Calmodulin

Calmodulin is a ubiquitous cellular protein which has an important role as a binding site for calcium. It is a 148 amino acid chain with a large amount of sequence homology with another calcium-binding protein, troponin C.

One molecule of calmodulin binds to four calcium ions. This does not occur until the cytosolic calcium concentration has been raised upon a cellular stimulus e.g. hormone-receptor binding. Binding induces a shape change in the molecule to an alpha-helix. The complex is then able to bind to and activate other proteins within the cell. These include:

• protein kinases which phosphorylate further proteins e.g.:
  • protein kinase C; roles include platelet aggregation
  • phosphorylase kinase; roles include stimulation of glycogenolysis
  • glycogen synthase kinase; stimulates glycogenolysis
  • myosin light chain kinase; smooth muscle contraction
• phospholipase A2; involved in production of arachidonic acid metabolites
• tubulin-binding protein; possible involvement in movement of mitotic spindle, sperm motility etc.