We have identified a mouse PDZ protein that interacts with the activin type IIA receptor (ActRIIA), which we named activin receptor-interacting protein 1 (ARIP1). By using yeast two-hybrid screening, we isolated a cDNA clone of ARIP1 from a mouse brain cDNA library. We detected two forms of ARIP1, ARIP1-long and ARIP1-short, which may be produced by alternative splicing. ARIP1-long had one guanylate kinase domain in the NH(2)-terminal region, followed by two WW domains and five PDZ domains (PDZ1-5). ARIP1-short had a deletion in the NH(2)-terminal region and lacked the guanylate kinase domain. Both forms interacted with ActRIIA through PDZ5. The COOH-terminal residues of ActRIIA (ESSL) agree with a PDZ-binding consensus motif, and ARIP1 recognized the consensus sequence. ARIP1 interacts specifically with ActRIIA among the receptors for the transforming growth factor beta family. Interestingly, ARIP1 also interacted with Smad3, which is an activin/transforming growth factor beta intracellular signaling molecule. The mRNA of ARIP1 was more abundant in the brain than in other tissues. Overexpression of ARIP1 controls activin-induced and Smad3-induced transcription in activin-responsive cell lines. These findings suggest that ARIP1 has a significant role in assembling activin signaling molecules at specific subcellular sites and in regulating signal transduction in neuronal cells.