Complex formation between junctin, triadin, calsequestrin, and the ryanodine receptor. Proteins of the cardiac junctional sarcoplasmic reticulum membrane.
Several key proteins have been localized to junctional sarcoplasmic reticulum which are important for Ca2+ release. These include the ryanodine receptor, triadin, and calsequestrin, which may associate into a stable complex at the junctional membrane. We recently purified and cloned a fourth component of this complex, junctin, which exhibits homology with triadin and is the major 125I-calsequestrin-binding protein detected in cardiac sarcoplasmic reticulum vesicles (Jones, L. R., Zhang, L., Sanborn, K., Jorgensen, A. O., and Kelley, J. (1995) J. Biol. Chem. 270, 30787-30796). In the present study, we have examined the binding interactions between the cardiac forms of these four proteins with emphasis placed on the role of junctin. By a combination of approaches including calsequestrin-affinity chromatography, filter overlay, immunoprecipitation assays, and fusion protein binding analyses, we find that junctin binds directly to calsequestrin, triadin, and the ryanodine receptor. This binding interaction is localized to the lumenal domain of junctin, which is highly enriched in charged amino acids organized into "KEKE" motifs. KEKE repeats are also found in the common lumenal domain of triadin, which likewise is capable of binding to calsequestrin and the ryanodine receptor (Guo, W., and Campbell, K. P. (1995) J. Biol. Chem. 270, 9027-9030). It appears that junctin and triadin interact directly in the junctional sarcoplasmic reticulum membrane and stabilize a complex that anchors calsequestrin to the ryanodine receptor. Taken together, these results suggest that junctin, calsequestrin, triadin, and the ryanodine receptor form a quaternary complex that may be required for normal operation of Ca2+ release.
Zhang
L
L
Department of Medicine and the Krannert Institute of Cardiology, Indiana University School of Medicine, Indianapolis, Indiana 46202, USA.
Kelley
J
J
Schmeisser
G
G
Kobayashi
Y M
YM
Jones
L R
LR
D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000378:metabolism*; D002135:Calcium-Binding Proteins; D002155:Calsequestrin / Q000378:metabolism; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D002846:Chromatography, Affinity; D004285:Dogs; D006352:Heart Ventricles; D007365:Intercellular Junctions / Q000737:chemistry*; D008565:Membrane Proteins; D006899:Mixed Function Oxygenases; D008958:Models, Molecular; D009124:Muscle Proteins / Q000235:genetics / Q000378:metabolism*; D009206:Myocardium / Q000737:chemistry*; D011233:Precipitin Tests; D011485:Protein Binding; D019837:Ryanodine Receptor Calcium Release Channel; D012519:Sarcoplasmic Reticulum / Q000737:chemistry*
D015220:Calcium Channels; D002135:Calcium-Binding Proteins; D002155:Calsequestrin; D002352:Carrier Proteins; D008565:Membrane Proteins; D009124:Muscle Proteins; D019837:Ryanodine Receptor Calcium Release Channel; C498269:TRDN protein, human; C070048:triadin; D006899:Mixed Function Oxygenases; D002118:Calcium