Macromolecular electron microscopy
Macromolecular Structure by Electron Microscopy speakers. (left to right) Eva Nogales, Helen Saibil, B.V.V. Prasad, Bettina Bottcher, Vinzenz Unger, Richard HendersonThis session included talks on isolated macromolecular complexes in solution and in two-dimensional crystals. The single particle field has been developing rapidly and is a remarkable topic for the IUCr, since it does not involve crystallography, except in the sense that 3D alignment of randomly oriented complexes is crystallization in the computer. B.V.V. Prasad presented a structural analysis of rotavirus, a double-shelled virus caught in the process of transcribing and releasing its RNA through the 5-fold vertices of its icosahedral shell. With a series of low resolution reconstructions of different states of the chaperonin GroEL, H. Saibil mapped out the domain rotations in the ATPase cycle and interpreted them by docking domain structures into the EM maps. At 10 Å resolution, a new inter-ring contact, involved in negative cooperativity, was seen in GroEL-ATP. B. Bottcher described elucidating the first complete secondary structure for a protein in a single particle complex. The 7.4 Å resolution map of Hepatitis B virus core particles revealed the fold of the α-helical capsid protein, recently confirmed by an X-ray crystallographic structure.
Moving to electron crystallography, V. Unger presented the structure of a gap junction at 7 Å resolution, with two hexameric rings forming a channel across two cell membranes. The transmembrane regions were clearly identified as 4-helix segments, and a plausible model was built with a large β-barrel forming the extracellular seal of the channel. High quality image phase maps allowed a complete secondary structure determination of the tubulin dimer described by E. Nogales at 3.7 Å resolution with the localization of bound GTP and taxol. The fold is close to that of the bacterial protein, whose structure was determined crystallographically.