Guanine nucleotide exchange factors of the Dbl family relay signals from membrane receptors to Rho family GTPases. We now demonstrate that a longer transcript of the Lbc gene encodes a chimeric molecule, which we have called AKAP-Lbc, that functions as an A-kinase-anchoring protein (AKAP) and a Rho-selective guanine nucleotide exchange factor. Expression of AKAP-Lbc in fibroblasts favors the formation of stress fibers in a Rho-dependent manner. Application of lysophosphatidic acid or selective expression of Galpha(12) enhances cellular AKAP-Lbc activation. Furthermore, biochemical studies indicate that AKAP-Lbc functions as an adaptor protein to selectively couple Galpha(12) to Rho. Thus, AKAP-Lbc anchors PKA and nucleates the assembly of a Rho-mediated signaling pathway.