Figure 1. Comparative analysis of SERCA‒SLN and SERCA‒PLB crystal structures.

(A) RMSD for each TM helix in SERCA‒SLN crystal structures 3w5a⁶ (purple) and 4h1w⁷ (pink) was calculated relative to TM helices in the SERCA‒PLB crystal structure 4kyt²². The inset shows RMSD of M4S4 divided into luminal and cytosolic segments R290-A306 (lum) and P312-K329 (cyto). (B) Superposed crystal structures of SERCA‒SLN (purple, pink) and SERCA‒PLB (green) reveal that SLN, but not PLB, mediates a large increase in the axial tilt angle (orange dashes) of M4S4. SERCA residue E309 (red spacefill) is the Ca²⁺ binding residue in transport site II in the TM domain that transmits the ‘full-occupancy’ signal to initiate ATP hydrolysis and phosphoenzyme formation at D351 in the phosphorylation domain.^23,24