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Fig. 5. Direct interaction between the C-terminal region of calsequestrin and the intraluminal loops of RyR. (A,B) In vitro binding assays were performed using (A) full-length CSQ-1 and (B) C-terminally deleted CSQ-1 (C17-CSQ-1). Intraluminal loops of RyRs [GST-RyR loop I (lanes 4, 5, 6) and GST-RyR loop II (lanes 7, 8, 9)] were tested for direct interaction in the presence of Ca²⁺ (lanes 4, 7; 0.5 mM Ca²⁺, lane 5, 8; 2 mM Ca²⁺) or absence of Ca²⁺ (lanes 6, 9; 1 mM EGTA). Lane 1, supernatant of CSQ-1 (A) or that of C17-CSQ-1 (B); lane 2, affinity beads alone; lane 3, GST control. (C) Simplified model of calsequestrin localization in C. elegans. Positively charged residues at the C-terminal end of C. elegans calsequestrin may directly interact with the negatively charged residues of the loops of RyRs to localize calsequestrin into the vicinity of the SR membrane. This could allow fast and localized release of Ca²⁺ through RyRs once the receptor is activated.

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