Calexcitin interaction with neuronal ryanodine receptors

Calexcitin (CE), a Ca2+- and GTP-binding protein, which is phosphorylated d uring memory consolidation, is shown here to co-purify with ryanodine recep tors (RyRs) and bind to RyRs in a calcium-dependent manner. Nanomolar conce ntrations of CE released up to 46% of the Ca-45 label from microsomes prelo aded with (CaCl2)-Ca-45. This release was Ca2+-dependent and was blocked by antibodies against the RyR or CE, by the RyR inhibitor dantrolene, and by a seven-amino-acid peptide fragment corresponding to positions 4689-4697 of the RyR, but not by heparin, an Ins(1,4,5)P-3-receptor antagonist. Anti-CE antibodies, in the absence of added CE, also blocked Ca2+ release elicited by ryanodine, suggesting that the CE and ryanodine binding sites were in r elative proximity. Calcium imaging with bis-fura-2 after loading CE into hi ppocampal CA1 pyramidal cells in hippocampal slices revealed slow, local ca lcium transients independent of membrane depolarization. Calexcitin also re leased Ca2+ from liposomes into which purified RyR had been incorporated, i ndicating that CE binding can be a proximate cause of Ca2+ release. These r esults indicated that CE bound to RyRs and suggest that CE may be an endoge nous modulator of the neuronal RyR.