Lecture 18: Intracellular transport
Flint et al., Chapter 12.
BSCI437
General remarks
 Viruses are cellular parasites
 Eukaryotic cells are functionally compartmentalized
 Viral components are 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
Intracellular trafficking: Considerations
 Relative sizes of viruses and host cells:
o Virus: diameters on the nm scale
o Cells: diameters on the m 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. (Box 12.1)
 Directional movement requires energy.
 Directional movement is generally achieved by:
o Protein channels (transporters, translocons, pores, portals)
 Move molecules across membranes
 Plasma
 Nuclear
 ER
 Golgi
 Mitochondria
o Motor proteins/cytoskeletal tracks
 Directional movement within the cytoplasm
 Myosin motors move cargo on actin fibers
 Dynein and kinesin motors move cargo on microtubules
Virus assembly within the nucleus (Fig. 12.1)
 Most DNA viruses assemble at the site of genome replication, the nucleus.
 Viral structural proteins enter vial normal cellular pathway of nuclear protein transport
 They carry Nuclear Localization Signals (NLS).
 NLS signals first described in viral systems
o

e.g. SV40 large T-antigen, Influenza NP protein, Polyoma virus VP proteins,
Adenovirus hexon proteins
o Often, protein complexes are first assembled in cytoplasm, then transported into
nucleus.
Entry via nuclear pores, described in Lecture 10.
Virus assembly at the plasma membrane (Fig. 12.2)
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.
Transport of viral membrane proteins to the plasma membrane (Fig. 12.4)
 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.
Translocation of viral membrane proteins into the ER (Fig. 12.6)
 Integral membrane proteins first inserted into ER membrane
o Ribosomes targeted to ER by Signal Recognition Protein (SRP)
o SRP interacts with ER bound SRP receptor
o Directs ribosome to Translocation channel
o Hydrophobic signal peptide associates with translocation channel
o Remainder of protein translocated into ER lumen
o Signal peptides cleaved off by signal peptidase
 Hydrophobic Stop Signals used to stop translocation of protein into ER lumen (Box 12.2)
o Results in transmembrane proteins
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
Vesicular transport to the cell surface (Fig. 12.9)
 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
 Targeting and fusion of transport and target vesicles require specific interactions among
Snare proteins
Snare proteins: ensuring targeting specificity (Fig. 12.11)
 Snare proteins bring membranes close together
o v-Snare: vesicular Snare




o 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.
Reactions within the Golgi apparatus (Fig. 12.12)
 N-linked oligosaccharaides are added in the ER lumen
 N-linked sugars are processed in a stepwise fashion in the Golgi
 O-linked sugars are added specifically in the Golgi
 Addition of fatty acid side chains (acylation) occurs in Golgi (Table 12.5)
Proteolytic cleavages occur in Golgi (Fig. 12.13)
e.g. retroviral TM (transmembrane) and SU (surface unit) subunits of Env
Interactions with internal cellular membranes (Fig. 12.20)
 Envelopes of many viruses are derived from internal membranes
 Majority assemble at the cytoplasmic faces of compartments of the secretory pathway
 Bottom line: wherever there is a membrane, there is a place for enveloped viruses to
assemble.
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.
 Some viruses inhibit transport beyond the compartment where they assemble.
o 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)
 Some viruses redirect transport system to decrease expression of their target proteins on
the plasma membrane.
o 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.