When it comes to nasty pathogens, Marburg virus is among the nastiest. Cousin to Ebola virus, Marburg causes fever, rash, delirium, and severe hemorrhaging, often ending in organ failure and death. It is rare in the wild, but was a central focus of weaponization by the Soviet Union, and remains a concern for terrorism experts who fear its lethal potential and resistance to treatment.
One reason that treatments have proved so elusive is because the virus is so hard to work with – hazmat suits, self-contained breathing gear, and electronically secured airlocks are all required for even the simplest of studies with live virus. But another reason is that the virion (the virus particle) is heterogeneous in shape, and that heterogeneity has confounded standard imaging techniques (X-ray crystallography, cryo-electron microscopy), which require purified, identical particles to obtain their highest resolution. Researchers have now got around that problem by using a sophisticated combination of imaging techniques that provide the first clear three-dimensional picture of the intact Marburg virion structure.
Marburg Virus Structure Revealed in Detail. (2011) PLoS Biol 9(11): e1001198. doi:10.1371/journal.pbio.1001198
Cryo-Electron Tomography of Marburg Virus Particles and Their Morphogenesis within Infected Cells. (2011) PLoS Biol 9(11): e1001196. doi:10.1371/journal.pbio.1001196
Several major human pathogens, including the filoviruses, paramyxoviruses, and rhabdoviruses, package their single-stranded RNA genomes within helical nucleocapsids, which bud through the plasma membrane of the infected cell to release enveloped virions. The virions are often heterogeneous in shape, which makes it difficult to study their structure and assembly mechanisms. We have applied cryo-electron tomography and sub-tomogram averaging methods to derive structures of Marburg virus, a highly pathogenic filovirus, both after release and during assembly within infected cells. The data demonstrate the potential of cryo-electron tomography methods to derive detailed structural information for intermediate steps in biological pathways within intact cells. We describe the location and arrangement of the viral proteins within the virion. We show that the N-terminal domain of the nucleoprotein contains the minimal assembly determinants for a helical nucleocapsid with variable number of proteins per turn. Lobes protruding from alternate interfaces between each nucleoprotein are formed by the C-terminal domain of the nucleoprotein, together with viral proteins VP24 and VP35. Each nucleoprotein packages six RNA bases. The nucleocapsid interacts in an unusual, flexible “Velcro-like” manner with the viral matrix protein VP40. Determination of the structures of assembly intermediates showed that the nucleocapsid has a defined orientation during transport and budding. Together the data show striking architectural homology between the nucleocapsid helix of rhabdoviruses and filoviruses, but unexpected, fundamental differences in the mechanisms by which the nucleocapsids are then assembled together with matrix proteins and initiate membrane envelopment to release infectious virions, suggesting that the viruses have evolved different solutions to these conserved assembly steps.