1996 McCoy Award Recipient
Timothy S. Baker
Professor Biological Sciences

Through the Looking Glass: The Beauty of Viruses Revealed by Cryoelectron Microscopy

Timothy S. Baker was born in Hackensack, New Jersey. He obtained a B.S. in chemistry from Duke University and a Ph.D. in biochemistry from the University of California, Los Angeles. Baker did postdoctoral research at Cambridge and Brandeis universities. He joined the Purdue biological sciences department in 1983. Baker has chaired Gordon Conferences on "Three Dimensional Electron Microscopy of Macromolecules" and has served on the editorial boards of the Journal of Ultrastructure Research and the Journal of Structural8iology. He is a member of the Advisory Committee for the National Scanning Transmission Electron Microscope Facility at Brookhaven National Laboratory. Baker's research on structural studies of biological macromolecules is described in more than 125 publications.

Abstract of talk
Viruses are among the most widely known and studied pathogens. They infect virtually every living organism, including bacteria, plants, insects, and mammals. Hundreds of different viruses have been identified, and while some are deadly, many cause mild symptoms or no disease at all. Everyone knows of the AIDS virus and of other viruses such as rhino (common cold), herpes, and papilloma (warts). Hence, it is no surprise that viruses are the subject of intense biochemical, genetic, and structural investigations aimed at understanding the molecular basis of viral function.

This presentation will summarize several studies in the past decade aimed at getting close-up views of these beautiful beasts with the underlying goal of trying to better understand how viruses function. Cryo-electron microscopy (cryoEM) and image reconstruction procedures allow us to obtain highly magnified images and three-dimensional structures of spherical viruses Representative examples of recent studies will highlighted to illustrate the types of information that can be obtained with current technology. For example, an infectious virus must be able to recognize and attach to a specific host cell in spite of the body's defensive attempt (the immune response) to block or limit infection. Recent collaborative cryoEM and x-ray crystallographic studies of human rhinoviruses have revealed details of how these viruses interact both with anti bodies and with cellular receptors.

Research
Professor Baker's research involves the coordinated application of high resolution cryo-electron microscopy (cryoEM) and three-dimensional (3D) image reconstruction techniques to study virus structure and to answer questions about how viruses infect a wide range of hosts including animals, plants, fungi, and bacteria.

Recent results have been obtained with several viruses in which structural information from cryoEM has been combined with biochemical and structural information obtained in collaboration with x-ray crystallographers and molecular virologists at Purdue. These studies provide the first detailed views of molecular interactions between viruses and antibodies and cellular receptor molecules, and have provided new insights about how the immune system recognizes viruses and how viruses recognize and attach to host cells. CryoEM: Highly magnified images (30,000- 50,000X) of frozen-hydrated viruses are obtained in a transmission electron microscope (TEM). The cryoEM technique requires that a very thin sample of purified virus in a buffer solution be rapidly frozen to -160 degrees Centigrade (-256 degrees Fahrenheit) or below and transferred into the microscope where images are recorded. Because the electrons used to image the virus sample also quickly damage the sample, the images must be recorded with very few electrons. CryoEM techniques are popular since they provide an objective and direct approach to both preserve and observe the "native," hydrated structure of biological specimens. The resulting low-dose TEM images have extremely low contrast and computer analysis and processing thus become essential. 3D Image Reconstruction: TEM images (micrographs) often include a field of view containing 100 or more virus particles. The image of a single virus particle is a 2D picture or representation of the 3D virus sample, and hence generally does not contain enough information to reconstruct the 3D structure of the virus. The micrograph is digitized to enter the image data into a computer where a battery of image processing programs are used to combine data from many virus particles in order to reconstruct the 3D structure of each type of virus studied. Virus Structure/Function: The structure of a virus (or any molecule) and the nature of its environment dictate the way the molecule functions. Knowledge of structure is therefore essential to understanding how viruses work. Most viruses are large and often quite complex biological macromolecules. The simplest viruses consist of a highly symmetric coat of 60 identical protein subunits that encapsidate the genetic information (DNA or RNA) needed for the replication of progeny viruses. Complex viruses such as HIV contain thousands of protein subunits (not all identical) and also a lipid membrane and glycoproteins (proteins with sugar groups attached). Viral Life Cycle: Though there is no norm, each virus participates in a series of well- coordinated events during its life cycle. To initiate infection, viruses must first recognize and bind to the appropriate target (host) cell. Rhino-viruses, for example, only infect cells in the upper respiratory tract. Once attached to the cell, viruses deliver their nucleic acid genome (the 'genetic payload') to the interior of the cell via a variety of mechanisms. The host cell is then 'tricked' into producing up to thousands of new viruses through steps of replication, translation, and assembly. In a lytic infection progeny viruses exit the cell and infect neighboring cells or spread (e.g. by sneezing) to a new host. Fortunately, hosts with a functioning immune defense system can overcome most infections or circumvent future infections. The first line of defense often involves recognition of and binding to viruses by antibodies.