The two sources for cytosolic in smooth muscle cells are the extracellular entering through calcium channels and the ions that are released from the sarcoplasmic reticulum. The elevation of cytosolic results in more binding to calmodulin, which then binds and activates myosin light-chain kinase. The calcium-calmodulin-myosin light-chain kinase complex phosphorylates myosin on the 20 kilodalton (kDa) myosin light chains on amino acid residue-serine 19, initiating contraction and activating the myosin ATPase. Unlike skeletal muscle cells, smooth muscle cells lack troponin, even though they contain the thin filament protein tropomyosin and other notable proteins – caldesmon and calponin. Thus, smooth muscle contractions are initiated by the -activated phosphorylation of myosin rather than binding to the troponin complex that regulates myosin binding sites on actin like in skeletal and cardiac muscles.