Figure 10. Unlatching is necessary for pore opening.

(a) Quiescent conformation (PDB ID: 4HKR) highlighting interactions between M4b and M3. The channel is shown in cartoon representation. On one of the Orai subunits, amino acids in the interface between M4b (orange) and M3 (green), which is highlighted by a dashed oval, are shown as sticks and colored accordingly. This interface exists for all six subunits and is stabilized by the pairing of M4-ext helices (pink). An arrow denotes the movement of M4b between (a) and (b). (b) Conformation of WT Orai_cryst showing the released latches and closed pore. The amino acids that had been in the interface between M4b and M3 in the quiescent conformation are drawn as sticks, with the dashed region showing the same region as in (a). (c) Open conformation (H206A Orai_cryst), depicted as in (b). (d) Unlatching is necessary for pore opening. A hybrid atomic model of the channel was generated using the conformations of the M1, M2 and M3 helices from the open conformation and the conformations of M4a/b and M4-ext from the quiescent conformation. In this model (shown in cartoon representation), molecular clashes exist between M4b and M3. Amino acids involved in the clashes are depicted as space-filling spheres (colored according to channel region: red, M2; green, M3; yellow, M4a; orange, M4b; pink, M4-ext). These steric hindrances would prevent opening of the pore while the latches are fastened. With unlatching, the repositioning of the M4b helix would allow the outward motion of M1-M3 that opens the pore.