A high affinity molecular interaction is demonstrated between calsequestrin and the sarcoplasmic reticular Ca2+ release channel/ryanodine receptor (RyR) by surface plasmon resonance. K D values of 92 nM and 102 nM for the phosphorylated and dephosphorylated calsequestrin have been determined, respectively. Phosphorylation of calsequestrin seems not to influence this high affinity interaction, i.e. calsequestrin might always be bound to RyR. However, the phosphorylation state of calsequestrin determines the amount of Ca2+ released from the lumen. Dephosphorylation of approximately 1% of the phosphorylated calsequestrin could be enough to activate the RyR channel half-maximally, as we have shown previously [Szegedi et al., Biochem. J. 337 (1999) 19].
