Renal, Na,K-ATPase structure from cryo-electron microscopy of two-dimensional crystals.

The molecular structure of Na,K-ATPase was determined by electron crystallography from two-dimensional crystals induced in purified membranes isolated from the outer medulla of pig kidney. The P2 type unit cell contains two protomers in the E2 conformation, each of them with a size of 65 x 75 x 150 Å3. The , ß, and subunits in the membrane crystals were demonstrated in the crystals with Western blotting and related to distinct domains in the density map. The subunit corresponds to most of the density in the transmembrane region as well as to the large hydrophilic headpiece on the cytoplasmic side of the membrane. The headpiece is divided into three separated domains. One of these gives rise to an elongated projection onto the membrane plane, while the putative nucleotide binding and phosphorylation domains form compact densities in the rest of the cytoplasmic part of the structure. Density on the extracellular face corresponds to the protein part of the ß subunit. Ten helices from the catalytic a subunit correspond to two groups of distinct densities in the transmembrane region. The structure of the lipid bilayer spanning part also suggests positions for the transmembrane helices from the ß and subunits. The overall structure of the subunit of Na,K-ATPase as determined here by cryo-electron microscopy is similar to the X-ray structure of Ca-ATPase. However, conformational changes between the E1 and E2 forms are suggested by different relative positions of cytoplasmic domain