General Colloquium:
January 27 - 4:00pm Phys 223
(Coffee at 3:30p.m. in room 242)

Professor Timothy S. Baker
Purdue University
Department of Biological Sciences

Title: "Electron Cryo-Microscopy and Image Reconstruction of Viruses - More than Meets the Eye"

Abstract

Electron cryo-microscopy (cryoEM) and three-dimensional (3D) image reconstruction techniques provide a powerful means to explore how viruses infect cells of a variety of hosts, including those from animals, plants, fungi, algae, and bacteria. These techniques have revealed the 3D structures of many viruses at low resolution (20-30Å), which has provided new insights about virus recognition of host cells, virus assembly, and the molecular mechanisms by which host antibodies recognize and neutralize viruses.

CryoEM techniques involve the preparation of thin, unstained specimens suitable for transmission electron microscopy. Aqueous samples are vitrified in a very thin layer (1000-2000 Å thick) by plunging the specimen into liquid ethane. The specimen is then maintained at -160°C or below in the microscope while images are recorded under low-irradiation conditions (<10e-/Å2) to minimize electron beam damage to the specimen. Micrographs are digitized and analyzed by computer to combine information from hundreds or thousands of individual particle images to reconstruct the 3D structure of each virus. CryoEM is popular because it provides a direct, objective approach to observe the "native" (fully hydrated) structure of biological specimens.

The seminar will highlight the principles of the cryoEM and image reconstruction techniques and also provide a brief sampling of many of the ongoing studies performed with a variety of different viruses. With some viruses, information obtained at low resolution (~20Å) from cryoEM has been combined with higher resolution information (3-4 Å) from X-ray crystallography. Such studies provide a detailed view of the molecular interactions that occur between viruses and other macromolecules such as antibodies and cellular receptor molecules. A current "grand challenge" in structural biology is to develop the means to conduct cryoEM analyses at much higher resolution (better than 10Å) with state-of-art instrumentation and computing. If time permits, some of the ongoing attempts to meet these challenges will be identified.