Entry into host cells Lecture 11
Overview: Once attached to a cell,
viruses must
• Enter the cell
– Active uptake
– Passive uptake
• Target themselves to the appropriate
compartment
– Cytosol
– Nucleus
• Make their genomes accessible for the
next step
– Disassembly.
Cellular Uptake of Macromolecules
• Some small molecules can freely traverse lipid
bilayer of cell membrane
– Water, gas, small hydrophobic molecules
• Most small metabolites are actively
transported in and out by pores or specific
receptors.
– Glucose, ATP, nucleosides, amino acids, proteins,
cations
• Larger molecules and particles require active
transport by
– Phagocytosis or Endocytosis
Phago- and Endocytosis
•Phagocytosis
•Large particles, e.g. bacteria and cell fragments
•Pinocytosis
•Fluid phase, non-specific
•Receptor-mediated endocytosis
•specific
Receptor-mediated endocytosis
•Ligands bind to membrane receptor proteins
•Migrate to clathrin coated pit
•Pit invaginates, then pinches off
•Clathrin coat falls off, now a vesicle
•Vesicle fuses with endosome – early endosome
•Contents transported to late endosome –
acidification
•Late endosomes fuse with lysosomes – degradation
(Fig. 5.16)
Cell membrane fusion
Cell membranes do not fuse all by
themselves…otherwise our bodies would be
just one gigantic syncytium.
•
Proceeds by specialized mechanisms
mediated by proteins
1. Help bring into close proximity
2. Targeting and docking proteins must interact
3. Water molecules must be removed (energetically
unfavorable)
Virus uncoating/disassembly
•Viruses use capsids (and sometimes envelopes) to protect
their genomes from the environment
•On entry into cells, genetic material must be released
•Effected by uncoating/disassembly
•Three general pathways
Uncoating at plasma membrane
Uncoating within endosomes
Uncoating at nuclear membrane
Uncoating at plasma membrane
•Generally used by
enveloped viruses.
•Receptor-ligand
interaction brings
envelope and cell
membranes into close
juxtaposition.
•Fusion generally
mediated by a second,
viral fusion protein
•Fusion proteins evolved
from cellular SNARES,
proteins that effect
fusion of intracellular
vesicles
Paramyxoviruses
HIV
Acid-catalyzed uncoating within the endosome
Many enveloped viruses undergo fusion within the
endosome.
• Example: Influenza
– At cell surface, virus attaches to sialic acid via viral HA
glycoprotein
– Virus-receptor complex internalized by clathrindependent endocytosis.
– In late endosome, pH = 5.0
– HA undergoes acid-catalyzed conformational change
– Exposes fusion peptpide/
– Viral and endosomal membranes fuse
– Viral ribonucleoprotein released into cytoplasm.
Entry of nonenveloped viruses
General points
• Entry into cells cannot be mediated by
membrane fusion
• Much more complex
• Many mechanisms
Disruption of the endosomal membrane
Example: Adenoviruses.
dsRNA genome in
icosahedral capsid.
•
Internalized by receptormediated endocytosis
•
In endosome, low pH exposes
penton base, which lyses
endosome
•
What remains of viron
enters cytoplasm
•
Docks with nuclear pore
complex
•
Capsid interacts with histone
H1 and importins
•
Capsid disassembles at
nuclear pore
•
DNA imported into nucleus
Formation of pore in the cell membrane
Example: picornaviruses,
ssRNA,icosahedral capsids.
• Virus interacts with cell surface
receptor (pvr for poliovirus)
• Internalized by endocytosis
• Interaction with pvr results in
conformational change
• VP1 and VP4 move from inside
virion to outside
• Exposes hydrophobic domains
• Result in pore through endosome
• Viral genome exits through pore
into cytoplasm
Lysosomal
uncoating
Example: Reoviruses:
dsRNA genomes,
double shelled
icosahedral capids
• Enter cell via receptormediated endocytosis
• VERY low pH of
lysosome induces
conformational change
• Outer capsids shell
uncoated
• Inner capsids revealed
• This penetrates
lysosomal vessicle
Transport into the Nucleus
General: Many viruses begin replication in the
nucleus
• They need to get from cytoplasm to nucleus
• Examples: adenoviruses, retroviruses, influenza,
Epstein-Barr virus
• Require nuclear localization signals
– Short amino acid sequences: 2 general motifs
• Hydrophobic – basic3-7 – hydrophobic
• Basic2/3 – X10 – basic3/5
Transport through nuclear pore