COMPLEX-FORMATION BETWEEN JUNCTION, TRIADIN, CALSEQUESTRIN, AND THE RYANODINE RECEPTOR - PROTEINS OF THE CARDIAC JUNCTIONAL SARCOPLASMIC-RETICULUM MEMBRANE
L. Zhang et al., COMPLEX-FORMATION BETWEEN JUNCTION, TRIADIN, CALSEQUESTRIN, AND THE RYANODINE RECEPTOR - PROTEINS OF THE CARDIAC JUNCTIONAL SARCOPLASMIC-RETICULUM MEMBRANE, The Journal of biological chemistry, 272(37), 1997, pp. 23389-23397
Several key proteins have been localized to junctional sarcoplasmic re
ticulum which are important for Ca2+ release. These include the ryanod
ine receptor, triadin, and calsequestrin, which may associate into a s
table complex at the junctional membrane. We recently purified and clo
ned a fourth component of this complex, junctin, which exhibits homolo
gy with triadin and is the major I-125-calsequestrin-binding protein d
etected in cardiac sarcoplasmic reticulum vesicles (Jones, L. R., Zhan
g, L., Sanborn, K., Jorgensen, A. O., and Kelley, J. (1995) J. Biol. C
hem. 270, 30787-30796). In the present study, we have examined the bin
ding interactions between the cardiac forms of these four proteins wit
h emphasis placed on the role of junctin. By a combination of approach
es including calsequestrin-affinity chromatography, filter overlay, im
munoprecipitation assays, and fusion protein binding analyses, we find
that junctin binds directly to calsequestrin, triadin, and the ryanod
ine receptor. This binding interaction is localized to the lumenal dom
ain of junctin, which is highly enriched in charged amino acids organi
zed into ''KEKE'' motifs. KEKE repeats are also found in the common lu
menal domain of triadin, which likewise is capable of binding to calse
questrin and the ryanodine receptor (Guo, W., and Campbell, K. P. (199
5) J. Biol. Chem. 270, 9027-9030). It appears that junctin and triadin
interact directly in the junctional sarcoplasmic reticulum membrane a
nd stabilize a complex that anchors calsequestrin to the ryanodine rec
eptor. Taken together, these results suggest that junctin, calsequestr
in, triadin, and the ryanodine receptor form a quaternary complex that
may be required for normal operation of Ca2+ release.