Coordinate regulation of Activin and Bone Morphogenetic Protein signaling by a lysosomal trafficking switch

BMP/TGF{beta} family ligands act mainly as factors that differentially initiate Smad1/5/8 or Smad2/3 signaling via heterotetrameric complexes comprised of two type I and two type II receptors (IIR). ActA (ActA) stands out as it activates Smad2/3 signaling through type I receptor ACVR1B, whereas it generates non-signaling complexes (NSCs) with ACVR1. In fibrodysplasia ossificans progressiva (FOP), a genetic disorder caused by missense mutations in ACVR1 (ACVR1.FOP), ACVR1.FOP*ActA*IIR complexes activate Smad1/5 signaling, mimicking those formed with BMPs. As the NSCs that ActA forms with ACVR1 are stoichiometrically identical with signaling complexes formed with ACVR1.FOP, we explored how NSCs differ from their signaling counterparts. We show that ACVR1*ActA*IIR complexes rapidly traffic to lysosomes, where their constituent components are degraded, reducing the cells responsiveness to BMPs along with ActAs availability. This property is specific to ActA as Activin B, AB, and AC do not form lysosomal trafficking complexes with ACVR1, but rather remain on the surface as NSCs. Lysosomal trafficking and degradation of ACVR1*ActA*IIR complexes is a novel regulatory mechanism of BMP/TGF{beta} signaling whose physiological roles remain largely unexplored.