Virus Structure Characteristics of capsids Virus envelopes Virion assembly
Capsids package viral genomes and transmit them to a new host cell Capsid rigid, symmetrical container composed of viral protein Nucleocapsid capsid with enclosed genome Virion complete, infectious virus particle possibly including an envelope Many virus capsids are surrounded by a lipid bilayer called an envelope
Virions are studied by electron microscopy and X-ray diffraction Photo courtesy of Jean-Yves Sgro, University of Wisconsin, Madison
Many viruses come in simple, symmetrical packages Composed of many copies of identical subunits (genomic economy) Can assemble spontaneously (self-assembly) Identical subunits give the capsid symmetry Shape = the geometry of its outline Symmetry = rotational and translation operations that describe it
Some viral capsids have icosahedral symmetry Icosahedron = a shape with 20 triangular faces Icosahedral capsids have fivefold, threefold and twofold rotational axes.
Simple capsids have repeating subunits with identical interactions. More complex capsids have repeating subunits interacting in a quasi-equivalent manner
How many subunits can be accommodated on the capsid surface? Quasi-equivalent icosahedral structures are composed of subunits present in multiples of 60 (60 x T). Triangulation number (T) can be determined by: T = h 2 + hk + k 2
Larger viruses come in more complex packages Capsids contain two types of structural subunits: hexons and pentons
Other structures, large and small, display icosahedral symmetry
Capsids with helical symmetry are organized as helical tubes composed of identical, repeating subunits
The symmetry of helical capsids is described by: The number of subunits per turn (µ) The displacement along the helical axis between one subunit and the next (p) The Pitch of the helix (P) is the distance along the axis corresponding to one turn P = µ x p
Virus envelopes Viral envelopes are derived from cellular membranes and are composed of: Lipid bilayers (with the same composition as the cellular membrane from which they were derived) Viral glycoproteins Envelopes can adopt a variety of shapes
Virus envelopes Viral envelopes are obtained by a process called budding Viral glycoproteins are inserted into membrane Nucleocapsids associate with glycoproteins and get wrapped in membrane
Virus envelopes Multiple modes of capsid assembly exist depending on: Size, shape and complexity of capsid Genomic composition May require scaffolding proteins Assist with formation of pro-capsid Are not included in final, mature virion
Virus envelopes Specific packaging signals direct incorporation of viral genomes into virions Core proteins may accompany the viral genome inside the capsid Formation of viral envelopes by budding is driven by interactions between viral proteins
Disassembly of virions Virions are primed to enter cells and release their genomes The release of the viral genome can occur by multiple mechanisms: Proteolytic cleavage of capsid proteins Unspooling of genome into cell Interaction of genome with cytoplasmic components
Fundamental concepts Viral genomes are enclosed in protein capsids. Some viruses also have a lipid envelope Small viruses have highly symmetrical capsids Icosahedral capsids have 5-, 3- and 2-fold axes of symmetry Icosahedral capsids are constructed with 60 x T subunits (where T is the triangulation number) Tubular capsids have helical symmetry Envelopes are derived from cellular membranes Scaffolding proteins and packaging sequences assist in the assembly of viruses
Key Terms Buckminsterfullerene Budding Capsid Core proteins Cryoelectron microscopy Ectodomain Envelope Genetic economy Helical symmetry Hexon Icosahedral symmetry Jelly-roll ß -barrel Matrix protein Myristate Negative staining Nucleocapsid Packaging sequence Palmitoylation Penton Positive staining Pseudovirion Quasi-equivalent Scaffolding proteins Self-assembly Signal sequence Synchrotron Transmembrane anchor Triangulation number Twofold axis Type I integral membrane protein Type II intregal membrane protein Virion X-ray diffraction