docPrinciples of Virology
download 
Report Transcription
Principles of Virology
Welcome to the lecture series Principles of Virology Thomas Kietzmann Principles in Virology Thomas Kietzmann TK Virus structure and entry into cells Viral genomes must be protected! • Assembly of a stable, protective protein shell • Specific recognition and packaging of the nucleic acid genome • Interaction with host cell membranes to form the envelope This is the task of virus envelope and core proteins 1 Principles in Virology TK Thomas Kietzmann Structure principles of virions Virion with envelope Virion without envelope Capsid proteins Capsid (core) proteins Virion with envelope Envelope proteins (virus) ssRNA dsDNA Membrane protein (host) Nucleoproteins Nucleocapsid Icosaeder Principles in Virology Nucleoproteins Membrane lipid bilayer (host) Icosaeder Thomas Kietzmann Capsid proteins Helix TK Functions of viral envelope proteins Delivery of the genome • Bind host cell receptors • Fusion with cell membranes • Uncoating of the genome • Transport of genome to the appropriate site • Induction of an immune response in the host 2 Principles in Virology Thomas Kietzmann TK Terminology • Subunit, single folded polypeptide chain • Structural unit (protomer, asymmetric unit), Unit from which capsids or nucleocapsids are built; one or more subunits • Capsid capsa is Latin for box, Protein shell surrounding genome • Nucleocapsid (core), Nucleic acid-protein assembly within virion • Envelope (viral membrane), Host cell-derived lipid bilayer • Virion, Infectious viral particle Principles in Virology Thomas Kietzmann TK Virions are metastable • Must protect the genome (stable) • Must come apart quickly on infection (unstable) • An infectious virion is a molecular machine - Has moving parts and does work 3 Principles in Virology Thomas Kietzmann TK Virion structure and function Structure (the virion) • Created by symmetrical arrangement of many identical proteins to provide maximal contact and non‐covalent bonding Function (genome delivery) • Structure is not usually permanently bonded together • Can be taken apart or loosened on infection to release or expose genome Principles of building virions: Symmetry is key Principles in Virology Thomas Kietzmann TK The symmetry rules They are elegant in their simplicity They provide key information for “self-assembly” Rule 1: • Each subunit has ‘identical’ bonding contacts with its neighbors • Repeated interaction of chemically complementary surfaces at the subunit interfaces naturally leads to a symmetric arrangement Rule 2: • These bonding contacts are usually non-covalent Reversible; error-free assembly 4 Principles in Virology Thomas Kietzmann TK Symmetry and self-assembly Many capsid proteins can self assemble into ‘virus-like particles’ (VLPs) The HBV and HPV vaccines are VLPs made in yeast Principles in Virology Thomas Kietzmann TK Helical symmetry Proteins form together with the viral genome a stable structure from a single protein subunit 5 Principles in Virology Thomas Kietzmann TK Enveloped virions of RNA viruses with (-) ssRNA genomes with helical capsids • Paramyxoviridae (measles virus, mumps virus) • Rhabdoviridae (rabies virus) • Orthomyxoviridae (influenza virus) • Filoviridae (Ebola virus) The nucleocapsid is the nucleic acid-protein assembly that is packaged within the virion Principles in Virology Thomas Kietzmann TK What about round capsids? All round capsids have precise numbers of proteins; multiples of 60 are common (60, 120, 180, 240, 960) Spherical viruses come in many sizes, but capsid proteins are 20-60 kDa average Capsids are icosahedrons Capsid subunits tend to be arranged as hexamers and pentamers Icosahedron: solid with 20 equilateral triangular faces; this allows formation of a closed shell with smallest number (60) of identical subunits 6 Principles in Virology TK Thomas Kietzmann Triangulation number, T The number of facets per triangular face of an icosahedron Each facet contains a capsid protein multimer Combining several triangular facets allows assembly of a larger face from same structural unit Capsids with T>1 have a 6-fold axis of symmetry Principles in Virology Thomas Kietzmann TK Capsids are icosahedrons • Made of 60 identical protein subunits • The protein subunit is the structural unit • Interactions of all molecules with their neighbors are identical (head-to-head, tail-to-tail) 7 Principles in Virology Thomas Kietzmann TK Capsids are icosahedrons • Three modes of subunit packing (orange, yellow, purple) • Pentamers & hexamers • Bonding interactions are quasiequivalent: all engage tail-to-tail and head-to-head 180 identical subunits Principles in Virology Thomas Kietzmann TK Quasiequivalence When a capsid contains more than 60 subunits, each occupies a quasiequivalent position The non-covalent binding properties of subunits in different structural environments are similar, but not identical 8 Principles in Virology Thomas Kietzmann TK Quasiequivalence Johnson, J.E. & Speir, J.A. (1997) Quasi-equivalent Viruses: A Paradigm for Protein Assemblies JMB, 269, 665-675 Caspar DLD, Klug A. Physical principles in the construction of regular viruses. Cold Spring Harbor Symp Quant Biol 1962;27:1. Principles in Virology T=1 (h,k) = (1,0) Thomas Kietzmann T=3 (h,k) = (1,1) T=4 (h,k) = (2,0) TK T=7 (h,k) = (2,1) 9 Principles in Virology TK Thomas Kietzmann Complex capsids Adenovirus • 150 nm • T=25 capsid, made of 720 copies of viral protein II + 60 copies of protein III • Fibers at 12 vertices Principles in Virology TK Thomas Kietzmann Complex capsids with two layers VP7 trimers, T=13 VP3 monomers, T=2 Reoviruses •T=13 •70 - 90 nm •two concentric shells 10 Principles in Virology Thomas Kietzmann TK Enveloped virions Capsids can be covered by host membranes • Envelope is a lipid bilayer derived from host cell Viral genome does not encode lipid synthetic machinery • The envelope is acquired during budding Can be any cell membrane, but is virus-specific • Nucleocapsids inside may be helical or icosahedral Principles in Virology Thomas Kietzmann TK Envelope proteins • Integral membrane glycoproteins • Ectodomain: atachment, antigenic sites, fusion • Internal domain: assembly • Oligomeric: spikes 11 Principles in Virology Thomas Kietzmann TK Herpesviridae • • • • 80 viral genes, >50% encode proteins of 2,000 Å virions Envelope proteins (13) Icosahedral nucleocapsid surrounding DNA genome (4) Tegument (20) --‐ delivery of proteins required early in infecIon Principles in Virology Thomas Kietzmann TK Herpes simplex capsid Holes for entry and exit of DNA The portal or opening for viral DNA is built at ONE of the 12, 5-fold vertices of the T=16 herpesvirus capsid 12 Principles in Virology Thomas Kietzmann TK Other virion components Enzymes • • • • • • polymerases, integrases, associated proteins proteases poly(A) polymerase capping enzymes topoisomerase RNAse Activators, components required for efficient infection Cellular components • Histones • tRNAs • Myristate • Lipid • cyclophilin A and many more Principles in Virology Thomas Kietzmann TK Initiation of infection 1. Virions initially randomly interact with cells – no specificity (Virions are inanimate: Driven by Brownian motion, laws of diffusion and electrostatics) 2. Attach to specific receptor molecules on cell surface - More than one receptor may be involved 3. This enables transfer of the genome into the cell 13 Principles in Virology Thomas Kietzmann TK Virion receptors • Essential for all viruses except those of yeasts (no extracellular phases) and plants (enter cells by mechanical damage) • Receptors and co-receptors • Different virions can bind the same receptor Adenovirus and Coxsackievirus B3 have a common primary receptor: The swine herpesvirus binds the same receptor as human poliovirus Viruses of the same family may also bind different receptors Picornavirus receptors Principles in Virology Thomas Kietzmann TK Two modes of viral entry of enveloped virions 14 Principles in Virology Thomas Kietzmann Sialic acid: Receptor for influenza virus: TK Sialic acids: N-acetylneuraminic acid; 9-O-acetyl-N-neuraminic acid α(2,6) preferentially used by human strains, α(2,3) by avian Principles in Virology Thomas Kietzmann TK Two modes of viral entry of non-enveloped viruses 15 Principles in Virology TK Thomas Kietzmann Viral endocytosis and intracellular transport Clathrin independent Principles in Virology Clathrin-dependent Thomas Kietzmann TK Receptors are critical for endocytosis of enveloped virions 16 Principles in Virology Thomas Kietzmann TK Fusion is regulated • Must not occur in the wrong location • Proteolytic cleavage activates the fusion protein for cleavage (class I) • Cleavage of a second protein (class II) activates the fusion protein • Low pH triggers fusion Principles in Virology Thomas Kietzmann TK 17 Principles in Virology Thomas Kietzmann TK Key terms and questions Why are virions considered to be metastable? What is quasiequivalence and what is the triangulation number? What is the advantage that capsid and or core proteins are noncovalently connected? Describe the two modes that enveloped virions use for entry into their host cells? How do non-enveloped viruses enter their host-cells? Simple virus capsids are found in two types of structural arrangements: helical and icosahedral. What are the key featues in the assembly of these two kinds of particles? 18
Common infection disease,
transmitted by sexual contact.
