Bone-morphogenetic-protein (BMP)/Smads signaling pathway plays crucial role during heart development and vessel angiogenesis. BMP signaling is induced by the binding of BMP ligands (eg. BMP4) to their receptors, which recruit and phosphorylate receptor-Smads (R-Smads, eg. Smad1, Smad5) that form nuclear-transporting complexes with Smad4 for transcriptional regulation. Smad6 is an inhibitory Smad expresses predominantly in atria-ventricular cushion and outflow tract of the developing mouse heart, and expands to valves and great vessels. At the cell surface level, Smad6 binds to BMP type I receptor to block R-Smads recruitment to the receptor. At cytosolic level, Smad6 block Smad1/Smad4 complex formation. In the nucleus, Smad6 represses transcription. How these three levels of regulation are coordinated to inhibit BMP signaling is not known. We previously showed that BMP ligand induces an acute Smad6 methylation at arginine 74 (R74) at the cell surface level by a methyltransferase PRMT1, and methyl-Smad6 dissociates from receptor to allow receptor-induced Smad1/5 phosphorylation and activation. We further identified a delayed methylation on arginine 81 (R81) of Smad6 in the cytosol by PRMT1. We found that R81 methylation is required for BMP signaling-induced recruitment of Smad6 to phosphor-Smad1; it is also required for Smad6 to disrupt phosphor-Smad1/Smad4 complex formation and the following nuclear transportation, as well as for Smad6 to suppress Smad1 targeting gene transactivation. Previous findings indicate that Smad6 binds to type I receptor and Smad1 through its C-terminal region. We examined how arginine methylation in the N-terminal region, regulates the binding properties of C-terminal Smad6. We found that N-terminal Smad6 stabilizes the interaction between C-terminal Smad6 and Smad1 and enhances Smad6 inhibitory function. Disruption of R81 methylation results in loss of inhibitory function because of an increase in binding between N-term and C-term Smad6 that results in a "closed" conformation. In summary, R81 methylation controls Smad6 activity and R81 methylation of Smad6 defines the duration and intensity of BMP-induced Smad1/5 signaling.
Autosomal Recessive Hypercholesterolemia Protein Interacts with and Regulates the Cell Surface Level of Alzheimer's Amyloid β Precursor Protein