Virus structure tutorial STEM

Introduction to Virus Structure Tutorial Jonathan King, Peter Weigele, Greg Pintilie, David Gossard (MIT) v.November, 2008 Virus Structure • Size – 17 nm – 3000 nm diameter • Basic shape – Rod-like – “Spherical” • Protective Shell - Capsid – Made of many identical protein subunits – Symmetrically organized – 50% of weight – Enveloped or non-enveloped • Genomic material – DNA or RNA – Single- or double-stranded Virus Structure • Virus capsids function in: – Packaging and protecting nucleic acid – Host cell recognition • Protein on coat or envelope “feels” or “recognizes” host cell receptors – Genomic material delivery • Enveloped: cell fusion event • Non-enveloped: more complex strategies & specialized structures Electron Microscopy Mitra, K. & Frank, J., 2006. Ribosome dynamics: insights from atomic structure modeling into cryo-electron microscopy maps. Annual review of biophysics and biomolecular structure, 35, 299-317. History • In 1953, Crick & Watson proposed … principles of virus structure – Key insight: • Limited volume of virion capsid => nucleic acid sufficient to code for only a few sorts of proteins of limited size – Conclusion: • Identical subunits in identical environments • Icosahedral, dodecahedral symmetry X-ray Crystallography of Viruses • Symmetry of protein shells makes them uniquely well-suited to crystallographic methods • Viruses are the largest assemblies of biological macromolecules whose structures have been determined at high resolution History con’t • In 50’s & 60’s Klug and others confirmed that several (unrelated) “spherical” viruses had icosahedral symmetry – (Used negative staining & electron microscopy) • Conclusion: – Icosahedral symmetry is preferred in virus structure Similarity to Buckminster Fuller’s Geodesic Domes Icosahedral Symmetry • 12 vertices • 20 faces (equilateral triangles) • 5-3-2 symmetry axes • 60 identical* subunits in identical environments can form icosahedral shell * asymmetric Caspar and Klug’s Icosahedral shell [...]... host cell types the virus can infect • Immunization against flu involves your immune system synthesizing antibody proteins that bind the HA protein Influenza virus composition of virus entry of influenza into cell Influenza hemagglutinin: a pH induced, conformationally controlled trigger for membrane fusion 100 Å displacemen of fusion peptide disordered loop low pH backbone is structured fusion peptide... extend like a spring during infection http://www.roche.com/pages/facets/10/viruse.htm http://hsc.virginia.edu/medicine/basic-sci/cellbio/jgruenke.html Trimer Structure • Long alpha helices form coiled coil structure • In mature trimers of HA0, each monomer is cleaved into HA1 and HA2 Evolution of dsDNA viruses • All known viruses, whether infecting bacteria or humans, may have evolved from a single... elsewhere • Predicts packing arrangements of larger capsids – Shift from T1 to T4 packing => 8-fold increase in volume Spherical viruses have icosahedral symmetry Homunculattice HK97 Asymmetric Unit Outside Inside Herpes Simplex Virus at 8.5 Å resolution from Wah Chiu and Frazer Rixon in Virus Research (200 Influenza • Infection depends on spike proteins projecting from capsid membrane called “Hemagglutinin... gold-bead labeled ant visualized by negative stain electron microscopy Bill Newcomb and Jay Brown, University of V Cryo-EM structure of purified Herpes portal prot Trus BL, Cheng N, Newcomb WW, Homa FL, Brown JC, Steven AC Structure and polymorphism of the UL6 portal protein of herpes simplex virus type 1 J Virol 2004 Nov;78(22):12668-71 ... ancestor, relatively early in the evolution of organisms Common steps in the assembly of all dsDNA viruses • Unique portal ring at one Vertex • Scaffolding proteins • Procapsid assembled empty of DNA • DNA pumped into procapsid through portal ring • DNA moves back through portal to enter cell P22 Pathway Herpes viruses also have a portal protein portal complex Herpes portal (UL6) tagged with gold-bead labeled... capsid – Using larger subunits helps very little – Using more subunits helps a lot • Not possible to form icosahedral shell (of identical units in identical environments) with more than 60 subunits • Viruses with more than 60 subunits were observed • Question: – How can >60 subunits form an icosahedral shell? – Will any number of subunits work? – If so, how would they be organized? Quasi-equivalence . Introduction to Virus Structure Tutorial Jonathan King, Peter Weigele, Greg Pintilie, David Gossard (MIT) v.November, 2008 Virus Structure • Size – 17. non-enveloped • Genomic material – DNA or RNA – Single- or double-stranded Virus Structure • Virus capsids function in: – Packaging and protecting nucleic acid – Host