Figure 12. Proposed sequence of channel activation.
(a) Quiescent conformation of Orai prior to binding of STIM (from PDB ID 4HKR, cylinders and ribbons). The pore (gray surface) is closed and the latches are fastened (two latches are indicated with dashed ovals). The M4 helices are bent at Pro288, delineating them into M4a and M4b. The M4b portions (orange) interact with the M3 helices (green), in six-fold fashion, and prevent the pore from opening by constraining the cytosolic region of the M3 helices. The interaction between M4b and M3 and the bend at Pro288 are stabilized by three sets of paired M4-ext helices. (b) An unlatched-closed conformation: structure of WT Oraicryst in which the pore is closed but the latches are released. Conformations of M1-M4a are indistinguishable from (a) (Figure 8D). When unlatched, mobile M4-ext regions are hypothesized to be available to interact with cytosolic regions of STIM that would become exposed as a result of depletion of Ca2+ from the ER. Spontaneous unlatching would not necessarily require STIM binding and does not necessarily open the pore. (c) Open conformation. The structure of H206A Oraicryst is shown (cylinders and ribbons), with approximate dimensions of the pore shown as a gray surface. Following store depletion, we hypothesize that STIM (blue shapes) engages with cytosolic regions of Orai and stabilizes the pore in an open conformation. Unlatching is required to allow the widening of the pore and the influx of Ca2+ (green spheres). Arrows between conformations denote equilibria and the horizontal rectangle indicates approximate boundaries of the plasma membrane. The depiction of the cytosolic region of STIM is conceptual and is not meant to imply stoichiometry or conformation.
