Calsequestrin, the major calcium sequestering protein in the sarcoplasmic 
reticulum of muscle, forms a quaternary complex with the ryanodine receptor 
calcium release channel and the intrinsic membrane proteins triadin and junctin. 
We have investigated the possibility that calsequestrin is a luminal calcium 
concentration sensor for the ryanodine receptor. We measured the luminal calcium 
concentration at which calsequestrin dissociates from the ryanodine receptor and 
the effect of calsequestrin on the response of the ryanodine receptor to changes 
in luminal calcium. We provide electrophysiological and biochemical evidence 
that: 1), luminal calcium concentration of >/=4 mM dissociates calsequestrin 
from junctional face membrane, whereas in the range of 1-3 mM calsequestrin 
remains attached; 2), the association with calsequestrin inhibits ryanodine 
receptor activity, but amplifies its response to changes in luminal calcium 
concentration; and 3), under physiological calcium conditions (1 mM), 
phosphorylation of calsequestrin does not alter its ability to inhibit native 
ryanodine receptor activity when the anchoring proteins triadin and junctin are 
present. These data suggest that the quaternary complex is intact in vivo, and 
provides further evidence that calsequestrin is involved in the sarcoplasmic 
reticulum calcium signaling pathway and has a role as a luminal calcium sensor 
for the ryanodine receptor.