5 - Virus Attachment, Entry and Uncoating Flashcards
Host range of virus
The variety of different species that the virus can infect
Tissue tropism
Different tissue or cell types that a virus infects once it is inside a susceptible animal
Forces responsible for viral attachment to host cells
- Noncovalent intermolecular forces (e.g. ionic bonds, H bonds, london dispersion forces) between a virus spike protein and its cellular receptor
- Shape complementarity is essential for attachment
- First contacts are weak binding (through nonspecific electrostatic interactions) and thus reversible
Why is the second stage of attachment irreversible
Because the consequences of irreversible attachment, which are penetration and uncoating, do not ever proceed backwards
Dissociation constant (KD)
- Measure of the strength of interaction between two molecules
- Typically 1x10-9 or less)
Affinity
The strength of the binding interaction between a single molecule and its ligand.
Avidity
The collective strength of multiple noncovalent intermolecular forces during an interaction among macromolecules
Two spike proteins of influenza virus
- Haemagglutinin (HA)
- Neuraminidase (NA)
Function of HA and NA
Attach and allow virus to be released
HA
- Has a receptor binding pocket and a fusion peptide, and crosses the viral envelope.
- HA spike protein binds to sialic acid (aka neuraminic acid)
Influenza A and B genome
8 piece segmented genome
Host receptor for influenza virus
α2,6-linked sialic acid that is most common on nonciliated human respiratory cells
Why are avian influenza viruses more virulent
Replicate lower is respiratory tract
Avian IVA vs human IVA
- The HA1protein of influenza A viruses that
live in bird populations bind better to α-2,3-linked sialic acids that are common in birds. - Avian influenza A cannot undergo person-to-person transmission until the HA knob evolves and acquires the right structure to bind preferentially to human respiratory epithelial α-2,6-linked sialic acids.
Animal virus interactions with receptors
Interact with glycosylated host receptors because essentially all surface-exposed host proteins are glycosylated
Penetration
Entry of the virion or subcomponents of the virion
into the host cell.
Second stage of the virus replication cycle
Penetration, uncoating and if necessary transport to the nucleus
Enveloped Togavirus penetration
- Fusion of enveloped virus at the plasma membrane, releasing the nucleocapsid into the cytoplasm and leaving viral spike proteins on the cell surface.
- Penetration occurs when the virus is internalized into an endocytic vesicle that fuses its envelope
with the endocytic membrane and releases its nucleocapsid into the cytoplasm. - Uncoating then occurs when the nucleocapsid disassembles and the (+) ssRNA genome can then be translated.
Clathrin pathway
- After a ligand in the extracellular media binds to its receptor, the ligand–receptor complex diffuses in the plane of the membrane until it encounters a small indentation where there is abundant clathrin on the cytoplasmic face of the indentation.
- The fibrous clathrin proteins then assemble into a cage-like structure that forms an indentation and ultimately pulls the receptor and its ligand into the cell.
- The clathrin-coated vesicle loses its clathrin and then the vesicle fuses with an early endosome, which is acidified
Three types of viral entry into cell
- Direct release of viral genome into cytoplasm through pore
- Fusion mediated
- Receptor mediated endocytosis
What causes progressive acidification of endosomes
Vacuolar ATPase (transports protons)
What occurs following acidification of the early endosomes
Viral uncoating is triggered which releases the nucleocapsid or genome into the cytoplasm
Examples of viruses that enter through clathrin dependent, receptor-mediated endocytosis
influenza A virus and human rhinovirus
Entry via Caveolae
- An alternative form of entry that does not involve clatharin
- After internalization, the virus traffics through the caveosome and from there to the endoplasmic
reticulum. - Vesicles formed by caveolin require a protein called dynamin and do not always become endosomes or fuse with the lysosome.
- Viruses that enter by the caveolin route are not exposed to low pH but are consequently exposed to the resident proteins in the endoplasmic reticulum
- These endoplasmic reticulum proteins are needed for uncoating, comparable to the need for low pH for viruses that enter through receptor-mediated endocytosis.
Example of ER protein
Hsp105 protein
Viral entry via phagocytosis
- Viruses enter through phagocytosis via macrophages or neutrophils that ingest viruses as part of immune response
- Ultimately exposes the virion to the contents of a
phagolysosome, triggering uncoating. - Eg. Herpesvirus
Two distinct roles of virions
To protect the virus genome during transmission and to release the virus genome into a host cell
What allows a virion to maintain its characteristic shape
- The intermolecular interactions among the structural proteins of a virion, and between them and the genome, maintain the virion in its characteristic shape.
- Virions exist in this metastable state until a host cell
triggers them to advance from attachment to the penetration and uncoating stage.
Host cell triggers that cause a virion to advance from attachment to prenetration and uncoating
- Binding to a cellular receptor
- A decrease in endosomal pH
- Proteolytic degradation by a host enzyme
- Or some combination of these factors
Picornaviruses
- Naked (non-enveloped) viruses that release their genomic contents through pore formation
- Examples include poliovirus and rhinovirus
- Naked viruses can enter cells by endocytosis and phagocytosis
- The capsid has to disassemble in order to release the genome, or the genome has to be extruded from the capsid.
Poliovirus and rhinovirus
- Poliovirus releases its genome following receptor-mediated endocytosis or as a consequence of binding to its cellular receptor.
- While it is extracellular, poliovirus may become exposed to the low pH of the human stomach.
- Rhinovirus, contracted by inhalation and thus not exposed to low pH until after endocytosis, releases its genome after acidification of an endocytic vesicle.
- In both cases, substantial protein rearrangement apparently occurs, with the VP4 protein, normally buried on the inside of the capsid, becoming exposed on the surface of the virion.
Enveloped virion and fusion
- An enveloped virion is covered by a protein-rich lipid bilayer derived from its
former host and contains proteins of both host and viral origin. - Penetration and uncoating by enveloped viruses necessarily includes fusion of the virion envelope with a host cell membrane.
- A component of one of the virus spike proteins (fusion protein) has a region
termed the fusion peptide that becomes catalytically active following irreversible attachment.
Influenza virus viral fusion
- Binding to a receptor or a change in pH causes a rearrangement in the viral spike protein, revealing the fusion peptide
- The viral fusion peptide causes scission of the cell membrane, draws the viral envelope into close apposition with the host cell membrane, creates a hemifusion structure, and then catalyzes pore formation so that the viral contents enter the cytoplasm
Influenza fusion peptide
A-helix with a hydrophobic surface that can penetrate host membrane lipids.
Env
HIV fusion peptide that is part of the HIV spike
Two parts of Env
Surface (SU) glycoprotein gp120, and transmembrane (TM) glycoprotein gp41
HIV penetration
- HIV virions fuse with the external surface of the plasma membrane
- When SU engages a CD4 host receptor molecule, it triggers a conformational change that initiates SU binding to a second host molecule, the co-receptor (such as CCR5 and CXCR4).
- Binding to the co-receptor then triggers a rearrangement of the gp120–gp41 complex so that a portion of gp41, known as the fusion peptide, inserts into the plasma membrane.
- Once the virion envelope and the plasma membrane fuse, the HIV nucleocapsid is released into the cytoplasm
Pharmaceuticals that block HIV entry
Target penetration by binding to extracellular helices of gpp41, preventing the conformational changes required for fusion
Dissociation of influenza virus RNA from ribonuclear proteins
By acidification of virion interior via viral ion channel M2 protein
Kinesins and dyneins
- Carry cargo along a microtubule
- Dyneins move cargo such as viruses toward the (−) ends of the microtubule and therefore toward the nucleus.
- Kinesins move cargo towards the + end
Location of genome expression and replication
- All eukaryotic RNA viruses except influenza and retroviruses (HIV) express and replicate their genomes in the
cytoplasm - Whereas all eukaryotic DNA viruses except poxviruses (vaccinia and variola) express and replicate their genomes in the nucleus
What is the outer layer of the nuclear envelope contigous with
The ER
How to macromolecules enter the nucleus
Through gated structures called nuclear pore complexes
Overall structure of a nuclear pore complex
Ring with filaments that extend toward the cytoplasm, and a basket structure that extends into the nucleus
Transport through nuclear pores
- Requires a nuclear localisation signal
- Large molecules such as virus genomes require active transport
Adenovirus
Adenoviruses have naked virions with prominent spikes, enclosing a linear dsDNA genome that must enter the nucleus for expression and replication.
Adenovirus entry and movement in cells
- After attachment, adenovirus is internalized through clathrin-coated pits and enters the endosome system.
- The virus spike proteins dissociate from the rest of the capsid early after
internalization. - Acidification of the late endosome releases several proteins including protein VI, which is cleaved by a viral protease activated by the chemical
conditions in the maturing endosome. - Protein VI ultimately causes lysis of the endosomal membrane and the release of the partially disassembled adenovirus capsid into the cytoplasm.
- In an extended penetration and uncoating stage that can last 40–60 min, the capsid moves along the microtubule cytoskeleton (via Dynein), ultimately reaching a nuclear pore complex.
- Docking with the nuclear pore complex causes conformational changes in both the capsid and the nuclear pore complex, which together result in the release of virus DNA into the nucleoplasm.
Reoviruses
- Reoviruses ( eg. Rotavirus) are unusual in that the infecting virion remains largely intact inside the host cell for all or some of the gene expression stages.
- Have segmented dsRNA genomes enclosed by three layers of protein.
- These naked viruses are internalized by endocytosis after which the outer capsid disassembles and the double-layered particle is present in the cytoplasm.
- The double-layered particle remains intact and serves as the first site of virus mRNA production, when enzymes in the core use the genomic RNA as a template to synthesize, cap, and tail viral mRNAs.
Reovirus structure
The genome segments are surrounded by the inner capsid, a second a double layered intermediate capsid particle, and third layer called the outer capsid.
