Fig. 6: The Ca²⁺ transport pathway of hSPCA1.

a The open cytosol-facing cavity of hSPCA1 in the CaE1 state. TM1, TM2, TM4, and TM6 are colored medium blue, saddle brown, forest green, and magenta, respectively. Water molecules and Ca²⁺ are shown as small red balls and the lime sphere, respectively. The black arrow indicates the possible Ca²⁺ entry pathway. b The closed cytosol-facing cavity of hSPCA1 in the CaE2P state. The red arrow (bottom) shows the distance from the Cα of Pro82 in the CaE1 state to the Cα of Pro82 in the CaE2P state. c The closed cytosol-facing cavity and open lumen-facing cavity of hSPCA1 in the early E2P state. The black arrow indicates the Ca²⁺ release pathway. d The semi-open cytosol-facing cavity and closed lumen-facing cavity of hSPCA1 in the E2~P state. The Ca²⁺-binding residues are colored forest green, magenta, or black. The surfaces are shown with colors ranging from dark cyan (most hydrophilic) to white (intermediate) to dark goldenrod (most lipophilic).