8.1-8.10 Flashcards
how many major types of organelles
8
two of the main transport routes for proteins
- exocytic pathway (secretory)
- endocytic pathway
new proteins destined for any location in exo/endocytic pathways must first be targeted where?
to the ER
2 ways out of the ER
- to fail to fold properly, retrograde transport, ubiquination, proteasomed
- exit via budding into transport vesicle
transport vesicles leave the __________ compartment and fuse with the __________ compartment
donor, acceptor
when do endosomes form
during endocytosis
true or false: transport must be bi-directional
true
where is transport not bidirectional?
lysosomes
____________ mechanisms return vesicle components to donor compartment
recycling
_____________ mechanisms returns resident proteins which have escaped
salvage
Pulse chase experiment
used to experimentally show the pathway that proteins take as they move through the secretory pathway
what does pulse chase monitor
progression of location change over time and size change over time
how are proteins labeled in pulse chase
radioactive amino acid
which is longer: the pulse or the chase?
the chase
regulated secretion
when cells accumulate proteins to be secreted in vesicles near the pm, releasing them upon stimulation
ex. of regulated secretion
- digestive enzymes (pepsinogen, trypsinogen)
- hormones (ADH, insulin)
- histamine
constitutive secretion
when cells continuously secrete a protein
ex. of constitutive secretion
- immunoglobulins
- yolk protein
- bacterial infection-promoting proteins
- insulin
most abundant membrane in most eukaryotic cells
ER membrane
what is the Golgi apparatus composed of
Golgi stack made of cis, medial, and trans cisternae
is cis-Golgi nearest entry or ext face
entry
is trans-Golgi nearest entry or exit face
exit
what are mannose oligosaccharides modified to sequentially in the Golgi
highly sialylated structures
will a protein in the trans Golgi have a greater or less degree of mannose than cis
less
TGN sorts for distribution to:
- lysosomes
- PM by regulated/constitutive secretion
endocytosis functions:
- internalization of nutrients
- regulating cell surface expression of receptors/transporters
- uptake/recycle of EC debris
- recovery of membrane from pm
which is most degradative: early endosome, late endosome, lysosome
lysosome
V-ATPases (H+-ATPases)
transport protons from cytosol into the organelle lumen
acidification of early endosomes
- dissociation of internalized ligand-receptor complexes
- return of receptors to pm
acidification of late endosomes
delivery of lysosomal enzymes from TGN
what cells are known as “professional phagocytes”
macrophages and dendritic cells
what do macrophages and dendritic cells phagocytose?
pathogens and senescent/apoptotic cells
steps of receptor-mediated endocytosis
- cell surface receptor bind ligands (accumulate at coated pits)
- pinch off to form vesicles that fuse with early endosomes
- acidity separates ligands/receptors, receptors recycle to PM
- ligand goes to lysosomes
where does transcytosis occur
epithelial cells lining intestine and other body cavities
transcytosis
vesicle forms via endocytosis on one membrane, transported through cell, exocytosed at dif membrane
ex. of transcytosis
infants taking Ig from mother’s milk by binding it to Ig gut receptors, then transferring it to other side and releasing into blood
steps in vesicle-mediated transport
- budding - coat/adaptor complexes, PI lipids
- scission
- uncoating
- tethering
- docking
- fusion
cargo selection often involves what?
binding of protein to a cytoplasmic protein (coat protein)
how does cargo selection occur for membrane proteins
via sorting signal in cytoplasmic tails
how does cargo selection occur for soluble proteins in ER lumen
bind receptor protein or bulk flow
COPI and COPII proteins are used when?
exocytic pathway
clathrin coats are used when?
endocytic pathway
when do v-SNARE proteins form complexes with t-SNARE proteins?
during docking
what happens when proteins using signal-mediate movement don’t have a signal
they stay in the given organelle as a resident component
what happens when proteins using bulk transport don’t have a signal
keep going through pathway - require signal to stop in compartment in pathway or divert to another pathway
easiest way to get proteins out of ECM
bulk transport (first need signal to ER)
where do COPII vesicles mediate transport from
ER to Golgi
where do proteins to be exported gather
at ER export sites
only route of vesicular exit from ER
COPII-coated vesciles
soluble COPII components
Sar1p, Sec23/24, Sec13/31
Sar1p
small GTPase, inactive in cytosol and bound to GDP
- (Sar1p-GDP)
Sec12p
intrinsic membrane protein, indirectly activates Sar1p by getting rid of bound GDP so it can bind to GTP
- (Sar-GEF)
GEF
guanine-nucleotide exchange factor
- exchanges GDP for GTP
what does Sar1p do when activated
moves to membrane to form PMP and is bound to GTP
Sec23/24 and Sec13/31
sets of heterodimers which are structural coat complexes
once Sar1p moves to the membrane, what happens
Sec23/24 joins, then Sec13/31
what does Sec23 do to Sar1p
following vesicle formation, stimulates Sar1p to hydrolyze bound GTP to GDP, which stimulates uncoating
- (Sar-GAP)
GAP
GTP-ase binding protein - hydrolyzes GTP to GDP
where do v-SNARES bind (COPII)
Sar1p and Sec 23/24
describe ER export signals
- usually c-terminus
- diacidic or diphenylalanine
where are ER export signals known to bind
sec23/24
vesicular tubular clusters
when COPII vesicles cluster following scission from ER
what amino acids comprise the KDEL signal
Lys, Asp, Glu, Leu
where is the KDEL signal found
c-terminus of most soluble ER-resident proteins
where are KDEL receptor proteins mostly found
compartments of VTCs and CGN
binding of a KDEL receptor to KDEL sequence triggers what?
formation of COPI vesicle that will return complex back to ER
retrieval signals of ER resident membrane-bound proteins
dibasic retrieval signals at their cytoplasmic tails (C or N term)
type I transmembrane proteins
N-terminus in the lumen, signal is a dilysine
type II transmembrane proteins
C-terminus in the lumen, signal is diarginine
pathway of COPI vesicles
- Golgi to ER
- TGN to CGN
- CGN to TGN
ADP-ribosylation factor
GTPase that helps coat formation for COPI vesicles
ARF-GDP
soluble cytosolic protein
what is ARF-GDP activate by
ARF-GEF
where is ARF-GEF located
Golgi membranes
function of ARF-GEF
helps GDP dissociate from ARF so GTP can bind
what does ARF-GTP do
undergoes conformational change, which exposes myristic acid at ARF’s N-terminus and enables it to anchor on c-face of Golgi membrane
is myristic acid at N or C-terminus
N-terminus
what does membrane-bound ARF-GTP recruit
coatomers (COPI coat complexes)
what are bound coaxers analogous to
Sec23/24 and Sec13/31
types of proteins recycled back to ER
- type I membrane proteins - dilysine retrieval signal in cytoplasmic tail
- soluble proteins with KDEL (receptor might bind to coatomer proteins)
what does ARF-GAP do
binds to ARF-GTP, stimulates it to hydrolyze GTP to GDP, leading to uncoating
2 popular models for forward transport through Golgi
- cisternal maturation model
- vesicle-mediated transport model
cisternal maturation model
entire cistern move through the stack, new cisteriae on cis side
vesicle-mediated transport model
stack remains stable, cargo moving from cisterna to cisterna in COPI vesicles
does the cisternal or vesicle model work with large cargo
cisternal
most Golgi enzymes are what?
type II transmembrane proteins with short N-term on c-face and catalytic C-term in lumen
retention signals of Golgi enzymes
membrane-spanning domain and its flanking aa residues
2 models for Golgi retention
- Kin recognition
- bilayer thickness model
Kin recognition
recognition of each other by the enzymes could generate aggregates too large to enter a COPI vesicle
bilayer thickness model
length of membrane-spanning domain keeps protein in Golgi, later parts of secretory pathway have increasing cholesterol, resulting in increased membrane thickness
