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Note: Only selected aspects of the complex subject of calcium signaling will be covered here. See also sections on phosphatidylinositol signals and calcium pumps.
Modulation of Cytosolic [Ca++]
Ca++-binding proteins within the ER lumen "buffer" the free Ca++ concentration, and increase the capacity for Ca++ storage. ER Ca++-binding proteins have 20-50 low-affinity Ca++-binding sites per molecule, consisting of acidic residues. Examples:
Ca++ concentration, within the cytosol or other cell compartments, may be monitored using indicator dyes or proteins that are either luminescent or change their fluorescence when they bind Ca++. Fluorescent indicators used with confocal fluorescence microscopy can provide high-resolution imaging and quantitation of Ca++ fluctuations within cells.
A transient increase in cytosolic Ca++ may be localized to the vicinity of one or a few activated Ca++-release or Ca++-entry channels. Such a localized Ca++ "puff" or "spark" may activate activate effectors that induce additional Ca++ release, leading to a more widespread increase in cytosolic Ca++. A “wave” of higher cytosolic Ca++ may spread to neighboring cells.
For example, see a website maintained by E. Niggli showing recordings of Ca++ sparks and waves, using fluorescent Ca++ indicators.
Ryanodine Receptor: A Ca++ Release Channel
A large Ca++-release channel in the membrane of muscle sarcoplasmic reticulum (SR) is called the ryanodine receptor, because of its sensitivity to inhibition by a plant alkaloid ryanodine. Skeletal and cardiac muscle contraction is activated when Ca++ is released from the SR lumen to the cytosol via the ryanodine receptor.
| T tubules are invaginations of muscle cell plasma membrane. Voltage-gated Ca++ channels in the T tubule membrane
interact with ryanodine receptors in the closely
apposed SR membrane.
Activation of the voltage-gated Ca++ channels, by an action potential in the T tubule, leads to opening of ryanodine-sensitive Ca++ release channels. Ca++ moves from the SR lumen to the cytosol, passing first through the transmembrane portion of the ryanodine receptor, and then through the ryanodine receptor's cytoplasmic assembly.
|At right are three views of a 3D reconstruction of the structure of the
channel, at 30 Å resolution,
based on micrographs obtained by cryo-electron microscopy at varied tilt angles.
See also an animation of conformational changes during channel opening & closing.
A somewhat higher resolution structure now available
indicates the presence of bent a-helices
adjacent to the lumen in the transmembrane
pore domain, but an atomic resolution structure
of the whole channel has not yet been
achieved. For diagrams see
by Ludtke et al. (journal subscription required).
|IP3 receptor Ca++
In many mammalian cells, IP3 (inositol-1,4,5-trisphosphate) triggers Ca++ release from the endoplasmic reticulum. The "second messenger" IP3 is produced, e.g., in response to hormonal signals, from the membrane lipid phosphatidylinositol. (Phosphatidylinositol signals are discussed in more detail elsewhere.)
|Explore at right the structure of
IP3. The receptor-bound IP3, from PDB 1N4K, structure solved by I. Bosanac, J. R. Alattia, T. K. Mal, J.
Chan, S. Talarico, F. K. Tong, K. I. Tong, F. Yoshikawa, T. Furuichi, M.
Iwai, T. Michikawa, K. Mikoshiba & M. Ikura in 2002.
Recommended display options:
View with different display settings, e.g., ball & stick, sticks, spacefill.
Note the stereospecific orientation
of substituted and un-substituted hydroxyl groups relative to the two sides of
Structures of cytosolic domains of the IP3 receptor, including the IP3 binding site, have been solved, but the pore structure of the IP3 receptor has not yet been determined at atomic resolution.
See image in a website for a low-resolution structure of the IP3 receptor (AIST, Japan, findings of C. Sato et al.).
Calmodulin, a Ca++-activated switch protein, mediates many of the signal functions of Ca++. Calmodulin cooperatively binds 4 Ca++.
At each binding site, Ca++ interacts with oxygen atoms, mainly of glutamate and aspartate side-chain carboxyl groups, and of the protein backbone, in a loop domain between two a-helices at right angles. This helix-loop-helix motif is called an EF hand (diagram p. 642).
There are four helix-loop-helix motifs in calmodulin, two at each end of the molecule, which has a dumbbell shape.
Ca++ binding to the 4 helix-loop-helix motifs promotes a conformational change that exposes hydrophobic residues along a concave patch on each of the 2 lobes. These residues are involved in protein-protein interactions. Ca++-calmodulin then changes conformation again as it wraps around the target domain of a protein.
A typical calmodulin-binding target domain is a positively charged, amphipathic a-helix, with polar and non-polar surfaces. Terminal methyl groups of methionine side-chains of calmodulin participate in binding to hydrophobic residues in target domains of some enzymes that are regulated by calmodulin. However the interaction of Ca++-calmodulin with some target proteins is different from what is described here.
Some proteins have bound calmodulin as part of their quaternary structure, even in the absence of Ca++. In either case, Ca++ binding to calmodulin may induce a conformational change that alters activity of the target protein.
Many enzymes are regulated by Ca++-calmodulin. For example:
View at right:
Defects in genes coding for Ca++ channel proteins, Ca++-ATPases, and intracellular Ca++ sensors are associated with disease or death.
Copyright © 1998-2007 by Joyce J. Diwan. All rights reserved.
Additional material on Ca++