High-resolution single particle electron cryomicroscopy of membrane protein complexes

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BACKGROUND: Cell membranes differentiate a cell from its external environment. Proteins that reside within cell membranes, known as membrane proteins, serve as the gateway to cells. These proteins transport chemicals across membranes and allow cells to communicate with each other and sense their external environment. Understanding how cells carry out these activities requires determining the 3-D structures of membrane proteins. Unfortunately, it is often difficult or impossible to determine the structures of membrane proteins by traditional methods such as X-ray crystallography or nuclear magnetic resonance spectroscopy. Single particle electron cryomicroscopy (cryo-EM) is a method uniquely capable of investigating the structures of large membrane proteins. In cryo-EM, proteins are flash frozen and imaged with an electron microscope. Computational image analysis allows information from many images to be combined into a 3-D map of the protein. We, and one or two other groups, have been able to use single particle cryo-EM to obtain resolutions of ~ 1 nm (one millionth of a millimeter) when studying membrane proteins. At these resolutions, the features that explain how the proteins function start to become visible.