Synthetic partial agonists reveal key steps in IP(3) receptor activation

Inositol 1,4,5-trisphosphate receptors (IP(3)R) are ubiquitous intracellular Ca(2+) channels. IP(3)binding to the IP(3)-binding core (IBC) near the N-terminal initiates conformational changes that lead to opening of a pore. The mechanisms are unresolved. We synthesized 2-O-modified IP(3) analogues that are partial agonists of IP(3)R. These are like IP(3) in their interactions with the IBC, but they are less effective than IP(3) in rearranging the relationship between the IBC and N-terminal suppressor domain (SD), and they open the channel at slower rates. IP(3)R with a mutation in the SD occupying a position similar to the 2-O-substituent of the partial agonists has a reduced open probability that is similar for full and partial agonists. Bulky or charged substituents from either the ligand or SD therefore block obligatory coupling of the IBC and SD. Analysis of ΔG for ligand binding shows that IP(3) is recognised by the IBC and conformational changes then propagate entirely via the SD to the pore.