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BIOL 130 > Unit 5 > Flashcards

Unit 5 Flashcards

1
Q

Relative volumes occupied by major membrane-enclosed organelles in liver cell

A
  • Mitochondria: 1700/cell
  • ER: 12%
  • Golgi: 3%
  • Vesicles: 1% and 200-400 per type
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2
Q

Origin of mitochondria

A

Anaerobic euk cell w/ membrane bound nucleus + ER —> engulfed aerobic bacterium

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3
Q

Evolution of nuclear + ER membranes

A
  • Precursors of euks believed to be organism (like bacteria) with no internal membranes
  • Plasma membrane carried out all membrane-related functions
  • Endomembrane system evolved as invagination of plasma membrane
  • Mitochondria + chloroplasts evolved as endosymbionts
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4
Q

Three ways to import proteins into organelles

A
  1. Through nuclear pores
  2. Across membranes
  3. By vesicles
  • Protein sorting = transfer or proteins into compartments where they are needed
  • Synthesis of virtually all proteins starts in cytosol on free ribosomes
  • All protein transport requires energy
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5
Q

Signal sequence

A
  • Stretch of AAs (15-60 AAs long) –> directs proteins to particular organelles (for nucleus, mito/chloro, peroxi, ER)
  • Usually removed after sorting
  • Delete or transfer sequence to another protein –> protein goes to wrong “address”
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6
Q

Nuclear envelope

A
  • Double membrane of nucleus
  • Close association w/ ER
  • Nuclear lamina: strand material inside (where genetic material is)
  • Nuclear pores: allow proteins into nucleus
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7
Q

Nuclear pore complex

A
  • Nuclear basket
  • Gateway proteins block passage
  • Very high traffic but highly selective (500 molecules through each of 3000-4000 pores/sec)
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8
Q

Transport of proteins into nucleus

A
  • Translation of protein finishes on free ribosomes in cytosol –> protein folds (signal sequence fully functional)
  • Nuclear import receptor (VERY LARGE) binds to nuclear localization signal
  • Protein and receptor enter nucleus –> then dissociate
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9
Q

Features of nuclear pore complexes

A
  • Small molecules (even small proteins) freely pass through
  • Passage of larger proteins is active (req. energy)
  • Nuclear localization signal –> AA sequence tags protein for nuclear transport (import)
  • Nuclear export signal tags protein for export
  • Proteins pass through nuclear pore complexes W/OUT unfolding
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10
Q

What moves into the nucleus?

A
  • Histones, proteins req. for transcription + DNA replication
  • dNTPs, rNTPs
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11
Q

What moves out of the nucleus?

A
  • Mature, properly processed mRNA
  • Ribosomal RNA (manufactured in nucleolus)
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12
Q

Protein import into mitochondria

A
  • Synthesis on free ribosomes in cytosol
  • Signal sequence binds to import receptor on outer mito membrane
  • Import receptor migrates to matching translocator in inner membrane
  • Protein folding is undone and protein is fed through straight (very energy intensive)
  • Localization sequence is cut off –> protein refolds in mito matrix
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13
Q

Transport across membranes (mito/chloro)

A
  • Mito/chloro have double membranes
  • Even though they have own genome + ribosomes –> most of their proteins encoded by nuclear genome –> must be imported
  • Signal sequence located at N terminus of protein
  • Proteins must be moved across both membranes at special sites where layers are in contact
  • Subsequent transport within organelle req. another signal sequence (exposed after first one removed)
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14
Q

ER + endomembrane system

A
  • ER is most extensive of endomembrane system
  • Serves as entry point for proteins for ER + rest of endomembrane system (Golgi, lyso/endosomes), cell surface, secretory proteins
  • Once in ER (in membrane/lumen) –> proteins NEVER re-enter cytosol
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15
Q

Transport into ER

A
  • Synthesis BEGINS on free ribosomes in cytosol
  • AS TRANSLATION OCCURS –> ER signal sequence recognized by SRP (they bind)
  • SRP binds to SRP receptor in ER membrane –> brings growing polypeptide (+ ribosome) to translocation channel –> SRP displaced + recycled
  • SRP receptor detaches + can go assist another protein transport
  • Signal sequence forces translocation channel open + remains there since it is hydrophobic
  • Ribosome still translating –> pushing peptides into ER
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16
Q

2 types of proteins transferred to ER

A
  • Water soluble: translocated completed across into ER lumen (destined for lumen of organelle/secretion)
  • Transmembrane proteins: translocated only partially across (destined for plasma membrane, ER membrane, membrane of another organelle) (not snipped off)
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17
Q

Water soluble proteins

A

Fully translocated into ER lumen, signal sequence cleaved off, folds inside

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18
Q

Single-pass transmembrane protein

A
  • Have a hydrophobic stop-transfer sequence –> also gets stuck in translocation channel –> rest protein translated outside of ER
  • Signal/start transfer sequence cleaved off
19
Q

Multi-pass transmembrane protein

A

Multiple hydrophobic start/stop transfer sequences dictating which parts of protein on cytosolic vs non-cytosolic side

20
Q

Temporary vesicles

A
  • Allow material to leave + enter cells
  • Move material b/w endomembrane compartments
  • Carry soluble proteins (in their lumens) to plasma membrane for secretion
  • Move membrane proteins (in their membranes) to be expressed on cell surface
  • NOT considered organelles
21
Q

Vesicle traffic

A
  • Outward from ER (membrane added to plasma membrane): Golgi –> other organelles? plasma membrane?
  • Inward (membrane subtracted from plasma membrane): plasma membrane –> endosomes (chemical changes happening) –> lysosomes

High specificity of destination

22
Q

Vesicle budding

A

Protein coated pit forms –> membrane bent inward to form vesicle –> vesicle closed + plasma membrane sealed off

23
Q

Clathrin coated vesicles

A
  • Mediate transport from outward face to Golgi + inward from plasma membrane
  • Forms basket that gives vesicle shape
  • Adaptins capture specific cargo molecules by trapping receptors that bind to them

Cargo binds to receptors –> adaptin recognizes + bind receptor + clathrin –> vesicle forms + broken off by dynamin (req. energy) –> coated removed –> naked transport vesicle (can now be processed)

24
Q

Vesicles finding their destination

A
  • Must recognize + dock w/ its specific organelle
  • Each transport vesicle displays molecular markers that identify its origin + cargo
  • Markers must be recognized by complementary receptors on target membrane
25
Q

Rab proteins

A

Family of monomeric GTPases, displayed on vesicle surface

26
Q

Tethering proteins

A
  • Displayed on cytosolic side of target membrane
  • Docking –> interaction btw Rab + tethering protein
27
Q

SNAREs

A

Transmembrane proteins on vesicle (v-SNARE) + target membrane (t-SNARE) –> consolidate docking + catalyze membrane fusion

28
Q

Vesicle docking

A

Tethering: tethering protein (on membrane) recognizes Rab protein (on vesicle) + brings vesicle close to membrane
- Docking: If v-SNARE + t-SNARE match –> interact (specificity) –> pull vesicle closer to membrane on both sides –> lipid bilayers fuse

MEMBRANE ASYMMETRY MAINTAINED

29
Q

ER processing

A

Most proteins covalently modified in ER
- Disulfide bonds
- Glycosylation (addition of sugar groups): various functions depending on protein, protect from degradation, help direct protein to proper organelle (act as transport signal for packing into appropriate vesicle), on cell surface –> cell-cell recognition

30
Q

Disulfide bonds

A
  • Covalent stabilization of protein structure found in secreted proteins (destined for more hostile extracellular environment)
  • Formed in ER (oxidizing environment)
31
Q

Glycosylation (ER)

A
  • Addition of sugar groups during protein translocation into ER
  • Various functions depending on protein: protect from degradation, help direct protein to proper organelle (act as transport signal for packing into appropriate vesicle), on cell surface –> cell-cell recognition
  • Already formed carbohydrates attached to membrane lipid dolichol
  • As growing polypeptide enters ER, carbohydrate group transfers to amino (NH2) groups of asparagine (Asn) side chains via membrane-bound enzyme (N-linked glycosylation)

IN ER LUMEN

32
Q

Proteins leaving ER

A
  • Only properly folded –> allowed to leave ER
  • If misfolded –> chaperone protein surrounds it + gives another chance to fix
33
Q

Unfolded protein response (UPR)

A
  • Sensors for misfolded proteins activated in ER lumen –> transcription regulators activated (drive transcription of particular genes) –> activation of chaperone genes + other genes that increase protein folding capacity + expansion of ER
  • If cell can’t keep up, UPR will trigger cell death –> apoptosis
34
Q

Golgi apparatus structure

A
  • Series of flattened sacs (cisternae)
  • Organized into functionally distinct compartments w/ cis (entry) face closest to ER, trans (exit) face at other end
  • Cis: newly formed
  • Trans: breaking away
35
Q

Functions of Golgi

A

Modification of new proteins arriving from ER (post-transcriptional modifs):
- Peptide chains shortened by proteases
- AAs modified
- CHO groups –> added in ER modified or removed
- Glycosylation: different CHO groups added to different AAs (O-linked glycosylation)

Most complex polysaccharide synthesis:
- Glycos amino glycans in extracellular matrix (animals)
- Pectins, hemicellulose (plant cell walls)

36
Q

Regulated secretion

A
  • Secretory proteins in high conc. in vesicles leaving Golgi
  • Extracellular signal (hormone/neurotransmitter) binds to receptor –> secretion
  • Cell is often “famous for secreting this particular protein” (main function)
37
Q

Constitutive secretion

A
  • Happening by default, all the time
  • Vesicle buds off Golgi –> non-specific proteins/membrane lipids inside –> secretion
  • Happening in all cells in an unregulated way
  • e.g. Getting new lipids to plasma membrane, secreting products of cell (characteristic molecules –> cell conditioning its medium, creating comfort for itself)
38
Q

Endocytic pathways

A
  • Taking substances into cell by surrounding them w/ membrane –> become membrane bound vesicle
  • 2 main types based on size: pinocytosis + phagocytosis
  • 1 more type in animal cells: receptor-mediated endocystosis
39
Q

Pinocytosis

A
  • “Cell drinking” / “sampling”
  • Tiny vesicles formed (endosomes)
  • Done by all euks
  • Solutes, macromolecules, fluid
  • ‘Bulk’ –> any molecules present in enclosed fluid enter cell
  • Non-specific + unregulated
40
Q

Phagocytosis

A
  • “Cell eating”
  • Much larger vesicles (phagosomes)
  • Done only by specialized cells –> phagocytes –> e.g. macrophages
  • Particles, other cells, debris
41
Q

Receptor-mediated endocytosis

A
  • Very selective concentrating mechanisms
  • Requires specialized receptors
  • Particular molecules (ligands) for which the membrane has receptors
  • Receptors grouped in patches of membrane called coated pits (e.g. clathrin)
42
Q

LDL example of receptor-mediated endocytosis

A

LDL binds to receptors –> clathrin coated vesicle forms –> uncoats –> fusion with endosome –> contents transfer to lysososme (receptor buds off of transport vesicles + returns to plasma membrane)

43
Q

Lysosomes

A
  • Site of cellular digestion (from endocytosis, phagocytosis, and autophagy (break down of worn out mito/organelles)
  • Have many acid hydrolases (work in acidic conditions)
  • Have proton pump
  • If mutation in acid hydrolase –> buildup of non-broken down material –> lysosomal storage diseases
44
Q

Roadmap of protein traffic

A
  • Escorted transport: cytosol –> nucleus
  • Transmembrane transport: cytosol –> mitochondria/ER
  • Vesicular transport: ER <–> Golgi –> endosomes, cell surface, secretory vesicles
  • Endocytosis: cell surface –> early endosome –> late endosome –> lysosome

BIOL 130 (6 decks)

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