Helical crystallization of soluble and membrane binding proteins

Helical protein arrays offer unique advantages for structure determination by cryo-electron microscopy (cryo-EM). A single image of such an array contains a complete range of equally spaced molecular views of the underlying protein subunits, which allows a low-resolution, isotropic three-dimensional (3D) map to be generated from a single helical tube without tilting the sample in the electron beam as is required for two-dimensional (2D) crystals. Averaging many unit cells from a number of similar tubes can improve the signal-to-noise ratio and consequently, the quality of the 3D map. This approach has yielded reconstructions that approach atomic resolution [Miyazawa et al., 1999, 2003; Sachse et al., 2007; Unwin, 2005; Yonekura et al., 2005]. Proteins that naturally adopt helical protein arrays, such as actin and microtubules, have been studied for decades. The wealth of information on how proteins bind and move along these cytoskeletal tracks, provide cross-talk between tracks, and integrate into the cellular machinery is due, in part, to multiple EM studies of the helical assemblies. Since the majority of proteins do not spontaneously form helical arrays, the power of helical image analysis has only been realized for a small number of proteins. This chapter describes the use of functionalized lipid nanotubes and liposomes as substrates to bind and form helical arrays of soluble and membrane-associated proteins.