Ultrastructure of the Na, K-ion pump

Na, K-ATPase has been analysed by electron microscopy to obtain information about the structure of the enzyme and its organization within the membrane. Following negative staining the membrane-bound enzyme was observed as surface particles which on the basis of their size and frequency and the enzymatic and chemical composition of the membranes are interpreted as protomers (alpha beta-units). Freeze-fracture electron microscopy revealed the enzyme as intramembrane particles. Quantitative electron microscope studies suggested that the intramembrane particles are oligomers of the protein unit that forms the surface particles. Following reconstitution of the enzyme into phospholipid vesicles it was demonstrated that similar intramembrane particles represent a protein unit which transports sodium and potassium. Vanadate and magnesium induced the formation of two-dimensional crystals in the membrane fragments of the purified Na, K-ATPase. Further information regarding the shape and dimensions of the protomer was obtained through analysis of electron micrographs of negatively stained crystals with optical diffraction and image reconstruction methods.