Endocytosis and intracellular vesicle traffic lecture

why do cells need endocytosis

-lipid bilayer is poorly permeable to ions and macromolecules

-small and large molecules can enter through protein pumps, carriers or channels

-macromolecules can enter only through membrane bound carriers = endocytosis

what is endocytosis

-process of uptake of outside material by invagination of plasma membrane

-followed by pinching off and intracellular vesicle formation

what are the types of cellular uptake mechanisms

-macropinocytosis

-clathrin-coated vesicle

-non coated vesicle

-caveolae

-phagocytosis

what is the function of phagocytosis

-ingestion/removal of bacteria

-professional phagocytic cells have receptors that can bind antibodies that tag bacteria

what is the mechanism of phagocytosis

-receptors that bind become activated

-F-actin polymerisation

-plasma membrane protrudes and zippers around particle and phagosome is sealed off

-F-actin disassembles

-phagosome is transported into cells

-fuses with lysosome (phagolysosome)

-degradation of contents

what is the function of macropinocytosis

-uptake of fluid for feeding and removal of large growth factor receptors from plasma membrane

-used by cancer cells for nutrient uptake

what do growth factors do

-stimulates actin driven protrusion of plasma membrane called ruffles that are taken up as large vesicles

how is macropinocytosis triggered

-triggered by some bacteria for uptake

what is the function of clathrin mediated endocytosis

-receptor mediated endocytosis

-allows cell to take up specific macromolecules which arent abundant in extracellular fluid

-via >25 receptors

-req clathrin and adapter molecules

what are the 2 subtypes of clathrin mediated endocytosis

-constitutive; iron & cholesterol uptake

-ligand induced: modulation of growth factor signalling

what is the structure of clathrin

-3-legged structure

what is the difference between clathrin mediated and non-clathrin mediated endocytosis.

clathrin-mediated:

-clathrin coated

-receptor driven

-well defined vesicles

non-clathrin mediated:

-no clathrin coat

-variety of mechanisms & functions

what is the mechanism of clathrin mediated endocytosis

-adaptin is recruited to membrane

-it recognises and binds cargo receptors that have captured their cargo molecules

-it also acts as recruits clathrin to cytoplasmic face

-clathrin recruited by adaptin and self assemble into polyhedral lattice on cytoplasmic surface

-causes membrane to curve inwards forming coated pit

-dynamin recruited and warps around neck of bud

-uses GTP hydrolysis to constrict and pinch off neck

-releasing coated vesicle into cytosol

-F-actin pushes vesicle away from mechanism and supports vesicle scission

-once vesicle is free in cytosol clathrin coat is removed

-fuses with target

what is caveolae

-small 50nm invaginations enriched in cholesterol and caveolin

what is the function of caveolin-mediated endocytosis

-plasma membrane reservoir to balance membrane tension

what is mechanism of caveolin-mediated endocytosis

-caveolin self associates to form a coat and together with cholesterol in the membrane

-causes membrane invagination

-internalisation of caveolae req dynamin and F-actin