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MCB2050 > Endomembrane System Part 2 > Flashcards

Endomembrane System Part 2 Flashcards

1
Q

Golgi Complex

A
  • ‘complex’ or ‘stack’ of flattened, membrane-bound cisternae (sacs) with dilated edges and numerous associated tubules and vesicles
  • number and distribution of them vary between different cell types (mammals contain one large one, and plants/yeast contain many small)
  • possesses several subcompartments (results in complex (stack) having distinct polarity: both structurally and functionally)
  • Location of Protein modification (N-linked glycosylation continued and phosphorylation of mannose
    groups (M6P) for protein targeting to lysosomes)
  • discovered by Camillo Golgi (1898)
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2
Q

Cisternae

A
  • embrane-bound sacs with dilated edges and numerous associated tubules and vesicles
  • 3+ make up the Golgi Cisternae
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3
Q

cis-Golgi network (CGN)

A
  • consists of complex, interconnected network of tubules and vesicles adjacent to ERES
  • located at cis face of Golgi complex
  • initial destination of COPII transport vesicles from ERES
  • serves as a ‘sorting station’…:
  • destination (‘acceptor’ compartment) of COPII vesicles coming ‘forward’ (anterograde transport) from ERES to CGN
  • site of COPI vesicle assembly for transport ’back’ (retrograde
    transport) from CGN to ER
  • ‘forward’ (anterograde) transport as CGN matures into next subcompartment of Golgi complex (i.e., CGN → cis cisternae)
  • destination of COPI vesicles moving ‘back’ (retrograde transport) from next subcompartment of Golgi complex (
    cis cisternae) to CGN
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4
Q

Golgi Cisternae (cis, medial and
trans)

A
  • series of three or more large, flattened cisternae (makes up most of the golgi)
  • main sections: cis, medial and trans cisternae
  • sites of Golgi metabolism (synthesis of complex polysaccharides used for cell wall and modification (glycosylation) of proteins/lipids & phosphorylation of mannose units in lysosomal-destined proteins)
  • Golgi cisternae act as ‘assembly line’ in N-linked glycosylation:
  • core oligosaccharides on proteins moving through Golgi modified sequentially (in various ways) by different enzymes in
    each subcompartment (cis, medial and trans Golgi cisternae possess unique glycosyltransferase and glycosidase enzymes)
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5
Q

trans-Golgi network (TGN)

A
  • interconnected network of tubules and vesicles ( » CGN)
  • located on trans face of Golgi complex
  • serves as ‘sorting station’ ( » CGN)…:
  • ‘forward’ (anterograde) transport as previous subcompartment of Golgi complex matures into TGN (i.e.,
    trans cisternae → TGN)
  • site of clathrin coat vesicle assembly for transport ‘forward’
    (anterograde transport) from TGN to endosomes
  • site of secretory vesicle and secretory granule assembly
    for transport ‘forward’ (anterograde) to pm (secretion into
    extracellular space)
  • site of COPI vesicle assembly for transport ’back’ (retrograde)
    to Golgi trans cisternae
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6
Q

Golgi matrix

A
  • consists of various Golgi peripheral and integral membrane proteins (ex. GRASPS)
  • mediates organization of Golgi complex (stack)
  • links Golgi complex to cytoskeleton (positioning and movement of Golgi (like all organelles/vesicles) in cell controlled by interaction with cytoskeleton)
  • cytoplasmic-facing domains interact to form ‘scaffold’ – link CGN, cis/medial/trans cisternae, and TGN together
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7
Q

GRASPs (Golgi reassembly and stacking proteins)

A
  • serve as ‘tethering proteins’ to link different Golgi subcompartments together – RNAi of GRAPS results
    Golgi complex disassembly
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8
Q

N-linked glycosylation

A
  • most glycoproteins (synthesized & N-linked-glycosylated in RER) moving through Golgi ( cis -> trans) subjected to additional
    glycosylation reactions
  • core oligosaccharides on proteins moving through Golgi modified sequentially (in various ways) by different enzymes in
    each subcompartment
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9
Q

a-mannosidase I

A
  • found within the cis cisternae, and removes 3 mannose sugars from core oligosaccharide of glycoprotein during N-linked glycosylation
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10
Q

Mannose-6-phosphate (M6P)

A
  • in cis cisternae, mannose units in core oligosaccharide(s) of soluble proteins destined for lysosomes are phosphorylated with these residues
  • N-acetylglucosamine phosphotransferase recognizes unique sequences in lysosomal-destined proteins
  • prevents lysosomal-destined proteins from being subjected to N-linked glycosylation reactions

Basically…
- proteins without this are packaged at TGN into
secretory transport vesicles/granules destined for
plasma membrane/extracellular space (via constitutive
and regulated secretion pathways) …..or reside in Golgi
- proteins with this are packaged at TGN into clathrin-
coated transport vesicle to endosomes and then
lysosomes (via the biosynthetic pathway)

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11
Q

Signal patch

A
  • targeting signal consisting of specific 3D arrangement of molecules (e.g., sugars) on folded protein’s surface
  • Distinct from the polypeptide-based targeting signal (e.g., NLS and signal sequence involved in nuclear and ER targeting, respectively)
  • M6P acts as a one of these
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12
Q

N-acetylglucosamine phosphotransferase

A
  • recognizes unique sequences (M6P)
    in lysosomal-destined proteins
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13
Q

Cisternal progression/maturation model

A
  • Golgi is a dynamic structure: each subcompartment continually moves (forward) fromcis to
    trans side of Golgi complex
  • Golgi complex persists ( structurally & functionally)
    because COPI transport vesicles continually move
    resident Golgi proteins ‘back’ (retrograde transport)
    to proper subcompartment

It’s essentially a constant flow of forward and backwards motion that just goes on forever lolol idk how to explain this do your own research pfft

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14
Q

COPI

A
  • COPI-coated vesicles move backwards (retrograde transport) between Golgi subcompartments
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15
Q

Lysosome

A
  • digestive organelle – degrades all types of macromolecules
  • plays key role in degradation of larger cellular components/organelles (autophagy)
  • contains ~60 different soluble acid hydrolyase enzymes
    enzymatically active only at low pH of lysosome interior     (lumen)
  • products of degradation are transported into cytoplasm
  • low pH in lysosomal lumen maintained by membrane-bound ATPase proton pumps
  • highly dynamic: lysosomes possess wide variety of shapes and sizes depending on organism/tissue/cell type
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16
Q

Autophagy

A
  • degradation of larger cellular components/organelles
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17
Q

Acid hydrolyase

A
  • responsible for breakdown of materials within lysosomes (contains ~60 different ones)
  • enzymatically active only at low pH of lysosome interior (lumen)
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18
Q

M6P receptor

A
  • integral transmembrane protein that mediates subsequent
    concentration of soluble lysosomal (‘cargo’) proteins into nascent clathrin-coated transport vesicles
  • in the TGN, soluble M6P-bearing lysosomal
    destined proteins (e.g., acid hydrolyases)
    recognized by M6P receptor
  • lumenal-facing domain of M6P receptor binds
    to M6P groups on soluble lysosomal-destined
    proteins in lumen of TGN
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19
Q

Clathrin-coated transport vesicle

A
  • transport materials to the lysosomes (proteins with M6P: TGN → clathrin-coated vesicle → late endosome → lysosome)
20
Q

AP1 and GGA (AP complex)

A
  • mediate vesicle ‘cargo’ section and serve as ‘linker’ for clathrin-coat vesicle assembly
  • cytoplasmic-facing domain of M6P receptor binds to AP1 and GGA adaptor coat proteins
  • recruitment of AP1/GGA adaptor proteins from cytoplasm to TGN surface mediated by GTPase Arf1
21
Q

Arf1

A
  • mediates recruitment of AP1/GGA adaptor proteins from cytoplasm to TGN surface
  • Arf1 binding to GTP causes conformational
    change; exposed lipid anchor in Arf1-GTP directs it from
    cytoplasm to outer leaflet of TGN membrane; initiates outward bending of membrane
22
Q

Clathrin: triskelion

A
  • clathrin triskelions recruited from cytoplasm self-assemble to form outer ‘scaffolding’ (cage-like lattice) of ‘coat’ on growing vesicle clathrin assembly promotes curvature (outward bending) of TGN membrane
  • one molecule of clathrin consists of
    three ‘light’ chain polypeptides & three
    ‘heavy’ chain polypeptides
  • individual clathrin triskelions initially assemble to form hexagons that lie flat on membrane (cytoplasmic) surface
  • triskelions subsequently self-assemble to form pentagons
23
Q

Dynamin

A
  • mediates release (scission) of clathrin-coated vesicle from TGN membrane into cytosol; results in breakdown of clathrin-coat upon release
  • dynamin recruited from cytosol to connection (‘stalk’)
    between growing clathrin-coated bud and TGN membrane
  • dynamin proteins assemble to form dynamin ring
    around stalk
  • GTP hydrolysis causes a conformational change in
    dynamin ring resulting in twisting and ‘pinching off’
    (scission) of nascent vesicle
24
Q

Scission

A
  • release of a vesicle from wherever it was forming, so it can begin its epic journey

Didn’t know what to write for this one either lolol so this was what you get

25
Q

gamma-GTP

A
  • incubation of cell with gamma-GTP (non-hydrolyzable analog of GTP) causes continued dynamin ring polymerization, resulting in long, extended ‘stalk’ and no scission of vesicle bud
26
Q

Late endosome

A
  • junction of biosynthetic and endocytic pathways
  • acidic interior (~pH 5.0-5.5 in lumen) of late endosomes causes M6P receptors to dissociate from soluble lysosomal ‘cargo’ proteins (acid hydrolases)
  • eventually, late endosome fuses with lysosome (late endosome trafficking and docking/fusion with lysosome mediated by organelle-specific Rab/Rab effectors and SNARES)
27
Q

Retromer transport vesicle

A
  • ‘empty’ M6P receptors ‘recycled’ back to TGN via retromer-coated vesicles (retromer complex ‘coat’ assembles on cytoplasmic surface of late endosome, and is destroyed after scission)
  • retromer vesicles (w/ various pm cargo and
    ‘empty’ M6P receptors) also traffic to
    plasma membrane
28
Q

Receptor-mediated endocytosis

A
  • M6P receptor-cargo protein complexes
    at pm retrieved by receptor-mediated
    endocytosis and delivered back to
    late endosome
29
Q

Constitutive secretion pathway (default pathway)

A
  • materials continually transported
    (via secretory vesicles) from TGN to pm
  • vesicles fuse (via Rabs & SNAREs) with
    pm and release (exocytosis) their lumenal
    soluble ‘cargo’ outside of cell
  • pathway for proteins not selectively
    sorted through biosynthetic pathway
    (i.e., targeted to late endosomes and
    lysosomes) OR by regulated secretion
30
Q

Phagocytosis

A
  • uptake of large, particulate materials from
    extracellular space by specialized cells
  • ex. Recognition and removal of bacteria by leukocytes
31
Q

Fc receptor

A
  • leukocyte plasma membrane Fc receptors recognize exposed Fc domain on antibodies bound to bacterium and signal re-assembly of actin microfilament network (pseudopods)
32
Q

Fc domain

A
  • bacteria identified by immune system as ‘foreign’ material and
    generates antibodies containing Fc domains against bacterial cell-surface components
33
Q

Pseudopod

A
  • alterations in cytoskeleton result in changes in shape ofof leukocyte; cell extensions = pseudopods, which allows engulfing of bactrium
34
Q

Phagosome

A
  • eukocyte pm engulfs (pseudopods) bacterium and fuse to form phagosome
35
Q

Bulk-phase endocytosis: Pinocytosis

A
  • non-specific uptake of extracellular fluids and plasma membrane proteins and lipids into small vesicles
36
Q

Receptor-mediated endocytosis

A
  • specific cell-surface (pm) receptor binds extracellular ligand(s) and receptor-ligand complexes subsequently concentrated and internalized in clathrin-coated transport vesicles

Examples of materials (macromolecules) internalized by receptor-mediated endocytosis…
- M6P receptor-bound, lysosomal proteins ‘escaped’ from TGN via secretory pathway
- Receptor complexes with bound cell signaling hormones (e.g., insulin) or growth factors (e.g., EGF)
- Iron (Fe3+ )-bound carrier protein ferrotransferrin recognized by transferrin receptor Cholesterol-containing, low-density lipoprotein (LDL) particle recognized by LDL receptor

37
Q

AP2

A
  • cytoplasmic protein serves as ‘linker’ during clathrin-coat vesicle assembly at pm
  • cytoplasmic-facing domain of receptor binds to AP2 adaptor ‘coat’ protein
  • receptor-ligand-AP2 complex accumulates in clathrin-coated pit
38
Q

Clathrin-coated pit

A
  • specialized regions (indentations) of pm where receptor-ligand complexes are concentrated
    and endocytic vesicles eventually form
  • inner (cytoplasmic) leaflet of pm at coated pit enriched in unique membrane phospholipids (membrane lipid microdomain)
39
Q

Membrane lipid microdomain

A
  • enriched in phosphatidylinositol (PI) (4,5) P 2 – serves as signal for recruiting AP2 with bound receptor-ligand into coated pit
40
Q

Phosphatidylinositol (4,5) P2

A
  • serves as signal for recruiting AP2 with bound receptor-ligand into coated pit (found plentifully in the membrane lipid microdomain)
  • one of AP2s binding domains
41
Q

Multivesicular late endosome: Multivesicular body

A
  • contains numerous intralumenal vesicles
    similar in size to transport vesicles, but opposite topology: MVB vesicles bud away from the cytoplasm (unlike COPI/II and clathrin vesicles - bud towards cytoplasm)
  • formed via inward budding of vesicles into late endosome interior
42
Q

ESCRT machinery

A
  • soluble (cytosolic) protein constituents recruited to MVB surface - mediate membrane ‘cargo’ protein selection and inward vesicle budding
  • endocytosed membrane proteins destined for degradation linked to mono-ubiquitin
  • disassembly of ESCRT complex by Vps4 ATPase
43
Q

Mono-ubquitination

A
  • signal for recognition by ESCRT protein Hrs and subsequent entry (packaging) into newly-forming MVB
    vesicle
  • Hrs also mono-ubiquitinated
44
Q

Hrs

A
  • Hrs recruits additional ESCRT proteins – assemble to mediate inward budding and scission of
    nascent vesicle into MVB lumen
45
Q

Vps4

A
  • mediates disassembly of ESCRT complex
46
Q

HIV Gag protein

A
  • HIV Gag protein functions similar to ESCRT Hrs

MCB2050 (5 decks)

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