Ligand-induced conformational changes via flexible linkers in the amino-terminal region of the inositol 1,4,5-trisphosphate receptor.

Cytoplasmic Ca2+ signals are highly regulated by various ion transporters, including the inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R), which functions as a Ca2+ release channel on the endoplasmic reticulum membrane. Crystal structures of the two N-terminal regulatory regions from type 1 IP(3)R have been reported; those of the IP(3)-binding core (IP(3)R(CORE)) with bound IP(3), and the suppressor domain. This study examines the structural effects of ligand binding on an IP(3)R construct, designated IP(3)R(N), that contains both the IP(3)-binding core and the suppressor domain. Our circular dichroism results reveal that the IP(3)-bound and IP(3)-free states have similar secondary structure content, consistent with preservation of the overall fold within the individual domains. Thermal denaturation data show that, while IP(3) has a large effect on the stability of IP(3)R(CORE), it has little effect on IP(3)R(N), indicating that the suppressor domain is critical to the stability of IP(3)R(N). The NMR data for IP(3)R(N) provide evidence for chemical exchange, which may be due to protein conformational dynamics in both apo and IP(3)-bound states: a conclusion supported by the small-angle X-ray scattering data. Further, the scattering data show that IP(3)R(N) undergoes a change in average conformation in response to IP(3) binding and the presence of Ca2+ in the solution. Taken together, these data lead us to propose that there are two flexible linkers in the N-terminal region of IP(3)R that join stably folded domains and give rise to an equilibrium mixture of conformational sub-states containing compact and more extended structures. IP(3) binding drives the conformational equilibrium toward more compact structures, while the presence of Ca2+ drives it to a more extended set.