M.Prasad NaiduMSc Medical Biochemistry,

Ph.D.Research Scholar

THE JOURNAL OF BIOLOGICAL CHEMISTRY

Vol. 276, No. 4, Issue of January 26, pp. 2311–2312, 2001

© 2001 by The American Society for Biochemistry and Molecular Biology, Inc.

Printed in U.S.A.

Extracellular Ca2+ is necessary for muscle contractions .

Intracellular Ca2+ mediates a large number of cellular responses with the high affinity andspecificity required for a regulatory second messenger.

Ca2+ acts by binding to specific intracellular proteins, which may be considered Ca2+ receptors .

The class of proteins that binds Ca2+ with high affinity and specificity now includes hundreds of members .

Studies focused on calmodulin because it was recognized as a regulator of many different target enzymes .

Calmodulin interactions with proteins are categorized into six distinct classes based on Ca2+ -independent and -dependent modes of binding and regulation .

Calmodulin has four Ca2+ -binding sites with two in a globular N-terminal domain separated by a flexible alfa-helix from a C-terminal globular domain containing the other two Ca2+ binding sites .

In the presence of Ca2+ each domain adopts an open conformation exposing a hydrophobic pocket that renders calmodulin functional for binding to target sequences .

The process of complex formation includes sequential interactions between the C-terminal hydrophobic pocket with a hydrophobic residue in the target N-terminal sequence followed by interactions between the N-terminal globular domain with the C-terminal sequence of the calmodulin-binding domain.

Calmodulin thus collapses and wraps around the peptide, resulting in the formation of a high affinity complex.

The high affinity binding of calmodulin to a target sequence is only partly responsible for enzyme activation as surface residues on calmodulin may subsequently interact with other areas of the enzyme .

Ca2+/calmodulin regulates 1.calmodulin-dependent protein kinases I, II,

IV, 2.myosin light chain kinases,3.calmodulin-dependent protein phosphatase,4.calcineurin.

Protein phosphatases play dynamic roles in diverse cellular processess .

Protein phosphatase IIB, or calcineurin, is a serine/threonine protein phosphatase activated by Ca2+/calmodulin and, thus,couples Ca2+ signals to specific cellular responses via protein dephosphorylation .

Calcineurin was initially identified in neuronaltissues, but it was quickly recognized that it had a broad tissue distribution and a highly conserved structure from yeast to man.

The immunosuppressive drugs, cyclosporin A and FK506 have provided tools to explore the roles of calcineurin in diverse Ca2+dependent signaling pathways .

Calcineurin has role in regulating transcription during development via dephosphorylation of the NF-AT transcription complex.

Ca2+ also regulates transcription through phosphorylation of several transcriptionfactors, including CREB and MEF2, by a Ca2+/calmodulindependent protein kinase cascade involving three distinct kinases.

Ca2+/calmodulin-dependent protein kinases I and IV have broad but overlapping substrate specificities and similar mechanisms of activation.

Both kinases are phosphorylated by a thirdkinase, which enhances Ca2+/calmodulin-dependent activity .

Another ubiquitous effector of Ca2+ signaling is the multifunctional Ca2+/calmodulin-dependent protein kinase II, which hasbroad substrate specificity with substrates found in nuclear, cytoskeletal, and membrane compartments of cells .

Multifunctional Ca2+/calmodulin-dependent protein kinase II, which has biochemical property of autophosphorylation mechanism that results in the trapping of calmodulin on the kinase and conversion of the enzyme to a Ca2+ independent form .

The dynamic and spatial aspects of autophosphorylation are an important element of regulation, allowing it to respond to transient cellular Ca2+ oscillations .

Ca2+/calmodulin-dependent protein kinase II in the neuronal synapse where its translocation and activation regulate a number of proteins in the postsynaptic cell.

In contrast to the Ca2+/calmodulin-dependent protein kinases and calcineurin that have broad substrate specificities, myosin light chain kinases are dedicated protein kinases for which the only known physiological substrate is the regulatory light chain of myosin II .

Myosin light chain kinases have role cell spreading and migration, cytokinesis, celladhesion, secretion, and cytoskeletal arrangements that affect plasma membrane ion movements .

Smooth muscle sarcoplasm contain a myosin light chain kinase that is calcium dependent .

The Ca 2+ activation of myosin light chain kinase requires binding of calmodulin .4Ca2+ to its Kinase subunit .

Calmodulin .4Ca2+ acivated light chain kinase phosphorylates the p – light chain , which then ceases to inhibit the myosin - F – actin interaction , the contraction cycle then begins.

Enzymes regulated by calcium or calmodulin

Adenylyl cyclase , guanylyl cyclase ,Cyclic nucleotide phosphodiesterase ,Calcium dependent protein kinase , phospho

protein phosphatse 2 B,Glycerol 3 phosphate dehydrogenase ,

Phosphorylase kinase , glycogen synthase ,Pyruvate carboxylase , pyruvate kinase ,Ca2+ Mg2+ ATPase ,Phospholipase A2 , Ca2+ phospholipid dependent

protein kinase