Role of Ca2+ – calmodulin complex in cell signaling

Role of Ca2+ – calmodulin complex in cell signaling

  • Calmodulin is a dumbbell shaped ‘calcium modulating’ protein that mediates most of the activities of Ca2+ ions.
  • The word ‘calmodulin‘ means – cal(cium) + modul(ate) + in(g).
  • Calmodulin is a ubiquitous eukaryotic regulator protein that is involved in many calcium-mediated processes (17kDa protein). 

Location:

  • Calmodulin is always found intracellularly.
  • Calmodulin 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).

Structure of calmodulin

  • It is homologous to the muscle protein troponin C in structure and function.
  • Calmodulin forms two globular domains connected by a flexible central linker.
  • Each domain binds two calcium ions in E-F hand motifs (calmodulin can bind a total of four Ca2+ ions via EF hand motifs).
  • The calcium binding sites are 12 amino acids long and contain many negatively-charged or polar amino acid residues such as aspartate, glutamate, and asparagine. Calmodulin is an acidic protein.
  • The side chains on these amino acids form ionic bonds with the Ca2+
  • Full occupancy of these calcium binding sites leads to a marked conformational change, which allows calmodulin to activate enzymes and ion channels.

When intracellular Ca2+ increases in response to some stimulus, calmodulin binds Ca2+. The binding of Ca2+ to calmodulin drives a conformational change in the protein which in turn activates the CaM kinase. The kinase then phosphorylates a number of target enzymes, regulating their activities.

Figure 1: Structure and funtion of calcium – calmodulin complex

Some other features of calmodulin

  • Many of the proteins that Calmodulin binds are unable to bind calcium themselves, and use calmodulin as a calcium sensor and signal transducer.
  • Calmodulin can also make use of the calcium stores in the endoplasmic reticulum, and the sarcoplasmic reticulum.
  • Calmodulin can undergo post-translational modifications, such as phosphorylation, acetylation, methylation and proteolytic cleavage, each of which has potential to modulate its actions.

Ca2+/calmodulin dependent protein kinase (CaM kinase) – serine–threonine kinases

CaMKs are enzymes activated by increases in the concentration of intracellular calcium ions (Ca2+).

Figure 2: Activation of CaM kinase by Ca2+ – calmodulin complex
  • On binding Ca2+/calmodulin, CaM kinase II activates itself by autophosphorylation.
  • The degree of activation being dependent on the oscillation frequency of Ca2+

Function: Phosphorylation of serine and threonine residues on the receiving proteins

  • Transfers phosphates from ATP to defined serine or threonine residues in other target proteins.
  • These proteins then go on to activate downstream processes such as intracellular signalling, smooth muscle contractions, neurotransmitter and hormone synthesis and release, and cell cycle regulation
  • Activated CaMK is involved in the phosphorylation of transcription factors and therefore, in the regulation of expression of responding genes.

There are two types of CaM kinase:

1) Specialized CaM kinases:

Example: myosin light chain kinase (Role in smooth muscle contraction)

  • Myosin Light Chain (MLC) Kinase is activated by a calmodulin when it is bound by calcium ions.
  • Activated MLC will phosphorylate the head of the myosin light chain which in turn cause smooth muscle contraction.
Figure 3:Myosin Light Chain (MLC) Kinase activation by calcium – calmodulin complex

2) Multifunctional CaM kinases: (CaM kinase II)

Play a role in neurotransmitter secretion, transcription factor regulation, and glycogen metabolism.

Neurotransmitter secretion: Acetylcholine stimulation of G protein coupled receptors in secretory cells of the pancreas and parotid gland induces an IP3 mediated rise in Ca2+ that triggers the fusion of secretory vesicles with the plasma membrane and release of their contents into the extracellular space.

Glycogen metabolism:  Activation of Phosphorylase kinase

Transcription factor regulation: phosphorylation of target transcription factors

Activation of specific transcription factors triggered by increased Ca2+ concentration.

 Two mechanisms are there:

  • Ca2+/calmodulin activate protein kinases that in turn phosphorylate transcription factors, thereby modifying their activity and regulating gene expression.
  • Ca2+/calmodulin activate a phosphatase that removes phosphate groups from a transcription factor.

Example of this mechanism: Activation of T cells of the immune system

Ca2+ ions enhance the activity of an essential transcription factor, NFAT (nuclear factor of activated T cells).

  • In unstimulated cells, phosphorylated NFAT is located in the cytosol.
  • Following receptor stimulation and elevation of cytosolic Ca2+, the Ca2+/calmodulin complex binds to and activates calcineurin (a protein-serine phosphatase).
  • Activated calcineurin then dephosphorylates key phosphate residues on cytosolic NFAT, exposing a nuclear localization sequence that allows NFAT to move into the nucleus and stimulate expression of genes essential for activation of T cells.
Figure 4:Calcineurin activation by Ca2+/calmodulin complex

Other Functions of calcium – calmodulin complex:

Calmodulin mediates many crucial processes: inflammation, metabolism, apoptosis, smooth muscle contraction, intracellular movement, short-term and long-term memory, and the immune response.

The Ca2+/calmodulin complex also plays a key role in controlling the diameter of blood vessels and thus their ability to deliver oxygen to tissues.

  • Endothelial cells contain a G protein–coupled receptor that binds acetylcholine and activates phospholipase C, leading to an increase in the level of cytosolic Ca2+.
  • After Ca2+ binds to calmodulin, the resulting complex stimulates the activity of NO synthase (an enzyme that catalyzes formation of NO from O2 and the amino acid arginine).
  • Because NO has a short half-life (2–30 seconds), it can diffuse only locally in tissues from its site of synthesis.
  • In particular NO diffuses from the endothelial cell into neighboring smooth muscle cells, where it activates an intracellular NO receptor with guanylyl cyclase activity.
  • The resulting rise in cGMP leads to activation of protein kinase G (PKG) which causes relaxation of the smooth muscle and thus vasodilation.

Figure 5: Regulation of contractility of arterial smooth muscle by nitric oxide (NO) and cGMP.

In addition to its effects on enzymes and ion transport, Ca2+/calmodulin regulates the activity of many structural elements in cells.

These include:

  • The actin – myosin complex of smooth muscle (under β- adrenergic control)
  • Various microfilament-mediated processes in non contractile cells (including cell motility, cell conformation changes, mitosis, granule release, and endocytosis).

Figure 6: Action of Ca2+ – calmodulin complex in signal transduction

  • The Ca2+ – calmodulin complex can activate specific kinases. Two of them are shown here.
  • These actions result in phosphorylation of substrates, and this leads to altered physiologic responses. This figure also shows that Ca2+ can enter cells through voltage- or ligand gated Ca2+
  • The intracellular Ca2+ is also regulated through storage and release by the mitochondria and endoplasmic reticulum.

Role in glucose metabolism

  • Activation of Phosphorylase kinase (in turn activate glycogen Phosphorylase) which leads to the release of glucose from glycogen catalysed by glycogen Phosphorylase.

Activation of calcium pumps

  • One of the functions of the Ca2+/calmodulin complex is to activate calcium pumps.
  • These pumps remove calcium from the cytoplasm by either pumping it out of the cell or storing it in the endoplasmic reticulum.
  • By controlling the amount of calcium in the cell, the downstream responses are regulated.

Table 1: Enzymes regulated by Ca2+ or calmodulin

 

 

 

 

 

 

 

 

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