An endogenous sulfated inhibitor of neuronal inositol trisphosphate receptors.

In cerebellum, inositol trisphosphate- (InsP(3)-) gated Ca channels play a key role in learning, though they exhibit a low sensitivity to InsP(3) compared to peripheral tissues. In the present study, the cerebellar InsP(3) receptor is shown to be associated with a novel inhibitor of InsP(3) binding. (3)H-InsP(3) binding studies indicated that this inositol trisphosphate receptor inhibitor (IRI) could completely inhibit InsP(3) binding to the purified cerebellar InsP(3) receptor and acted as a competitive inhibitor. Gel filtration of IRI showed a predominant peak at 6500 Da, though this peak appeared to be an aggregate (with a monomeric molecular mass of approximately 1500 Da). Mass spectrometry of IRI showed a predominant peak at 1635 m/z, consistent with this low molecular mass estimate. The inhibitory activity of IRI was prevented by pretreatment with aryl sulfatase, suggesting the presence of a critical sulfo ester in IRI. IRI was insensitive to proteases and organic extraction but bound to concanavalin A, suggesting that IRI is a sulfated glycan. IRI was present in cerebellum but below the level of detection in aorta. IRI was also present in the neuronal cell line N1E115 (which exhibits a low sensitivity to InsP(3)). We conclude that IRI is a novel endogenous sulfated inhibitor of the InsP(3) receptor that modulates the sensitivity of the InsP(3) receptor and thus may explain the low InsP(3) sensitivity of neurons.