Sandbox Reserved 820: Difference between revisions

Each monomer is divided in <scene name='56/568018/Monomer_structure/5'>3 thioredoxin domains (TRX)</scene>: <scene name='56/568018/Monomer_structure/7'>the N-term</scene>, <scene name='56/568018/Monomer_structure/6'>the middle</scene> and the <scene name='56/568018/Monomer_structure/8'>C-term</scene> domains. Each of these has a regular structure: a <scene name='56/568018/Beta_sheet/4'>5 strands beta sheet core</scene>  surrounded by <scene name='56/568018/Alpha_helix/3'>4 alpha helices</scene>.<ref name="Martin">PMID:7788290</ref>

Finally, the C-term Asp-rich extremity  is intrisically disordered.

There are two types of dimerization: the  

The front-to-front form is stabilized by intermolecular interactions between the  

<scene name='56/568018/Dimer/3'>α2 helix of the domain I</scene> of each CASQ2.<ref name="Crystal Structure of calsequestrin from rabbit skeletal muscle sarcoplasmic reticulum (Wang et al., 1998)">http://www.nature.com/nsmb/journal/v5/n6/abs/nsb0698-476.html</ref> The intermolecular salt bridges are built between <scene name='56/568018/Dimer/13'>Glu 55 and Lys 49</scene>.<ref name="Crystal Structure of calsequestrin from rabbit skeletal muscle sarcoplasmic reticulum (Wang et al., 1998)">http://www.nature.com/nsmb/journal/v5/n6/abs/nsb0698-476.html</ref>  This dimerization induces the formation of an electronegative pocket which involves the following amino acids: Glu 39, Glu 54, Glu 78, Glu 92, Asp 93 and Asp 101 for the first monomere and Glu 199, Asp 245, Asp 278, Glu 350 and Glu 348 for the second one.<ref name="Crystal Structure of calsequestrin from rabbit skeletal muscle sarcoplasmic reticulum (Wang et al., 1998)">http://www.nature.com/nsmb/journal/v5/n6/abs/nsb0698-476.html</ref>  

The back-to-back form is stabilized by intermolecular interactions between the <scene name='56/568018/Oligomere_and_ligand/7'>α3 helix of the domain I</scene>, <scene name='56/568018/Oligomere_and_ligand/6'>α4 helix of the domain II</scene><ref name="Crystal Structure of calsequestrin from rabbit skeletal muscle sarcoplasmic reticulum (Wang et al., 1998)">http://www.nature.com/nsmb/journal/v5/n6/abs/nsb0698-476.html</ref>, and it has also been proved that the C-term domain is involved<ref name="c term">NCBI Structure Ressource: CASQ2 calsequestrin 2 http://www.ncbi.nlm.nih.gov/Structure/cdd/cddsrv.cgi</ref> (<scene name='56/568018/Oligomere_and_ligand/9'>together</scene>). The intermolecular salt bridges are built between Glu 215 and Lys 86, Glu 216 and Lys 24, Glu 169 and Lys 85.<ref name="Crystal Structure of calsequestrin from rabbit skeletal muscle sarcoplasmic reticulum (Wang et al., 1998)">http://www.nature.com/nsmb/journal/v5/n6/abs/nsb0698-476.html</ref> The dimerization is also favored by a hydrogen bond between Ala 82 and Asn 22. This dimerization creates a very electronegative pocket at the C-terminal region which enables the binding of Ca²⁺.<ref name="Crystal Structure of calsequestrin from rabbit skeletal muscle sarcoplasmic reticulum (Wang et al., 1998)">http://www.nature.com/nsmb/journal/v5/n6/abs/nsb0698-476.html</ref>

Each monomere of CASQ2 can bind between <scene name='56/568018/Oligomere_and_ligand/12'>18 to 50 Ca2+</scene>. The Ca²⁺ ions bind to two or more acidic amino acids like <scene name='56/568018/Oligomere_and_ligand/13'>Glutamate</scene> or <scene name='56/568018/Oligomere_and_ligand/15'>Aspartate</scene>. These amino acids are mainly oriented outside and in the C-terminal region. It had been shown that Ca²⁺ions mainly binds an Asp-rich region on the disordered C-terminal domain. <!-- METTRE DU VERT MAIS LE CT N'EST PAS DISPONIBLE cf: http://www.rcsb.org/pdb/explore/remediatedSequence.do?structureId=2VAF&bionumber=1 -->As CASQ2 form homooligomers, Ca²⁺ can be bound in the electronegative pockets created by the <scene name='56/568018/Oligomere_and_ligand/16'>front-to-front</scene> and <scene name='56/568018/Oligomere_and_ligand/17'>back-to-back</scene> dimer interactions.<ref name="The asp-rich region at the carboxyl-terminus of calsequestrin binds to Ca²⁺‡ and interacts with triadin (Shin et al., 2000)">The Asp-rich region at the carboxyl-terminus of calsequestrin binds to Ca²⁺ and interacts with triadin (Shin et al., 2000) http://www.sciencedirect.com/science/article/pii/S0014579300022468</ref>

CASQ2 can also bind other ions like Mg²⁺ or H⁺. The affinity for Mg²⁺ is lower than the affinity for Ca²⁺ however the concentration of Ca²⁺ decreases. When the pH is low, the calcium-binding capacity of CASQ2 decreases as H⁺ ions occupy the acidic sites.<ref name="Calsequestrin and the calcium release channel of skeletal and cardiac muscle (Beard et Al., 2004)">PMID:15050380</ref>

 

<!-- Source ASP-rich: The asp-rich region at the carboxyl-terminus of calsequestrin binds to Ca2+‡ and interacts with triadin (Shin et al., 2000) Lien: http://www.sciencedirect.com/science/article/pii/S0014579300022468 -->

CASQ2 can be anchored into the membrane of SR thanks to two integral proteins: the triadin and the junctin.<ref name="Regulation of Ryanodine Receptors by Calsequestrin: Effect of High Luminal Ca2+ and Phosphorylation (Beard et Al., 2005)">PMID:15731387</ref> Triadin and juctin can bind to CASQ2 on their KEKE motifs (amino acids 210 and 224 for the triadin).<ref name="Regulation of Ryanodine Receptors by Calsequestrin: Effect of High Luminal Ca2+ and Phosphorylation (Beard et Al., 2005)">http://www.ncbi.nlm.nih.gov/pubmed/15731387</ref> Both proteins bind CASQ2 on its Asp-rich region of the C-terminal region.<ref name="Regulation of Ryanodine Receptors by Calsequestrin: Effect of High Luminal Ca2+ and Phosphorylation (Beard et Al., 2005)">http://www.ncbi.nlm.nih.gov/pubmed/15731387</ref>

Triadin and Junctin can also interact with Ryanodin Receptor ([[3im5]]).<ref name="Regulation of Ryanodine Receptors by Calsequestrin: Effect of High Luminal Ca2+ and Phosphorylation (Beard et Al., 2005)">http://www.ncbi.nlm.nih.gov/pubmed/15731387</ref>

The binding site of CASQ2 to Ryanodin Receptor (RyR) is unknow.<ref name="Regulation of Ryanodine Receptors by Calsequestrin: Effect of High Luminal Ca2+ and Phosphorylation (Beard et Al., 2005)">http://www.ncbi.nlm.nih.gov/pubmed/15731387</ref>

 

As CASQ2 binds to triadin and junctin, it induces the inhibition of RyR and then the inhibition of calcium release in the cytoplasm. On the contrary, as CASQ2 unbinds triadin and junctin, it induces the activation of Ryr and an efflux of Ca²⁺ from the SR to the cytoplasm.<ref name="Calsequestrin and the calcium release channel of skeletal and cardiac muscle (Beard et Al., 2004)">http://www.ncbi.nlm.nih.gov/pubmed/15050380</ref> CASQ2 is free when the concentration of Ca²⁺ is higher than 1 mM in the SR lumen.<ref name="Regulation of Ryanodine Receptors by Calsequestrin: Effect of High Luminal Ca2+ and Phosphorylation (Beard et Al., 2005)">http://www.ncbi.nlm.nih.gov/pubmed/15731387</ref>

== Regulation of CASQ2 ==   

CASQ2 can be phosphorylated by three different kinases: casein kinase I (CK I), casein kianse II (CK II) and ε protein kinase C1 (εPKC1).<ref name="Calsequestrin and the calcium release channel of skeletal and cardiac muscle (Beard et Al., 2004)">http://www.ncbi.nlm.nih.gov/pubmed/15050380</ref> CK II is located in the SR and is able to phosphorylate Ser 378, Ser 382 and Ser 386. These residues are on the C-terminal domain.<ref name="Calsequestrin and the calcium release channel of skeletal and cardiac muscle (Beard et Al., 2004)">http://www.ncbi.nlm.nih.gov/pubmed/15050380</ref> The consensus sequence recognized by CK II is Ser/Thr-X-X-Asp/Glu.<ref name="Calsequestrin and the calcium release channel of skeletal and cardiac muscle (Beard et Al., 2004)">http://www.ncbi.nlm.nih.gov/pubmed/15050380</ref> More there are acidic residues after this consensus sequence, more the probabilty of phosphorylation increases.<ref name="Calsequestrin and the calcium release channel of skeletal and cardiac muscle (Beard et Al., 2004)">http://www.ncbi.nlm.nih.gov/pubmed/15050380</ref>

"The phosphorylation and de-phosphorylation of CASQ2 my provide an off/on switch for CASQ2 to regulate Ca2+" <!-- A reformuler, mais bon...! --> But there is not any prove yet.<ref name="Calsequestrin and the calcium release channel of skeletal and cardiac muscle (Beard et Al., 2004)">http://www.ncbi.nlm.nih.gov/pubmed/15050380</ref>