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Cover  These micrographs, taken in the laboratory of Humberto Fernandez-Moran, were among the first to reveal the complexity of structure in mitochondrial membranes. Shown here are mitochondria from bovine heart, which were isolated on a sucrose gradient and prepared by surface spreading on 1% potassium phosphotungstate, pH 7.2, without prior fixation. The mitochondria are obviously disrupted, yet the lower-magnification image on the left shows an external membrane that remains intact in some places and many internal membranes or "cristae," both in place and spilling out into the surrounding space (arrow at top of left panel).

These images are from a study that correlated electron microscopic and biochemical data to define a submitrochondrial particle on the surface of the cristae membranes. In this paper (Fernandez-Moran, H., Oda, T., Blair, P.V., and Green, D.E. [1964]. J. Cell Biol. 22, 63-100) the authors identified regularly arrayed projections from the membranes, which they called "elementary particles." These structures consisted of 1) a spherical or polyhedral head piece (80-100 Å units in diameter), 2) a cylindrical stalk (~50 Å long and 30-40 Å wide, and 3) a base piece (40 × 110 Å). These features are evident in the micrograph on the right (EP and arrows in the right panel), which shows the area indicated by the arrow at the top of the panel on the left at a magnification of approximately ~420,000×.

E. Racker and colleagues later showed that the stalk and base of the elementary particle includes a transmembrane domain, which carries protons across the membranes of the cristae (the F0 portion of the molecule). The head comprises the F1 ATPase, which synthesizes ATP when protons pass through F0 down an electrochemical gradient. Both the F0 and F1 portions are composed of multiple protein subunits.

These images of the elementary particles on the mitochondrial cristae clearly demonstrated the asymmetric orientation of membrane proteins. Images like these initiated the intense biochemical investigations that have led to our deeper understandng of the ways in which chemo-osmosis is coupled to ATP synthesis to accomplish efficient oxidative phosphorylation in a wide range of cells. The structure of the elementary particles opened people's minds to the complexity and asymmetric orientation of membrane proteins and helped initiate study of oligomeric protein complexes within membranes more broadly. The figure is reproduced from The Journal of Cell Biology, 1964, 22, 63-100 by copyright permission of The Rockefeller University Press.---Richard McIntosh and Kathryn Howell
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