On ligand binding, forms a receptor complex consisting of two type II and two type I transmembrane serine/threonine kinases. Type II receptors phosphorylate and activate type I receptors which autophosphorylate, then bind and activate SMAD transcriptional regulators. Receptor for activin A, activin B and inhibin A. Mediates induction of adipogenesis by GDF6. ATP + L-threonyl-[receptor-protein] = ADP + H(+) + O-phospho-L-threonyl-[receptor-protein] ATP + L-seryl-[receptor-protein] = ADP + H(+) + O-phospho-L-seryl-[receptor-protein] Interacts with AIP1. Part of a complex consisting of AIP1, ACVR2A, ACVR1B and SMAD3 (By similarity). Interacts with type I receptor ACVR1 (By similarity). Interacts with BMP7 (By similarity). Belongs to the protein kinase superfamily. TKL Ser/Thr protein kinase family. TGFB receptor subfamily.