BMP receptors, unlike TGF-beta receptors are known to form hetero-oligomeric complexes in the endoplasmic reticulum and are transported as oligomers to the plasma membrane where they bind ligand. However, evidence for ligand-induced heteromeric BMP receptor complexes on the cell surface has
also been published, leading to a model where both pre-formed and ligand-induced receptor oligomers are encountered on the plasma membrane. Based on the latter, a theory has been formulated that suggests that the signaling outcome from pre-formed and ligand-induced BMP receptor complexes is different. The mechanism that might explain this theory must involve different ways of internalization and trafficking of the BMP receptor complexes.