Host cell I and II

Host cell I and II思维导图：包含Cellular homeostasis，Proteasome - antigen presentation or protein recycling ，These quality control measures generally lead to the degradation of mis-folded or foreign proteins等等

Host cell I and II

• Intracellular transport
  • Mediating transport
    • Movement within lipid bilayer: clustering of receptors and signalling proteins facilitates by distinct lipid composition
    • Vesicular transport : mediated via distinct proteins “coat” the vesicles and target them to the appropriate location.
    • Cytoskeletal transport: mediated via microbules in a bi-directional manner
      • Microtubules and microtubule motor proteins
        • Kinesin drives transport from ER to PM
        • Dynein drives transport from Pm to ER
        • Pox virus utilisation of the cytoskeleton: moves from ER to PM via microtubules
        • Exit cells via actin
  • Transferring Cargo via membrane fusion
    • SNARE proteins mediated contact and fusion ‘mixing’ of vesicle membrane with target membrane
    • V-SNARE : vesicle snare
    • t-SNARE: target SNARE
  • Glycosylation in Golgi Apparatus
    • Plays a vital role in protein localisation, function and reognition
• Secretory pathway
  • Used to move material aroun the cell
    • Various locations, including the PM and outside the cell
  • Proteins destined for the PM, endosomes/ lysosomes or secretion
    • Synthesised by the RER
    • Contain specific amino acid sequence (motif) which target them to lumen of RER
    • Proteins may be glucocylated while passing through the ER
    • Then bud from ER in vesicles
  • ER vesicles travels to cis face of Golgi Apparatus, then proceeds to trans face
    • Sometimes gaining additional modifications during this time which target proteins to the final location
  • Can do forward - antrograde. Can do backward - retrograde
• Location of the motifs
  • Motif
    • Recognised by adaptor proteins embedded in the organelles membrane or by associated adaptor proteins
    • Each has different composition and property
    • Some have defined location within the proteins coding sequence whilst other do not
      • ER signals sequence: will embedded in ER; only found in N-terminus
        • Import into the ER
          • This motif is a stretch of 12-16 hydrophobic amino acids at the N-terminus flanked by basic ‘charged’ residues
          • These are termed ‘ signal sequence or signal peptide’
          • Once translocation into the ER has occurred the signal peptide is removed by the enzyme signal peptidase
      • Nuclear localisation motif (NLS): in the middle part of the amino acids sequences
        • Import into the ER
          • If the protein contains additional hydrophobic regions, these will embed the proteins within the membrane
          • Multiple ‘membrane spanning’ or ‘transmembrane’ domains may exist within a protein
          • These proteins then become tightly associated with the membrane
          • The greater the hydrophobicity of the protein the greater the affinity for membranes
      • ER retention motif (KDEL/KXXX): only found in C-terminus
        • Export from the ER
          • COPI: retrograde (back to ER)
            • Luminal proteins contain the motif - KDEL at their C terminus
            • Recognised by the KDEL receptor that interacts with COPI
            • Membrane-bound proteins contain a - KXXX motif within their cytoplasmic c-terminus (interacts directly with COPI)
          • COPII: anterograde (coats the vesicle leave the ER)
• Cellular homeostasis
  • All cell in our body have innate mechanisms of quality control
    • Ensures that all proteins and constituents are functional
    • Ensures that any abuse (toxins or infection) are monitored and reacted to - digestion, degradation of molecules, particle, foreign microbes
  • These quality control measures generally lead to the degradation of mis-folded or foreign proteins
    • Unfold protein response - translation restriction and cellular stasis
      • Protein maturation requires protein chaperones to correctly fold the proteins
      • With in ER
        • Some of the chaperones will recognise misfiled proteins, export these proteins from the ER
        • These unfold proteins become ubiquitinated and sent to the proteasome for degradation
    • Proteasome - antigen presentation or protein recycling
      • Correct folding of proteins is critical in targeting them to their intended location
      • Unfold and mis-fold proteins are:
        • Secreted into the cytosol
        • Target for destruction by ubiquitination: attachment of several ubiquitin molecules
        • Degraded by the proteasome:
          • Small peptides and aa are produces: recycling of amino acid
          • Ubiquitin and degraded produces are released back into the cytosol
          • Energy dependent process
      • Proteasome produce peptides for antigen presentation during immune response (CTL and MHC I
      • Ubiquitin - proteasome pathway
        • A complex of three protein E1,2,3, which together constitute a ubiquitin ligase, recognise a protein destined for degradation and takes this proteins with a chain of ubiquitin termed polyubiquitylation
      • Following polyubiquitylation, the protein is carried to a proteasome where is a de-ubiquitylated and degraded into oligopeptides
        • Degraded further into amino acids or used for antigen presentation by the immune system
        • A specialised proteasome is used to process proteins for antigen presentation
      • Ubiquitin itself is a relatively short protein of 76 aa residues whose aa sequence and structure are almost totally conserved among all eukaryotic cell
    • Endocytic Pathway
      • Endocytosis observed in all Euk. Cell
        • Regular occurrence for recycling membrane molecules
        • Can be mechanisms for: cell feeding, digestion, degradation of molecules. Particles, foreign microbes
      • Bring extracellular material into the cell
        • Pinocytosis
          • Intakes of solutes
        • Phagocytosis
          • Intakes of particles via engulfment by membrane protrusion
        • Receptor-mediated endocytosis
          • Intakes of specific molecules which bind to receptors on the cell surface
          • Cathrin-mediated Endocytosis
            • Mediated via vesicles coated in the molecules - clathrin
              • Consist of three polypeptides chains that forms a triskelion
              • Triskelion assemble together to from a ‘cage-like’ structure (vesicles)
            • Generally occurs once a ligand engages with its receptor and promotes association between the cargo molecules, the receptor, adaptor proteins and clathrin to drive the reaction
            • The GTPase dynamin is then required to pinch the vesicle from the membrane
            • Once formed the clathrin-coated vesicles is then transported to the endosome
      • Endosomes aid in the process of endocytosis
        • Move material for degradation in lysosomes
          • Different classes
          • Early, late and recycling
      • Virus entry
        • Viruses utilise the process of endocytosis to gain entry into cells
        • Essentially they act as a ligand and bind to a cell surface receptor to promote the accumulation of clathrin and endocytosis
      • Lysosomes
        • Bud from the Golgi apparatus
          • Maturation pathway: early endosome - late endosome - lysosome
        • Functions
          • Involved in intracellular digestion / degradation pathway
          • Acid pH
          • Contains
            • Enzymes
            • Reactive Oxygen intermediates
    • Autophagy - recycling of organelles
      • Second degradation pathway in lysosome
      • A process where the cell degrades/ digest its own constituents and recycle them (expect nucleus)
      • Autophagosome - enclosures of an organelle with a double membrane
      • Dependent on a large number of cellular proteins and occurs in a highly regulated manner
      • Autophagesome fuses with a lysosome and the internal contents are degraded by hydrolyase and thus released
      • Appears to be anti-pathogen response but also exploited by pathogens
      • Plays a key role in the detection of pathogens and subsequent activation of the immune response
        • Can stimulate our innate and adaptive response
          • Deliver antigen to MHC I and II
          • Stimulate cytokine production
      • Conversely many pathogens manipulate the antophagy resopnse
        • Can enable replication
        • Can enable immune evasion