Calmodulin



Calmodulin (CaM) (an abbreviation for CALcium-MODULated proteIN) is a calcium-binding protein expressed in all eukaryotic cells. It can bind to and regulate a number of different protein targets, thereby affecting many different cellular functions.

Function
CaM mediates processes such as inflammation, metabolism, apoptosis, smooth muscle contraction, intracellular movement, short-term and long-term memory, nerve growth and the immune response. CaM is expressed in many cell types and can have different subcellular locations, including the cytoplasm, within organelles, or associated with the plasma or organelle membranes. Many of the proteins that CaM binds are unable to bind calcium themselves, and as such use CaM as a calcium sensor and signal transducer. CaM can also make use of the calcium stores in the endoplasmic reticulum, and the sarcoplasmic reticulum. CaM undergoes a conformational change upon binding to calcium, which enables it to bind to specific proteins for a specific response. CaM can bind up to four calcium ions, and can undergo post-translational modifications, such as phosphorylation, acetylation, methylation and proteolytic cleavage, each of which has potential to modulate its actions. Calmodulin can also bind to edema factor toxin from the anthrax bacteria.

Structure
Calmodulin is a small, acidic protein approximately 148 amino acids long (16706 Dalton) and, as such, is a favorite for testing protein simulation software. It contains four EF-hand "motifs", each of which binds a Ca2+ ion. The protein has two approximately symmetrical domains, separated by a flexible "hinge" region. Calcium participates in an intracellular signalling system by acting as a diffusible second messenger to the initial stimuli.

Mechanism
Calcium is bound via the use of the EF hand motif, which supplies an electronegative environment for ion coordination. After calcium binding, hydrophobic methyl groups from methionine residues become exposed on the protein via conformational change. This presents hydrophobic surfaces, which can in turn bind to Basic Amphiphilic Helices (BAA helices) on the target protein. These helices contain complementary hydrophobic regions. The flexibility of Calmodulin's hinged region allows the molecule to "wrap around" its target. This property allows it to tightly bind to a wide range of different target proteins.

Dynamic features
Compared to the X-ray crystal structure, the C-terminal domain solution structure is similar while the EF hands of the N-terminal domain are considerably less open. The backbone flexibility within calmodulin is key to its ability to bind a wide range of targets.

Family members

 * Calmodulin 1
 * Calmodulin 2
 * Calmodulin 3
 * Calmodulin-like 1
 * Calmodulin-like 3
 * Calmodulin-like 4
 * Calmodulin-like 5
 * Calmodulin-like 6

Other calcium-binding proteins
Calmodulin belongs to one of the two main groups of calcium-binding proteins, called EF hand proteins. The other group, called annexins, bind calcium and phospholipid (e.g., lipocortin). Many other proteins bind calcium, although binding calcium may not be considered their principal function in the cell.