Kinase-related protein (telokin) binds to dephosphorylated smooth myosin near the junction between the rod and the catalytic head region (S-I). This interaction is prevented by MLCK-catalysed phosphorylation of myosin and conversely, the rate of myosin phosphorylation is in turn inhibited by KRP in vitro. As a consequence of this, in vivo KRP might slow down the rate of myosin phosphorylation by myosin light chain kinase (MLCK) and, therefore, tension development. When the intracellular Ca2+ level is decreasing, the KRP can also accelerate muscle relaxation by lowering the ratio of phosphorylated to phosphorylated myosin. KRP is also an important structural regulator of myosin filaments. Smooth muscle myosin, under physiological conditions in vitro, can adapt two relatively and different stable conformations. When the myosin is in the extended conformation, it is active and able to combine with other myosin molecules to form thick filaments which are fundamental for effective contraction. Upon ATP binding, the rod part of unphosphorylated myosin molecule folds into thirds, so that the head –rod junction is brought close to the middle of the rod and stabilized there, presumable by interacting with both the 20 KDa light chains and the neck region. This interaction is prevented by the MLCK-dependent light chain phosphorylation, resulting in the unfolding of myosin monomers and their rapid polymerization into filaments.