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The Institute for Enzyme Research, University of Tokushima, 3-18-15 Kuramoto, Tokushima 770-8503, Japan

(Requests for offprints should be addressed to K Tsuchida at Fujita Health University; Email: tsuchida{at}fujita-hu.ac.jp)

(Z H Liu is now at School of Basic Medical Sciences, Jilin University, 2 Xinmin Street, Changchun 130021, China)
(K Tsuchida is now at Division for Therapies Against Intractable Diseases, Institute for Comprehensive Medical Science, Fujita Health University, Toyoake, Aichi 470-1192, Japan)
(T Matsuzaki is now at Department of Biology, Faculty of Sciences, Kyushu University Graduate School, Fukuoka 812-8581, Japan)
(Y L Bao is now at The Institute of Genetics and Cytology, Northeast Normal University, 5268 Renmin St, Changchun 130024, China)

Activin type II receptors (ActRIIs) including ActRIIA and ActRIIB are serine/threonine kinase receptors that form complexes with type I receptors to transmit intracellular signaling of activins, nodal, myostatin and a subset of bone morphogenetic proteins. ActRIIs are unique among serine/threonine kinase receptors in that they associate with proteins having PSD-95, Discs large and ZO-1 (PDZ) domains. In our previous studies, we reported specific interactions of ActRIIs with two independent PDZ proteins named activin receptor-interacting proteins 1 and 2 (ARIP1 and ARIP2). Overexpression of both ARIP1 and ARIP2 reduce activin-induced transcription. Here, we report the isolation of two isoforms of ARIP2 named ARIP2b and 2c. ARIP2, ARIP2b and ARIP2c recognize COOH-terminal residues of ActRIIA that match a PDZ-binding consensus motif. ARIP2 and its isoforms have one PDZ domain in the NH₂-terminal region, and interact with ActRIIA. Although PDZ domains containing GLGF motifs of ARIP2b and 2c are identical to that of ARIP2, their COOH-terminal sequences differ from that of ARIP2. Interestingly, unlike ARIP2, overexpression of ARIP2b or 2c did not affect ActRIIA internalization. ARIP2b/2c inhibit inhibitory actions of ARIP2 on activin signaling. ARIP2 is widely distributed in mouse tissues. ARIP2b/2c is expressed in more restricted tissues such as heart, brain, kidneys and liver. Our results indicate that although both ARIP2 and ARIP2b/2c interact with activin receptors, they regulate ActRIIA function in a different manner.

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