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. 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 (By similarity). Interacts with MAPK12 (By similarity). Interacts with DLL1; enhances DLL1 protein stability, and promotes notch signaling in endothelial cells (By similarity).