Bone morphogenetic protein type I receptors are single pass, type I transmembrane proteins. They belong to a class of receptor serine/threonine kinases that bind members of the TGF beta superfamily of ligands-the Bone morphogenetic proteins. Bone morphogenetic protein type I receptors are single pass, type I transmembrane proteins. They belong to a class of receptor serine/threonine kinases that bind members of the TGF beta superfamily of ligands-the Bone morphogenetic proteins. There are three type I BMP receptors: ACVR1, BMPR1A and BMPR1B.