Chapter 15 Flashcards
ways that eukaryotic cells segregate chemical processes
membrane-enclosed organelles
phase separation of protein complexes (biomolecular condensates)
membrane contact sites, non vesicular communication
nucleus characteristics
outer and inner membrane
outer membrane continuous with ER
nuclear inter membrane space continuous with ER lumen
communicates with cytosol through nuclear pores
rough ER characteristics
site of new membrane synthesis
ribosomes on the cytosolic side synthesize proteins that are sorted into the ER membrane or lumen
smooth ER characteristics
steroid hormone synthesis; lipid synthesis
Ca+2 stores-uptake and release in response to extracellular signals (neuron signaling)
free ribosomes function
synthesis of cytosolic proteins
Golgi apparatus function
modifies proteins and lipids from the ER on their way to other cell compartments or EC space (sorting)
lysosome characteristics
breaks down damaged organelles and endocytosed macromolecules
signaling
acidic
Peroxisome function
breaks down lipids
detoxification
membrane invagination mechanism
nuclear membrane and ER believed to have evolved from invagination of the PM
endosymbiosis
mitochondria and chloroplasts may have evolved from uptake of aerobic bacteria
three mechanisms of protein import into organelles
transport through nuclear pores
transport across membranes by translocators
vesicular transport
sorting signal
N-terminal sorting sequence directs protein to organelle where it is required
no sequence = proteins stay in cytosol
15-60aa long
often cleaved after sorting
protein sorting to nucleus and mitochondria
proteins synthesized in cytosol delivered directly to nucleus and mitochondria
protein sorting within endomembrane system
ER synthesizes proteins/lipids and receives proteins from the cytosol
some retained in ER, but most packaged in vesicles for transport to Golgi, then to lysosomes, endosomes, inner nuclear membrane, and PM
peroxisome protein sorting
use direct from cytosol and indirect (vesicular via ER)
inner nuclear membrane proteins
binding sites for chromosomes and anchorage for nuclear lamina
nuclear pores function
allow mRNA and ribosomal subunits to move out and nuclear proteins to move in
allow protein transport in folded form
nuclear pores structure
~30 proteins have short disordered repeats that extend to the pore center and create a web
prevents large molecules but allows small hydrophilic molecules through
nuclear localization signal
polybasic motif (several positively charged Lys and Arg) that directs proteins from cytosol to nucleus
nuclear import receptor
cytosolic protein directs proteins through the nuclear pore
energy for nuclear transport supplied by:
GTP hydrolysis
GTP hydrolysis in nuclear transport
-nuclear protein binds to receptor and complex enters nucleus
-Ran-GTP displaces imported protein
- receptor/Ran-GTP complex leaves nucleus
-GTP is hydrolyzed and Ran GDP dissociates from the receptor (Ran-GDP has less affinity for receptor)
-receptor is free to pick up another protein for translocation
function of Ran-GAP accessory protein
GTPase activating protein
only found in cytosol
facilitates GTP->GDP hydrolysis
promotes binding of import receptor to cargo
Ran-GEF function
Guanine exchange factor
present in the nucleus
promotes exchange of GDP with GTP and cargo dissociation from receptor
what concentrations must be maintained for nuclear protein transport
high concentration of Ran-GTP in nucleus to displace imported protein
high concentration of Ran-GDP in cytosol, produced by GTP hydrolysis
maintained by accessory proteins
protein transport into mitochondria/chloroplasts
signal sequence binds to receptor in outer membrane
receptor, protein, and translocator diffuse within outer membrane until contact with second translocator in inner membrane
two translocators transport protein simultaneously across both membranes (UNFOLD in the process)
Chaperons function in mitochondrial protein transport
help pull protein through the membrane and refold it
signal peptidase function
cleaves signal sequence
two types of proteins entering ER
water-soluble proteins: pass through membrane into lumen, eventually reach lumen of specific organelle or EC space
transmembrane proteins: embedded into ER membrane, eventually transported to PM or membrane of another organelle
cytosolic ribosomes vs ER bound ribosomes
cytosolic ribosomes stay free in the cytosol
ER bound ribosomes attached to cytosolic side of ER
all ribosomes return to common pool and interchange between free/attached depending on the code of mRNA it is translating
polyribosome
many ribosomes bind to each mRNA at the same time
N-terminal ER signal sequence effect
read first by ribosome, directs ribosome to bind to cytosolic surface of ER
signal recognition particle (SRP)
binds to ER signal sequence and ribosome
slows down synthesis until complex is attached to ER surface
SRP receptor
embedded in ER membrane
binds to SRP/ribosome/mRNA/polypetide complex
displaces SRP
polypeptide passes through protein translocator into ER lumen, speeding up synthesis
translocation of water soluble protein into ER lumen
opens upon binding with signal sequence
translocates polypeptide as it is being synthesized by ribosome
signal peptidase cleaves sequence and translocated polypeptide released into ER lumen
(cleaved signal sequence remains in ER membrane)
single-pass transmembrane protein synthesis in ER membrane
start transfer sequence initiates translocation through translocator
protein passes through translocator until stop transfer is reached
start transfer cleaved off and protein is released from translocator leaving it anchored within the membrane
stop transfer sequence
stretch of hydrophobic aa within the protein that keeps that section of protein within the ER membrane
multi-pass transmembrane protein insertion into ER membrane
combination of start and stop transfer sequences initiates and ends translocation of protein
neither sequence cleaved off
peroxisomal protein transfer characteristics
most enter via selective transport from cytosol
short (3aa) import signal recognized by receptor protein for delivery
contain translocator
proteins do not unfold to enter
SOME proteins arrive via vesicular transport
lipids transported to mitochondria at ___________ by ______________
membrane contact sites (MCS); lipid transfer proteins (LTPs)
(also can be transported by vesicles)
LTP structure
form tunnels with hydrophobic core
ERMES
ER mitochondria encounter structure
a multi protein complex that allows lipid transfer and tethering of ER and mitochondria
where do clathrin-coated vesicles bud from
PM in endocytosis and from Golgi
what are COP-coated vesicles involved in
transporting molecules from ER to the Golgi and from Golgi back to ER
COP-II coated vesicles bud from where
bud from ER and go to Golgi
COP-I vesicles bud from where
from Golgi to ER
protein coat function
shape membrane into curved budding vesicle
capture cargo for transport
shed off when budding complete
Clathrin coated vesicle transport process
cargo receptors in membrane bind to cargo and adaptin on the other side
adaptin binds cargo receptors to clathrin
dynamin hydrolyzes GTP and pinches off vesicle from membrane
clathrin mediated endocytosis
important for recycling vesicles needed for neurotransmitter release
effects of dynamin mutations
paralysis in animal models
Rab (GTPase) function in vesicle docking
initial recognition; located on vesicle, identifies the vesicle and attaches to tethering protein on target membrane
docking process of vesicles
after Rab binds to tethering protein, the complemetary SNARE proteins wrap around each other to fuse the membranes together and remove water molecules interacting with vesicle
v-SNARE versus t-SNARE
v on vesicle
t on target membrane
secretory pathway order
ER -> Golgi -> (PM for secretion) OR (lysosome for degradation)
Disulfide bonds formation
form between S atoms of adjacent cysteine side chains
takes place by oxidation in ER lumen ONLY
important for secreted proteins to maintain function and conformation
N-linked glycosylation
short oligosaccharides attached to proteins (synthesize glycoproteins)
starts in ER completed in GOLGI
protects proteins from degradation, guides protein to correct transport vesicle, form glycocalyx
process of N-linked glycosylation
large oligosaccharide linked to dolichol (lipid) is transferred to growing polypeptide chain when Asn is produced
attached to amino (NH2) group of asparagine
further processing and differentiation happens in the Golgi
Chaperone proteins function in ER
help correctly fold newly synthesized proteins in ER
how are proteins that function in the ER kept there
ER retention signal
what happens to misfolded proteins, disease that is a result of this
transported to cytosol for degradation by proteasomes
cystic fibrosis: misfolded Cl- channels that are still functional are degraded due to extreme quality control
Unfolded protein response (UPR)
happens due to buildup of misfolded proteins in the ER
can cause increased folding capacity of ER, transcription/translation reduction, and possibly triggers cell death
Golgi function
oligosaccharide modification
sort to lysosome or cell surface
cis Golgi sends proteins with ER retention signal back to ER
movement within and to golgi
proteins travel from the ER to Golgi in COP-II coated vesicles
movement within Golgi by vesicles or maturation
move from cis to trans face
constitutive secretion
continual secretion from cells
operates in all cells regardless of signals
regulated secretion
operates in specialized secretory cells
produce large quantities of cargo, stored in secretory vesicles (concentrated by aggregation)
extracellular signal stimulates secretion
endocytic pathways
phagocytosis, pinocytosis, receptor mediated endocytosis
phagocytosis
ingestion of large particles (microorganisms, debris) via phagosomes
clathrin required, selective process
purpose: nutrition and defense
phagosomes fuse with lysosomes
pinocytosis
used to “sample” EC environment
fluid uptake containing solutes
endocytic vesicles fuse with endosomes which fuse with lysosomes
nonselective
MAINTAIN membrane volume, balance exocytosis
receptor mediated endocytosis
highly specific
endocytose specific molecules by forming clathrin coated endocytic vesicle
increases cargo concentration
hijacked by viruses
import of cholesterol process
cholesterol packaged in LDL particles, which are secreted to the blood, binds receptors on cells, ingested by clathrin mediated vesicles, enters endosome (acidic), receptor and LDL dissociate, LDL to lysosome, broken down and frees cholesterol to cytosol
what would be caused by mutations in LDL receptor
unable to uptake LDL/cholesterol from blood, high blood cholesterol, treated with statin
endosomes characteristics and function
early/late endosomes sort incoming molecules
acidic due to H+ pumps breaks apart receptor/ligand complexes
sends to same location (recycles), lysosome (degradation), or different location (transcytosis)
digestion in the lysosome
contain acid hydrolases (hydrolytic enzymes) that function at low pH
H+ pump maintains acidity
lysosomal membrane proteins
highly glycosylated for protection
receives proteins from endomembrane system (ER - Golgi)
tagged with mannose-6-phosphate
autophagy
degradation of unnecessary or dysfunctional cellular components (selective)
autophagosomes envelop target component
lipophagy
specialized autophagy of lipid droplets
regulates lipid content
TAG -> supplies free fatty acids to sustain energy/ ATP levels
triggered by nutrient depletion/starvation
function of IRE1 in UPR
transmembrane with kinase and nuclease domains on cytosolic side activated > adjacent kinases phosphorylate each other > enables RNase domains > RNase cleaves specific mRNA at 2 positions to excise introns
spliced mRNA exons joined (RNA ligase) > translated into transcription regulatory protein that enters nucleus and activates transcription of genes to expand ER and increase folding capacity
PERK function in UPR
transmembrane kinase activated by misfolded protein accumulation > phosphorylates itself > phosphorylates translation initiation factor (inactivating it) > inhibits overall protein synthesis, increases transcription of genes for UPR proteins
ATF6 function in UPR
transmembrane protein with transcription regulator (TRP) to be delivered to nucleus > vesicle forms and sends protein to Golgi > proteases cleave cytosolic domains (TRP) which are then free in cytosol > enter nucleus and activates transcription of genes encoding UPR proteins
