Ca2+ dependent Cell Signaling

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Ca2 dependent Cell Signalingthrough
Calmodulin-activatedProtein Phosphatase and
Protein Kinases

• M.Prasad Naidu
• MSc Medical Biochemistry,
• Ph.D.Research Scholar

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• THE JOURNAL OF BIOLOGICAL CHEMISTRY
• Vol. 276, No. 4, Issue of January 26, pp.
  23112312, 2001
• 2001 by The American Society for Biochemistry
  and Molecular Biology, Inc.
• Printed in U.S.A.

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• Extracellular Ca2 is necessary for muscle
  contractions .
• Intracellular Ca2 mediates a large number of
  cellular responses with the high affinity and
• specificity required for a regulatory second
  messenger.
• Ca2 acts by binding to specific intracellular
  proteins, which may be considered Ca2 receptors .

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• 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 .

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• 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 .

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

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• 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.

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• 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 .

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• Ca2/calmodulin regulates
• calmodulin-dependent protein kinases I, II, IV,
• myosin light chain kinases,
• calmodulin-dependent protein phosphatase,
• calcineurin.

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• 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 .

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• Calcineurin was initially identified in neuronal
• tissues, 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 Ca2dependent signaling
  pathways .

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• Calcineurin has role in regulating transcription
  during development via dephosphorylation of the
  NF-AT transcription complex.
• Ca2 also regulates transcription through
  phosphorylation of several transcription
• factors, including CREB and MEF2, by a
  Ca2/calmodulindependent protein kinase cascade
  involving three distinct kinases.

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• 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 third
• kinase, which enhances Ca2/calmodulin-dependent
  activity .

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

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• 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 .

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• 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.

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• 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 .

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

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• 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.

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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

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