Unit 5 Flashcards
(44 cards)
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
2
Q
Origin of mitochondria
A
Anaerobic euk cell w/ membrane bound nucleus + ER —> engulfed aerobic bacterium
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
4
Q
Three ways to import proteins into organelles
A
- Through nuclear pores
- Across membranes
- 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
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”
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
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)
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
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
10
Q
What moves into the nucleus?
A
- Histones, proteins req. for transcription + DNA replication
- dNTPs, rNTPs
11
Q
What moves out of the nucleus?
A
- Mature, properly processed mRNA
- Ribosomal RNA (manufactured in nucleolus)
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
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)
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
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
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)
17
Q
Water soluble proteins
A
Fully translocated into ER lumen, signal sequence cleaved off, folds inside
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
Rab proteins
Family of monomeric GTPases, displayed on vesicle surface
26
Tethering proteins
- Displayed on cytosolic side of target membrane
- Docking --> interaction btw Rab + tethering protein
27
SNAREs
Transmembrane proteins on vesicle (v-SNARE) + target membrane (t-SNARE) --> consolidate docking + catalyze membrane fusion
28
Vesicle docking
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
ER processing
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
Disulfide bonds
- Covalent stabilization of protein structure found in secreted proteins (destined for more hostile extracellular environment)
- Formed in ER (oxidizing environment)
31
Glycosylation (ER)
- 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
Proteins leaving ER
- Only properly folded --> allowed to leave ER
- If misfolded --> chaperone protein surrounds it + gives another chance to fix
33
Unfolded protein response (UPR)
- 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
Golgi apparatus structure
- 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
Functions of Golgi
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
Regulated secretion
- 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
Constitutive secretion
- 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
Endocytic pathways
- 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
Pinocytosis
- "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
Phagocytosis
- "Cell eating"
- Much larger vesicles (phagosomes)
- Done only by specialized cells --> phagocytes --> e.g. macrophages
- Particles, other cells, debris
41
Receptor-mediated endocytosis
- 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
LDL example of receptor-mediated endocytosis
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
Lysosomes
- 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
Roadmap of protein traffic
- 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
