Seems to be a coreceptor in inhibin signaling, but seems not to be a high-affinity inhibin receptor. Antagonizes activin A signaling in the presence or absence of inhibin B. Necessary to mediate a specific antagonistic effect of inhibin B on activin-stimulated transcription (By similarity). Interacts with INHA; the interaction is not confirmed by standard receptor binding assays. Interacts with ACVR1B; the interaction appears to be ligand-dependent as it is diminished by inhibin B and activin A. Interacts with ACVR2A, ACVR2B, ACVRL1 and BMPR1B (By similarity). Interacts with HECTD1 (By similarity). Expressed in pituitary gland, testis and liver. Isoform 2 is expressed pituitary gland and testis. It is uncertain whether Met-1 or Met-2 is the initiator.