We recently showed that the cardiogenic homeodomain factor Nkx-2.5 served as a positive acting accessory factor for serum response factor (SRF) and that together they provided strong transcriptional activation of the cardiac alpha-actin promoter, depending upon intact serum response elements (SREs) (C. Y. Chen, J. Croissant, M. Majesky, S. Topouz, T. McQuinn, M. J. Frankovsky, and R. J. Schwartz, Dev. Genet. 19:119-130, 1996). As shown here, Nkx-2.5 and SRF collaborated to activate the endogenous murine cardiac alpha-actin gene in 10T1/2 fibroblasts by a mechanism in which SRF recruited Nkx-2.5 to the alpha-actin promoter. Activation of a truncated promoter consisting of the proximal alpha-actin SRE1 occurred even when Nkx-2.5 DNA-binding activity was blocked by a point mutation in the third helix of its homeodomain. Investigation of protein-protein interactions showed that Nkx-2.5 was bound to SRF in the absence of DNA in soluble protein complexes retrieved from cardiac myocyte nuclei but could also be detected in coassociated binding complexes on the proximal SRE1. Recruitment of Nkx-2.5 to an SRE depended upon SRF DNA-binding activity and was blocked by the dominant negative SRFpm1 mutant, which allowed for dimerization of SRF monomers but prevented DNA binding. Interactive regions shared by Nkx-2.5 and SRF were mapped to N-terminal/helix I and helix II/helix III regions of the Nkx-2.5 homeodomain and to the N-terminal extension of the MADS box. Our study suggests that physical association between Nkx-2.5 and SRF is one way that cardiac specified genes are activated in cardiac cell lineages.