Isoform 1 regulates endocytosis of activin type 2 receptor kinases through the Ral/RALBP1-dependent pathway and may be involved in suppression of activin-induced signal transduction. Isoform 2 and isoform 3 show a stimulatory affect on activin-induced signal transduction and enhance activin type 2 expression at the cell surface. Binds (via the PDZ domain) to isoform 2A of SYNJ2 (via the unique motif in the C-terminus) (By similarity). Interacts (via C-terminus) with RALBP1. Interacts (via PDZ domain) with ACVR2A (via C-terminus) and ACVR2B (via C-terminus). Forms a ternary complex with ACVR2A and RALBP1 (PubMed:11882656, PubMed:16648306). Interacts with MAPK12 (By similarity). Interacts with DLL1; enhances DLL1 protein stability, and promotes notch signaling in endothelial cells (By similarity). Isoform 1 and isoform 2 are widely expressed, notably in brain, heart, lung, liver, kidney, skeletal muscle, ovary and testis. Isoform 3 is detected only in heart, spleen and testis. Up-regulated between 12 and 24 hours after treatment with activin A and lipopolysaccharide (LPS). Down-regulated by calcium ionophore A23187.