Coexistence of different forms of Na,K-ATPase in two-dimensional membrane crystals

Two-dimensional membrane crystals of renal Na,K-ATPase were analyzed by electron microscopy and image processing. The particular property of the crystals in this work was that they showed unit cell parameters similar to the previously studied p21 crystals but lacked the dyad axis as observed in nominal 0 degrees-projections. A three-dimensional reconstruction revealed that structural differences between alpha beta-units of the enzyme gave rise to the asymmetry. A high degree of two-fold rotational symmetry was observed in the middle of the structure while the protein units had different three-dimensional shapes at levels above and below the central sections. The simultaneous coexistence of different forms of Na,K-ATPase suggests that the conformational flexibility of the enzyme plays an important role in the pumping process.