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Biological Sciences
Tanya Da Sylva

Cellular uptake of particles and macromolecules •Two separate categories: –Endocytosis –Phagocytosis Phagocytosis: “Cell eating” •Uptake of relatively large particulate matter –(>0.5 mm in diameter) •For food, eg. Unicellular heterotrophic protists •For protection •Eg., macrophage, neutrophil–immune cells •Entrapment (pseudopods) -> engulfment -> Digestion -> Absorption –Macrophage Receptors on cell binds particle (bacteria, dead cells, etc.) –macrophages have Fc-receptors that binds to antibodies (IgG) coating (‘opsonize’) pathogens Phagocytosis by macrophages is induced in both innate(fast and first) and adaptive (antibody) immunity Phagocytosis •IgG(antibody) binds FcR (Fc receptor that lies on the plasma membrane) •receptors cluster into lipid rafts or microdomains •clustering signals makes tails of Fc come together and helps recruitment of signalling proteins like src and PI3K •PI3K phosphorylates PI (phosphatidylinositol) on plasma membrane to make PIP3 PI is composed of a Inositol Ring (head) Diglyceride (tail) •PIP3 recruits PH-domain containing proteins: this protein domain has a high affinity to PIP3 (will bind to it) •PH-proteins bind to actin proteins and cause the underlying actin cytoskeleton to rearrange •actin rearrangement causes large membrane extensions which engulf particle •IgG-opsonized particle: bacteria/ virus or infected cell that is covered with antibodies: IgG molecules •recognized by IgG receptors that rearranges the underlying cytoskeleton which causes engulfment of the particle and internalization of the opsonized particle •particle internalized into membrane-bound compartment called phagosome Difference between endosome and phagosome: endocytosis is the budding on the membrane and is pinched off and phagocytosis there are pseudopods, the membrane moves forwards and takes it in. •Phagosome fuses with lysosome: -> Phagolysosome •Acid hydrolases kill/digest bacteria •Some pathogens “escape” lysosomes –Not killed and can proliferate in macrophages Modes of bacteria escaping the lysosomes/phagosome: 1) Escape: Listeria monocytogenes 2) Prevent fusion with lysosomes: Mycobacterium tuberculosis 3) Survive in Phagolysosome: Coxiella burnetii Posttranslational Uptake of Proteins •Membrane bound organelles that import proteins though one or more membranes: talking about nucleus, mitochondria, chloroplast , peroxisomes done in the cytosol by free ribosome and doesn’t need a signal peptide. •Proteins imported by these organelles contain amino acid “addresses” (signals) •Unlike in RER, proteins are imported post-translationally Uptake of proteins into Peroxisomes •Peroxisomes have only two compartments in which proteins can be placed –Boundary membrane –Internal matrix •Proteins bound for peroxisome have peroxisomal targeting signal (PTS) •Receptors bind to proteins in the cytosol and shuttle them to surface where receptor and ligand travel through peroxisome membrane to peroxisomematrix –Exact process unknown Uptake of proteins into chlo
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