For many viruses, the final stage of assembly involves structural transitions that convert an innocuous precursor particle into an infectious agent. The three-dimensional structure of the P22 tail machine determined by electron cryo-microscopy and image reconstruction reveals how the five types of polypeptides present as 51 subunits are organized into this molecular machine through twelve-, six- and three-fold symmetry, and provides insights into molecular events during host cell attachment and phage DNA translocation. As human sensory equipment relies heavily on visual input, our brains find it much easier to process information from an image—the question is how to “represent data in a way that humans can analyse and understand it,” according to Forbes Dewey, Professor of Mechanical Engineering and Bioengineering at the Massachusetts Institute of Technology (Boston, MA, USA) and co-director of the International Consortium for Medical Imaging Technology. Using SDS-PAGE and mass spectrometry to analyze wild-type capsids, UL25-null capsids, and denaturant-extracted capsids, we conclude that (i) the C-capsid-specific component is a heterodimer of UL25 and UL17; and (ii) capsids have additional populations of UL25 and UL17 that are invisible in reconstructions because of sparsity and/or disorder. neapolitanus carboxysomes were re-examined by cryo-electron tomography and scanning transmission electron microscopy (STEM). Electron microscopic analysis demonstrated that, compared to capsids lacking a tegument, these capsids (called T36 capsids) had tufts of protein located at the vertices. During productive infection, newly synthesized viral DNA enters preformed capsids.
Cryo-electron tomography is opening new vistas in molecular and cellular structural biology (Baumeister and Steven, 2000; Subramaniam and Milne, 2004). Our structure represents a unique bacteriophage reconstruction with its tail fibers and ligand-binding domains shown in relation to its tail apparatus. The idea of their free occurrence within the crista—or inner—membrane became progressively replaced by the concept of dedicated supercomplexes, which were proposed to be stable interactions of single complexes. In addition, some of the products presented a density layer underlying and resolved from the viral membrane, which may represent detachment of the matrix protein to facilitate the fusion process. Many viruses also incorporate single-copy proteins that function as movement, maturation, or infectivity proteins. Like all enveloped viruses, herpesviruses enter cells by fusing their envelopes with the host membrane. A thin film layer of glass-like ice is formed in the holes so the molecules or objects of interest smaller than the film thickness can be studied suspended in their original medium.
As with other herpesviruses, the virion of herpes simplex virus 1 (HSV-1) is composed of a nucleocapsid that is surrounded by an amorphous layer of proteins called the tegument and enclosed in an envelope studded with glycoproteins (1, 2). The viral genome is packaged into the procapsid through a unique portal vertex (5,–7). Dr. Because radiation damage puts stringent limitations on the electron dose one can use for imaging, the signal-to-noise ratio in these images is low, and one needs to average over many images. Whereas the overall topology of the process of herpesvirus nuclear egress resembles cellular vesicle trafficking, little is known about the nanostructural details that lead to formation, scission, and fusion of INM-derived vesicles. This multivalent binding enhances BPP-1 affinity compared to purified Mtd in a receptor binding pattern characterized by avidity (4). The random conical tilt data collection method could be used to compensate for the lack of randomness in orientations, by tilting the grids in the microscope to one specific angle (8).
We will show that a single cell produces ~2700 enveloped R7041∆US3 virions in average.