are expressed in several smooth muscle-containing tissues, as well as in skeletal muscle and brain. In vitro binding studies using bacterial fusion proteins revealed that the DNA-binding domain of SRF interacts with a region of Barx2b located amino-terminal of the homeobox domain. The results of these studies support the hypothesis that interaction of SRF with different homeodomain-containing proteins may play a critical role in determining the cell-specific functions of SRF.