A fundamental biologic principle is that diverse biologic signals are channeled through shared signaling cascades to regulate development. Large scaffold proteins that bind multiple proteins are capable of coordinating shared signaling pathways to provide specificity to activation of key developmental genes. Although much is known about transcription factors and target genes that regulate cardiomyocyte differentiation, less is known about scaffold proteins that couple signals at the cell surface to differentiation factors in developing heart cells. Here we show that AKAP13 (also known as Brx-1, AKAP-Lbc, and proto-Lbc), a unique protein kinase A-anchoring protein (AKAP) guanine nucleotide exchange region belonging to the Dbl family of oncogenes, is essential for cardiac development. Cardiomyocytes of Akap13-null mice had deficient sarcomere formation, and developing hearts were thin-walled and mice died at embryonic day 10.5-11.0. Disruption of Akap13 was accompanied by reduced expression of Mef2C. Consistent with a role of AKAP13 upstream of MEF2C, Akap13 siRNA led to a reduction in Mef2C mRNA, and overexpression of AKAP13 augmented MEF2C-dependent reporter activity. The results suggest that AKAP13 coordinates G␣ 12 and Rho signaling to an essential transcription program in developing cardiomyocytes.We previously reported cloning and characterization of a human 5.3-kb brx-1 (breast cancer nuclear hormone receptor auxiliary factor 1) transcript that encoded a 170-kDa Dbl family member (1) and later localized the gene to chromosome 15q24-25 (2). Larger transcripts of the gene were subsequently isolated (3), and based on its AKAP region, the gene we initially called BRX is now known as AKAP13.The central Dbl homology (DH) 3 or guanine nucleotide exchange region (GEF) is present in all native transcripts of AKAP13 (Fig. 1). The lbc oncogene (4), derived from two chromosomes, 15 and 7 (5), contains the GEF region of AKAP13 but lacks the N terminus and carboxyl regions. The GEF domain of AKAP13 was shown to bind RhoA and activate Rho family GTPases (3, 5, 6). Rho GTPases have been reported to influence the cell cytoskeleton and sarcomere development in cardiomyocytes (7-9). RhoA, a target of AKAP13, influences at least 11 downstream effectors (10), including two factors essential for cardiomyocyte differentiation, serum response factor (SRF) and 11,12). AKAP13 was also shown to play a role in cardiac hypertrophy (13) via MEF2 (myocyte enhancer factor-2). Despite the established role for RhoA in cardiac development, the importance of AKAP13 in the developing heart has not been described.The function of the carboxyl region of proteins encoded by AKAP13 has remained enigmatic. The carboxyl region of the brx-1 transcript encoded an LXXLL nuclear receptor-interacting domain, and BRX-1 protein was found to specifically interact with nuclear hormone receptors (1, 14, 15). Homodimerization of the carboxyl region has been reported to regulate Rho-GEF activity of the protein (16). These observations suggest that the carboxyl r...