The envelope glycoproteins (Env) of human and simian immunodeficiency viruses mediate virus binding to the cell surface receptor (CD4) on target cells to initiate infection (Figure 1). Using cryo-electron tomography combined with 3-D image classification and averaging, Sriram Subramaniam, Ph.D., head of the biophysics section in the laboratory of cell biology at the NCI and his colleagues have reported on the 3-D structure of Env.
By fitting known crystal structures into the density maps derived by electron tomography, they derived molecular models for the native Env, unbound and bound to the surface receptor. They demonstrated that binding results in a major reorganization of Env, leading to closer contact between the viral and target cell membranes.
The SARS coronavirus (SARS-CoV) spike is the largest known viral spike molecule (Figure 2). Daniel Beniac, Ph.D.’s group at the Public Health Agency of Canada are using cryo-EM and image processing (single-particle analysis) to investigate conformational changes that occur in the entire spike of intact virions when they bind to the viral receptor. They have shown that ACE2 binding results in structural changes that appear to be the initial step in viral membrane fusion. The SARS-CoV spike provides an ideal model system to study receptor binding and membrane fusion in the native state.
Investigators in the lab of Hong Zhou, Ph.D., professor of microbiology, immunology, and molecular genetics at UCLA, have reported on the 3-D structure of cytoplasmic polyhedrosis virus with 3.88 Å resolution using cryo-EM and single-particle analysis (Figure 3). This is reportedly the highest obtained resolution so far using cryo single-particle analysis. They confirmed the resolution of their analysis by identifying characteristic protein structures with well established dimensions such as the 5.8 Å pitch of an α helix and the 3.8 Å spacing along the polypeptide chains in an α sheet.
Recent developments in cryo electron microscopy—single-particle analysis and dual-axis tomography—have made it possible to investigate the 3-D structure of viral proteins and the complex interactions of these proteins with other macromolecular structures such as the molecular epitopes that are recognized by the immune system.
Cryo-EM has enabled important discoveries about the relationship of structure and function in the complex processes involved in viral replication, assembly, and infection. It has been shown to be a reliable and complementary adjunct to x-ray and NMR analysis. Unlike x-ray and NMR, which must look at large numbers artificially produced and purified proteins under conditions that are far from physiological, cryo-TEM offers the advantage of looking at the total functional entity in its natural context, thereby providing a unique and powerful window on the molecular machinery of biological systems.