Protein trafficking and assembly:

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Protein trafficking and assembly Production of
Viral Progeny
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Extravagant diversity in viral morphology. -
Arises via the different proteins that are
produced and assembled.
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Different viral genomes Blah Blah Blah
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Acquisition of viral envelope
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Map of HIV genome (9000 bases). Green gag
(capsid proteins) pol (reversetranscriptase)
env (envelope proteins) Red Other genes for
producing proteins reqd for viral life-cycle. -
some of which modify the host cell processes
(e.g. protein trafficking down regulation of
host protein synthesis).
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• Modes of Viral capsid introduction
• Membrane fusion and cytosolic deposit
• B) Trigger Host Receptor-mediated endocytosis
  cytosolic deposit
• C) Non-enveloped forms a pore in membrane and
  deposits RNA into cytosol
• D) Combination Rec. mediated Endo. Cytosis
  endosome escape, Nuclear deposit

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• Example of Influenza uncoating strategy
• Receptor docking at PM
• Induction of Receptor-mediated Endocytosis
• Acidification of the endosome lumen triggers
• membrane fusion
• Genome extruded through lysed endosome.

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How do cells know where proteins belong?

• Signal sequences!
• Some times proteins will have several signal
  sequences to direct its
• destination.
• e.g.) a PM protein must first have a signal to
  get to the ER and then a signal to get trafficked
  (via vesicles) to the PM.
• Most signal peptides are removed by signal
  peptidases.

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• Nuclear Transport
• - Occurs through a Nuclear Pore complex
• - made up of gt100 proteins
• Allows free diffusion of molecules lt 9nm in
  diameter
• However, proteins gt25nm diameter pass through as
  well, but require
• energy to do so.
• Also, proteins can be accumulated against a
  concentration gradient
• inside of the nucleus.

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• Nuclear Import Receptors
• Soluble cytosolic proteins that recognize and
  bind to the NLS
• Also bind to the nucleoporins at the extended
  fibrils
• Fibrils contain short amino acid repeats with Phe
  Gly (i.e. FG-repeats) that serve as Import
  receptor binding sites.

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• Movement in and out of the nucleus is controlled
  by a molecular
• switch - Ran.
• Associated with a cytosolic GTPase Activating
  Protein (Ran-GAP).
• Also, has a nuclear Guanine Exchange Factor
  (Ran-GEF).
• Thus, the cytosol predominately has Ran(GDP)
  nucleus Ran(GTP)

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However, some viruses skip the export step.
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• In this case all proteins required to synthesize
  the
• capsid have been imported into the nucleus,
  assembled,
• and then exits via vesicularization directly
  into the ER.

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Membrane Proteins Anyone?
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• Membrane spanning proteins need an additional
  signal.
• Stop-transfer peptide signal which directs the
  translocon to open and release the peptide
  laterally.
• Initial ER signal acts as the start-transfer
  signal, but is cleaved

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All N-linked oligosaccharides in the ER start
out the same. - Carbohydrates undergo
extensive modification almost
immediately. eg.) - glucose trimming
re-glucosylation -
Mannose trimming ER - Mannose trimming
Golgi - complex glycosylation golgi
core glycosylation (shaded in gray) of
five sugars remain in virtually all
glycosylated proteins.
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Protein glycosylation There is one OS
trans. per translocon. It catalyzes the
transfer of the core oligosaccharide
from dolicohol to the NXs/t sequence of the
nasent polypeptide.
Oligosaccharyl transferase
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Electron micrographs of the separate types of
coated vessicles
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Trafficking direction is mediated in part by the
appropriate coat.
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Viral proteins must have evolved to mimic
adaptin-binding molecules
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