Abstract
Calsequestrin is by far the most abundant Ca$^2+$-binding protein
in the sarcoplasmic reticulum (SR) of skeletal and cardiac muscle.
It allows the Ca$^2+$ required for contraction to be stored at
total concentrations of up to 20mM, while the free Ca$^2+$ concentration
remains at approximately 1mM. This storage capacity confers upon
muscle the ability to contract frequently with minimal run-down in
tension. Calsequestrin is highly acidic, containing up to 50 Ca$^2+$-binding
sites, which are formed simply by clustering of two or more acidic
residues. The Kd for Ca$^2+$ binding is between 1 and 100 microM,
depending on the isoform, species and the presence of other cations.
Calsequestrin monomers have a molecular mass of approximately 40
kDa and contain approximately 400 residues. The monomer contains
three domains each with a compact alpha-helical/beta-sheet thioredoxin
fold which is stable in the presence of Ca$^2+$. The protein
polymerises when Ca$^2+$ concentrations approach 1mM. The polymer
is anchored at one end to ryanodine receptor (RyR) Ca$^2+$ release
channels either via the intrinsic membrane proteins triadin and junctin
or by binding directly to the RyR. It is becoming clear that calsequestrin
has several functions in the lumen of the SR in addition to its well-recognised
role as a Ca$^2+$ buffer. Firstly, it is a luminal regulator
of RyR activity. When triadin and junctin are present, calsequestrin
maximally inhibits the Ca$^2+$ release channel when the free
Ca$^2+$ concentration in the SR lumen is 1mM. The inhibition
is relieved when the Ca$^2+$ concentration alters, either because
of small changes in the conformation of calsequestrin or its dissociation
from the junctional face membrane. These changes in calsequestrin's
association with the RyR amplify the direct effects of luminal Ca$^2+$
concentration on RyR activity. In addition, calsequestrin activates
purified RyRs lacking triadin and junctin. Further roles for calsequestrin
are indicated by the kinase activity of the protein, its thioredoxin-like
structure and its influence over store operated Ca$^2+$ entry.
Clearly, calsequestrin plays a major role in calcium homeostasis
that extends well beyond its ability to buffer Ca$^2+$ ions.
- 15050380
- acid
- acid,
- amino
- animals,
- calc,
- calcium
- calcium-binding
- calsequestrin,
- carrier
- cellular
- channel,
- data,
- function
- homology,
- humans,
- inter,
- ium,
- junctions,
- membrane
- mixed
- molecular
- muscle
- muscle,
- myocardium,
- oxygenases,
- proteins,
- receptor
- release
- reticulum,
- ryanodine
- sarcoplasmic
- sequence
- sequence,
- skeletal,
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