The role of calsequestrin, triadin, and junctin in conferring cardiac
ryanodine receptor responsiveness to luminal calcium.
Biophys. J., 86 (4):
2121--2128 (April 2004)Abstract
The level of Ca inside the sarcoplasmic reticulum (SR) is an important
determinant of functional activity of the Ca release channel/ryanodine
receptor (RyR) in cardiac muscle. However, the molecular basis of
RyR regulation by luminal Ca remains largely unknown. In the present
study, we investigated the potential role of the cardiac SR luminal
auxiliary proteins calsequestrin (CSQ), triadin 1, and junctin in
forming the luminal calcium sensor for the cardiac RyR. Recordings
of single RyR channels incorporated into lipid bilayers, from either
SR vesicle or purified RyR preparations, were performed in the presence
of MgATP using Cs+ as the charge carrier. Raising luminal Ca from
20 microM to 5 mM increased the open channel probability (Po) of
native RyRs in SR vesicles, but not of purified RyRs. Adding CSQ
to the luminal side of the purified channels produced no significant
changes in Po, nor did it restore the ability of RyRs to respond
to luminal Ca. When triadin 1 and junctin were added to the luminal
side of purified channels, RyR Po increased significantly; however,
the channels still remained unresponsive to changes in luminal Ca.
In RyRs reassociated with triadin 1 and junctin, adding luminal CSQ
produced a significant decrease in activity. After reassociation
with all three proteins, RyRs responded to rises of luminal Ca
by increasing their Po. These results suggest that a complex of CSQ,
triadin 1, and junctin confer RyR luminal Ca sensitivity. CSQ apparently
serves as a luminal Ca sensor that inhibits the channel at low luminal
Ca, whereas triadin 1 and/or junctin may be required to mediate
interactions of CSQ with RyR.
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