Calmodulin

Calmodulin

Calmodulin

Calmodulin (CaM) is a multifunctional intermediate calcium-binding messenger protein expressed in all eukaryotic cells. It is an essential component of the calcium signal transduction pathway, where it acts as a calcium sensor and signal transducer. Calmodulin is involved in a wide range of cellular processes, including muscle contraction, intracellular movement, cell division, and the regulation of metabolic pathways.

Structure

Calmodulin is a small, highly conserved protein composed of 148 amino acids. It has a dumbbell-shaped structure with two globular domains connected by a flexible linker. Each domain contains two EF-hand motifs, which are helix-loop-helix structures that bind calcium ions.

EF-hand motif

EF-hand Motifs

The EF-hand motif is a common structural domain found in calcium-binding proteins. In calmodulin, each EF-hand can bind one calcium ion, allowing the protein to bind a total of four calcium ions. The binding of calcium induces a conformational change in calmodulin, enabling it to interact with various target proteins.

Function

Calmodulin functions as a calcium sensor and signal transducer. Upon binding calcium, calmodulin undergoes a conformational change that allows it to interact with and regulate a variety of target proteins, including kinases, phosphatases, ion channels, and other enzymes.

Calmodulin Binding Sites

Calmodulin binding sites

Calmodulin interacts with its target proteins through specific binding sites. These sites are often characterized by a basic amphiphilic helix that fits into the hydrophobic pocket of calmodulin. The interaction is calcium-dependent, meaning that calmodulin must be bound to calcium to effectively bind its targets.

Role in Cellular Processes

Calmodulin plays a critical role in various cellular processes:

• Muscle Contraction: Calmodulin regulates muscle contraction by activating myosin light chain kinase (MLCK), which phosphorylates myosin light chains, facilitating interaction with actin filaments.

Calmodulin bound to MLC Kinase

• Cell Cycle Regulation: Calmodulin is involved in the regulation of the cell cycle by modulating the activity of various kinases and phosphatases.

• Signal Transduction: Calmodulin acts as a key mediator in calcium signal transduction pathways, influencing cellular responses to external stimuli.

Calmodulin in Plants

Calmodulin in Sorghum bicolor

In plants, calmodulin is involved in various physiological processes, including growth, development, and response to environmental stimuli. It plays a role in the regulation of plant-specific processes such as stomatal movement and gravitropism.

Calmodulin Domains

N-terminal Domain

Calmodulin N-terminal

The N-terminal domain of calmodulin contains two EF-hand motifs and is responsible for binding the first two calcium ions. This domain undergoes a conformational change upon calcium binding, which is crucial for its interaction with target proteins.

C-terminal Domain

Calmodulin C-terminal

The C-terminal domain also contains two EF-hand motifs and binds the remaining two calcium ions. The C-terminal domain is important for the stability of the calcium-calmodulin complex and its interaction with target proteins.

Related Pages

• Calcium signaling
• EF-hand
• Signal transduction
• Muscle contraction

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