Lecture 12 - endomembrane system part 7 Flashcards
What are the two secretion pathways in the endomembrane system?
- constitutive secretion pathway
- regulated secretion pathway
What do secretion pathways in the endomembrane system do?
transport cargo from TGN to pm and extracellular space
What is the constitutive secretion pathway?
materials continually transported via secretory vesicles from TGN to pm
vesicles fuse with pm and release (exocytosis) their luminal soluble cargo outside cell
vesicle is incorporated into pm
the default pathway
What is the regulated secretion pathway?
materials at TGN packaged into secretory granules
in response to cellular signal: granules stored in cytoplasm target to and fuse with pm and release (exocytosis their luminal cargo outside of cell
What is the endocytic pathway?
operates in opposite direction to secretory pathways, moving into the cell
What are the two main processes for internalization (bringing material in)?
- phagocytosis - uptake of large particulate materials from extracellular space by specialized cells
- endocytosis
What are the steps of phagocytosis of the recognition and removal of bacteria by leakocytes?
- bacteria identified by immune system as foreign material and generates antibodies against bacterial cell-surface components.
antibody Fab domain binds bacterial cell surface protein/sugars - leukocyte pm Fc receptors recognize exposed Fc domain on antibodies bound to bacterium
- Fc receptors signal reassembly of actin microfilament network
- alterations in cytoskeleton result in changes in shape of leukocyte - cell extensions - pseudopods - leukocyte pm engulfs pseudopods bacterium and fuse to form a phagosome
- phagosome fuses with lysosome
- fusion is blocked by mycobacterium tuberculosis (bacteria hijacks phagocytic machinery to infect leukocyte)
- bacterium digested and nutrients released into cytoplasm of leukocyte
What are the two types of endocytosis?
- bulk-phase endocytosis - non-specific uptake of extracellular fluids and pm proteins and lipids into small vesicles
- receptor-mediated endocytosis - specific pm receptor binds extracellular ligands and internalized into clathrin-coated vesicle
What are some materials internalized by receptor-mediated endocytosis?
- M6P receptor bound lysosomal proteins escaped from TGN via secretory pathway
- receptor complexes with bound cell signalling hormones or growth factors
- iron bound carrier protein ferrotransferrin recognized by transferrin receptor
- cholesterol-containing LDL particle recognized by LDL receptor
What are the steps to receptor-mediated endocytosis?
- transmembrane receptor at pm activated by binding to specific extracellular ligand
- cytoplasmic facing domain of receptor binds to AP2 adaptor coat protein - cytoplasmic protein serves as linker during clathrin coat vesicle assembly at pm
- receptor ligand AP2 complex accumulates in clathrin-coated pit
- AP2 at cytoplasmic face of coated pit forms inner layer of coat, AP2 also recruits clathrin triskelions from cytoplasm
- triskelions self assemble to form outer scaffolding on the coat
- triskelion hexagon to pentagon formation acts to curve the pm - clathrin-coated vesicle bud pinches off from pm via dynamic and GTP hydrolysis
- soon after budding, clathrin coat disassembles from vesicle (AP2 and triskelions)
- nascent uncoated endocytic vesicle referred to as early endosome
- early endosome fuses with late endosome (mediated by Rabs and SNARES)
- acidic interior of late endosome lumen causes dissociation of receptor-ligand complexes
- late endosome with free soluble ligands fuse with lysosome - mediated by Rabs and SNARES
- free receptors recycled back to cell surface for additional rounds of endocytosis OR in the lysosome - ligands are degraded by soluble lysosomal acid hydrolyases
What is a clathrin-coated pit?
specialized regions of pm where receptor-ligand complexes are concentrated and endocytic vesicles eventually form
What does the clathrin-coated pit contain
the inner leaflet of pm at coated pit is enriched in unique membrane phospholipids - serves as signal for recruiting AP2 with bound receptor-ligand into pit
What are the binding domains of AP2?
- PI(4,5)P2
- cytoplasmic domains of pm transmembrane receptors
- clathrin
What is the fate of the receptors after receptor-mediated endocytosis?
free receptors recycled back to cell surface or delivered to lysosome interior for degradation
What are the steps to deliver free receptors into lysosome after receptor-mediated endocytosis?
- sorting at the multi-vesicular body (MVB) of nascent resident lysosomal membrane proteins from TGN (via biosynthetic pathway) from membrane proteins destined for degradation in lysosomes
- MVB vesicles selectively contain membrane proteins destined for degradation in lysosome interior
- membrane cargo protein selection and inward vesicle budding is mediated by ESCRT machinery - endocytosed membrane proteins destined for degradation linked to mono-ubiquitin
- serves as signal for recognition by ESCRT protein Hrs and subsequent entry into newly-forming MVB vesicle - Hrs recruits additional ESCRT proteins
- assemble to mediate inward budding and scission of nascent vesicle into MVB lumen - disassembly of ESCRT complex by Vps4 ATPase
- ESCRT comp released into cytoplasm, recycled - MVB fuses with lysosome
- mediated by Rab and SNARES
- intraluminal vesicles with endocytosed membrane proteins released into lysosome interior and degraded by acid hydrolases
- lysosomal membrane proteins in MVB membrane move laterally into lysosomal membrane
