Lecture 18: Intracellular
transport
Flint et al., Chapter 12.
General remarks
• Viruses are cellular parasites
• Eukaryotic cells are functionally
compartmentalized
 Viral components must be synthesized in
different intracellular compartments
• To assemble mature virions, virus components
must be brought together for assembly from the
different compartments.
• Intracellular trafficking/sorting of viral nucleic
acids, proteins, and glycoproteins is an essential
prelude to virus particle assembly.
Cellular compartments:
considerations
•
•
•
•
•
•
Nucleus
Cyotoplasm
Plasma membrane
Internal membranes
Mitochondria and other organelles
Viruses have evolved to assemble
wherever a compartment exists
Virus assembly within the nucleus
•
•
Most DNA viruses assemble at the site of genome replication, the nucleus.
Viral structural proteins enter vial normal cellular pathway of nuclear protein
transport
Interactions with internal cellular membranes
• Envelopes of many viruses are derived from internal
membranes
• Majority assemble at the cytoplasmic faces of compartments
of the secretory pathway
• Bottom line: where there’s a membrane, there’s a place for
enveloped viruses to assemble
Fig. 12.20
Virus assembly at the plasma membrane
Enveloped viruses generally assembled at the plasma membrane.
Integral membrane proteins are first transported to plasma membrane
Internal proteins and nucleic acid genomes must also be actively transported to site of
assembly
(Fig. 12.2)
Intracellular trafficking:
Considerations
• Relative sizes of viruses and host cells:
– Virus: diameters on the nm scale
– Cells: diameters on the mm scale
• Cell vol. 103 – 104 x greater than virus.
• Viral components can’t find one another by
passive diffusion.
• Therefore, viral components must be
actively transported to assembly sites.
Getting from point A to point B.
Directional movement requires
energy.
• Directional movement is
generally achieved by:
– Protein channels (transporters,
translocons, pores, portals)
• Move molecules across
membranes
–
–
–
–
–
Plasma
Nuclear
ER
Golgi
Mitochondria
– Motor proteins/cytoskeletal
tracks
• Directional movement within the
cytoplasm
– Myosin motors move cargo on
actin fibers
– Dynein and kinesin motors
move cargo on microtubules
(Box 12.1)
Vesicular transport to the cell surface
• Viral membrane proteins travel to the cell surface through a series of
membrane-bound compartments and vesicles.
• ER to Golgi to plasma membrane.
• Trafficking requires cellular and viral proteins
(Fig. 12.9)
Transport of viral membrane
proteins to the plasma membrane
• They hitch rides on the cellular secretory pathway:
• ER  cis Goli network  Golgi cisternae trans Golgi
network  plasma membrane
• Maturation of proteins and structures occurs sequentially
in different components.
• Oligomerization occurs in ER and cis Golgi network
• Oligosaccharides are trimmed, and further modified in
Golgi cisternae
• Proteins can be cleaved and further modified in trans
Golgi network
• Transported from trans Golgi to plasma membrane.
Snare proteins: ensuring
targeting specificity
•Snare proteins bring
membranes close together
• v-Snare: vesicular
Snare
• t-Snare: target Snare.
•v- and t-Snares are highly
specific for one another
•Come together, bring
vesicle and target
membranes in close
proximity to one another
•Exclusion of water lowers
energy barrier to membrane
fusion
•Upon fusion, contents of
vesicle emptied into target
area
(Fig. 12.11)
Translocation of viral membrane
proteins into the ER
• Integral membrane proteins first inserted into ER
membrane
–
–
–
–
Ribosomes targeted to ER by Signal Recognition Protein (SRP)
SRP interacts with ER bound SRP receptor
Directs ribosome to Translocation channel
Hydrophobic signal peptide associates with translocation
channel
– Remainder of protein translocated into ER lumen
– Signal peptides cleaved off by signal peptidase
• Hydrophobic Stop Signals used to stop translocation of
protein into ER lumen (Box 12.2)
– Results in transmembrane proteins
Translocation of viral membrane
proteins into the ER
(Fig. 12.6)
• Hydrophobic Stop Signals used to stop
translocation of protein into ER lumen
• Results in transmembrane proteins
(Box 12.2)
Reactions within the ER lumen
•Proteins can be modified within the ER
lumen.
•Glycosylation (modification with sugars)
•Disulfide bond formation (between Cys
residues)
•Protein refolding by molecular chaperones
•Oligomerization – most viral membrane
proteins are complexed with one another
Reactions within
the Golgi apparatus
(Fig. 12.12)
Proteolytic cleavages occur in Golgi
e.g. retroviral TM
(transmembrane) and SU
(surface unit) subunits of Env
Fig. 12.13
Maturation
of Influenza
HA0 protein
during
transit
through the
secretory
pathway
Fig. 12.4
Viral genome transport
• Viral genomes must also be transported to
sites of virus particle assembly
• Sequence specific viral proteins interact
with viral genomes
• These interact with the cellular transport
systems to ensure delivery of genomes to
the proper compartment
Viral inhibition/alteration of
intracellular transport
• It is often in the interest of the virus to
inhibit components of the transport
machinery
• Many viruses inhibit transport of MHC
class I molecules: prevents cell from
signaling to the immune system that it is
infected.
Viral inhibition/alteration of
intracellular transport
• Some viruses inhibit transport
beyond the compartment
where they assemble.
• e.g. Poliovirus assembles in
the ER. Poliovirus 2BC and
3A proteins inhibit vesicular
transport beyond the ER.
Force accumulation of virus
components in the
compartment where they
assemble.
Fig. 12.16
Viral inhibition/alteration of
intracellular transport
• Some viruses redirect transport system to
decrease expression of their target
proteins on the plasma membrane.
• e.g. HIV Nef and Vpu redirect CD4 away
from plasma membrane.
• This helps to ensure that new virus won’t
fuse back into infected cell.