MYPT1 is a non-canonical AKAP that tethers PKA to the MLCP signaling node

The activity of myosin light chain phosphatase (MLCP) is fine-tuned by the phosphorylation status of the MLCP target subunit 1 (MYPT1), which is determined by the antagonistic effects of Rho kinase (ROCK) and cAMP/cGMP-dependent protein kinases (PKA and PKG). PKA is composed of two regulatory (PKA-R, of which four variants exist) and two catalytic (PKAcat) subunits. PKA is targeted to the vicinity of its substrates by binding to A kinase anchoring proteins (AKAPs). MYPT1 is part of a complex signaling node that includes kinases and other enzymes involved in signal transduction. We hypothesized that MYPT1 might function as an AKAP to target PKA to the MLCP signaling node. Using a combination of immunoprecipitation, affinity pulldown and in situ proximity ligation assay (PLA) in human platelets and endothelial cells, we show that MYPT1 directly interacts with all four PKA-R variants and mapped the interaction to a 200 residues long central region of MYPT1. The interaction does not involve the docking and dimerization domain of PKA-R typically required for binding to AKAPs. Using peptide array overlay we identified K595, E676 and the PKA/ROCK kinase substrate motif R693/R694/S695/T696 as critical for the interaction. Substitution of S695, T696 or both by aspartic acid or the corresponding phosphorylated residue abolished binding. Our findings reveal that MYPT1 functions as a non-canonical AKAP to anchor PKA to the vicinity of non-phosphorylated S695/T696, where PKA-R would prevent PKAcat, and potentially also ROCK, from interacting with and phosphorylating MYPT1.