Attachment and Entry Flashcards
Viruses are too large to diffuse across cellular membranes. Have large capsids/ envelopes which are to protect virus, and must be brought into cell
Initially must adhere to a specific receptor on the cell surface.
What is the next step after attachment?
Uncoating
Binding to receptor may trigger a conformational change, that prime capsid for uncoating
Alternatively, receptor may direct virus particle into endocytic pathway, where uncoating is triggered by low pH or action of proteases - usually for cytoplasm replicating viruses
For viruses that undergo replication in the nucleus,
import can occur either through use of the nuclear
pore complex or during cell division, when the nuclear
membrane breaks down.
Steps required:
endocytosis
membrane fusion
vesicular trafficking
transport to nucleus
What is the significance of viruses have specific receptor types?
The cell receptor may determine the host range and
tissue tropism of the virus.
Primate and mice have very similar receptor types. Polio cannot bind to mice receptors, as they are sufficiently different from primates which have the specific CD155 which polio uses for entry
What is the difference between a receptor and co-receptor?
Receptor is usually the primary binding site for a virus e.g CD4 HIV
Co-receptor may be necessary for fusion. e.g CXCR4 in HIV
However the naming is not strict. Some viruses can enter just using the co-receptor, and sometimes the co-receptor is bound to firstly
These naming conventions are used loosely
What is an example of technology which is used to identify a new receptor? e.g for MERS
DNA-mediated transformation
MERS binds to respiratory cells
Add different genes for respiratory cell receptors to receptor-negative other cells e.g liver cells
Add MERS - see if virus is able to bind
If virus is able to bind, suggests that the receptor gene that was given is the target of the virus
A monoclonal antibody may be used to try and identify a new virus receptor
What are potential pitfalls of this?
Monoclonal may not specifically bind to the receptor, but instead may be binding as the receptor is similar to another membrane protein.
Need to perform DNA-mediated transformation to see if naiive cells can be infected once the receptor is introduced
What are difference in mechanisms of uptake of macromolecules from extracellular fluid?
Phagocytosis
Macropinocytosis
Receptor-mediated endocytosis
Phagocytosis - large molecules such as bacteria, are engulfed by extensions of the plasma membrane
Macropinocytosis - small molecules can disrupt the plasma membrane, and result in invagination of plasma membrane - usually extracellular fluid. Happens in all cell types. viruses such as vaccinia, herpes, ebola enter this way (non-enveloped)
Receptor-mediated endocytosis - ligand binds to receptor and initiates plasma membrane ruffling, allowing molecules to enter - e.g HIV (enveloped)
Virus has entered a cell
During the uncoating step, how is nucleocapsid broken down?
Ribosomes bind to parts of the nucleocapsid, causing it to breakdown, and release nuclear material into the cytoplasm
how to viral/ subviral particles move around inside cells?
molecules larger than 500kDa are too large to move by passive diffusion - bump into cellular organelles
Movement relies on cellular cytoskeleton of protein filaments - microtubules, intermediate filaments, and microfilaments utilising myosin motors
Either travelling directly in cytoplasm, or in a membrane vesicle such as an endosome
how to viral/ subviral particles move around inside cells?
molecules larger than 500kDa are too large to move by passive diffusion - bump into cellular organelles
Movement relies on cellular cytoskeleton of protein filaments - microtubules, intermediate filaments, and microfilaments utilising myosin motors
Either travelling directly in cytoplasm, or in a membrane vesicle such as an endosome
Herpes viruses move in neuronal cells via interaction of tegument with dynein motors
Polio is carried within endocytic vesicles between neural cells, attached to receptor CD155
Specific mechanisms for attachment/ release from microtubules and filaments is not fully udnerstood
What are different strategies for viral nuclear material to enter the nucleus?
Unclear how viruses know to enter the nucleus - there a nucleus localisation signal which are amino acid sequences that engage with viruses, allowing nuclear entry
Nuclear material enter direct through nuclear pore complex - e.g influenza virus genome segments
Viral capsid can dock onto nuclear pore complex - viral nuclear material can be disassembled or minimally disassembled to allow entry of nuclear material. DNA viruses as their nuclear material is too large to enter fully formed through pores
Virion nucleocapsid enter directly - virion binds to nuclear pore complex, causing a conformational change, and allows nuclear entry
Nuclear pore complex - usual function is to allow nuclear proteins to enter, and RNA molecules to exit freely
