Chapter 15 Flashcards
the endomembrane system includes which membranes/ organelles ? (5)
- nuclear membrane
- ER
- golgi
- endosomes
- lysosomes
it’s hypothesized the endomembrane system evolved by which process ?
infolding of the plasma membrane
how did the mitochondria and chloroplast evolve ? and what are some key features that distinguish them from other organelles
endosymbiosis (engulfed bacteria by eukaryotic cell)
- bacteria like DNA
- double membrane
- ribosomes
proteins contain a specific ____ _____ that does what
sorting signal ( signal sequence)
- directs their delivery to locations outside the cytoplasm
proteins without a sorting signal remain in the _____
cytosol (default location)
where are free vs bound ribosomes
free = in the cytosol
bound = attached to ER (RER)
what is the signal sequence for ER localization ?
N-terminal 7-10 hydrophobic amino acids
what is the signal sequence for ER retention ?
C-terminal KDEL sequence
what is the signal sequence for mitochondrial import ?
embedded + charged amino acids form amphipathic a-helix
what is the signal sequence for nuclear localization ?
+ charged K (Lys) and R (Arg) amino acids
sorting signals are recognized by complimentary _____ _____ and do what
sorting receptors found in cytosol that guide each protein to its appropriate destination
what are the 3 ways soluble proteins can enter their destination organelle?
- transport thru nuclear pores
- transport across membranes
- transport by vesicles
soluble vs membrane bound proteins
soluble - found in hydrophilic enviro
MB - found in lipid bilayer
describe nuclear gated pores (3)
- transport from cytosol to nucleus
- pass thru via nuclear pore complexes
- don’t need to unfold to enter
nuclear pore complexes act as _____ _____
selective gates
nucleus contains …. (4)
-nuclear DNA
- two membranes (outer is continuous with ER membrane )
- nuclear lamins = support
- nuclear envelope that surrounds nucleoplasm
how do large molecules move thru NPCs?
they need nuclear import receptors (NIR)
- the receptors recognize the signal sequence (nuclear localization sequence) in order to bind
what happens once the nuclear import receptor (NIR) is bound to a large protein?
-goes to nuclear pore and interacts with cytosolic fibrils
-moves thru mesh of pore proteins and once inside nucleus the NIR dissociates and returns to cytosol (must convert back to GDP)
how is energy obtained for nuclear transport ? and how is it found in the cytosol ?
hydrolysis of GTP by Ran-GTPase
in cytosol as Ran-GDP
what is transmembrane transport and what does it use
transport between cytosol and:
-mitochondria
-chloroplasts
-peroxisomes
-ER
uses protein translocators = membrane proteins that transport specific proteins across a membrane
co vs post-translational translocation
co-translational = transport DURING translation
post-translation= transport AFTER translation
how do the mitochondria and chloroplasts import proteins into them?
post-translational transmembrane transport
describe mitochondrial transmembrane transport steps
- each membrane has a translocator
- proteins with a mitochondrial signal sequence interact with import receptor on outer membrane
- import receptor associates with a protein translocator
- once bound moved laterally on the outer membrane until it reaches a translocator on the inner membrane
- both translocators transport the proteins across both membranes, unfolding it in the process
- chaperone proteins help with entry and re folding of the protein once inside
- the signal sequence is cleaved off by an enzyme in the matrix
are the amino acids (signal sequence) part of the mature mitochondrial protein?
NO - cleaved off in matrix
describe peroxisome transmembrane transport
peroxisome=breakdown of fatty acids
- driven by ATP hydrolysis
- requires at least 23 distinct proteins called Peroxins
- proteins DO NOT need to unfold to enter
rough vs smooth ER
rough ER- has membrane bound ribosomes attached and allows co-translational transport of proteins into ER lumen
smooth ER- no membrane bound ribosomes / important for budding of transport vesicles
proteins are imported into the ER via ______
co-translational translocation
describe protein transport in the ER: the ribosome cycle (8)
- cytoplasmic ribosome begins translation of mRNA
- SRP binds to the ribosome and signal sequence (N-terminus 7-10 hydrophobic AAs) pausing translation elongation
- SRP directs ribosome to the SRP receptor
- SRP receptor is next to a translocator, both on ER membrane
- N-terminal signal sequence goes in first and opens channel pore in protein translocator complex on the surface of ER and the polypeptide is translocated through this pore AS its being translated into the ER lumen
- signal sequence remains bound to translocator while polypeptide is threaded through as a loop but the SRP and SRP receptor dissociate from translocator
- signal peptidase will cleave off the signal sequence
- once the C-terminal end of protein is thru the translocator the protein is released
what is the start-transfer signal of ER translocation
when the signal sequence opens the translocator and signals the start of translation/ threading into ER lumen
what is the stop-transfer sequence in ER translocation
hydrophobic AA that causes change in translocator, halting translocation and discharging part of protein laterally into bilayer
describe the double pass of proteins in ER translocation
- don’t have N-terminal signal sequence / signal sequence is located internally
•start-transfer and stop-transfer sequences become two transmembrane domains of the protein
5 major functions of the ER
- synthesis of transmembrane proteins
- synthesis of proteins secreted/delivered to lumens of ER, golgi, endosomes, lysosomes
- N-linked glycosylation
- production of all membrane lipids (SER)
- calcium storage
describe vesicular transport
transport vesicles carry proteins from the lumen of one place to the lumen of another
endocytic & exocytic (secretory) pathways
-cargo molecules (those in lumen and membrane bound) perform budding, turn into a transport vesicles and fuse to target
what does budding produce
it produces coated vesicles that are covered on their cytosolic surface by special proteins
what are the 3 special proteins that cover the cytosolic surface of vesicles and what do they do
- clathrin
- COPI
- COPII
they select different cargo and shape the transport vesicles
what is clathrin
- is on cargo from golgi to endosomes&lysosomes and cell membrane
- as well as cell membrane to endosomes (endocytosis)
what is COP1
when vesicles move back from golgi to ER as well as in between golgi cisternae
-as well as leaving golgi to go to cell membrane
what is COP2
on vesicles from ER to golgi
describe the formation of clathrin coated vesicles (budding)
cargo molecules interact with cargo receptors
- adaptin (adaptor) molecules bind to cargo receptors and recruit clathrin
- this forms a coated pit (a convex basket on the cytosolic surface of the membrane)
- dynamin assembles on neck of the forming bud and destabilizes the lipid bilayers (pinching off)
- once budding is complete (pinched off) clathrin and adaptins leave the vesicle
transport vesicles are very ____
specific (specific cargo to a specific location)
what is the process by which cargo proteins are released from transport vesicles into the ECF? (fusion) (3 steps)
tethering: (initial recognition)
- rab proteins on vesicle recognized by the tether protein on cytosolic surface of target membrane
- pulls vesicle to membrane
docking: (2nd level of recognition)
- v-SNARE (vesicular) and t-SNARE (target) interact / firmly docks vesicle to membrane
*v-SNARE already bound to vesicle
*t-SNARE stays bound to target membrane
- v and t act like twist ties pulling the vesicle in even closer so the two bilayers are really close
fusion:
- once within 1.5 nm phospholipids fuse together to form a continuous bilayer
which SNARE binds to Rab protein
t-SNARE
for N-linked glycosylation (on NH2 group of _____) sugars are added _____ translation to the ____ side only
asparagine
during
lumen
describe what happens to proteins when they are folded correctly vs incorrectly (UPR)
correctly - vesicles carrying protein bud off and head to golgi
incorrectly
-chaperones will try and refold or tagged with ubiquitin to get degraded in proteasome
- if too many misfolded proteins accumulate it causes the ER to enlarge and make more chaperones
• if they still can’t be refolded apoptosis is triggered this is the UPR (unfolded protein response)
what does the golgi do as a secretory pathway (3)
- received proteins and lipids from ER
- glycosylation of proteins (O-linked) and lipids
- modification/ sorting/ packaging into vesicles for delivery
what are the 3 destinations once the golgi packages proteins and lipids
- endosomes
- plasma membrane
- extracellular fluid (ECF)
proteins enter the ____ face of the golgi and leave out the ____ face
enter cis travelling from cisternae to cisternae and leave thru trans by budding off
how are proteins recognized to go from golgi back to the ER?
ER retention signal ( C-terminal KDEL sequence)
which coat proteins would be coating vesicles leaving the golgi going to the cell membrane
COP1 and clathrin
two types of secretion
-constitutive
•all eukaryotic cells
•no signal needed
•happens all the time
•grows plasma membrane
-regulated
•only specialized secretory cells
(controlled)
•needs a signal and won’t secrete until it knows the contents are needed
• proteins will gather at plasma membrane until are needed (receive signal)
endocytic-exocytic cycle
must be constant addition of new membranes to cell surface via exocytosis in order to keep cell surface area and volume constant
endocytosis
transport into cell from plasma membrane
- often sorted in endosomes
-pino/phagocytosis/receptor mediated
pinocytosis
(cell drinking)
- fluid and solutes ingested via pinocytic vesicles
- is continuous
clathrin coated pits -> clathrin coated vesicles
phagocytosis
(cell eating)
-large particles (pathogens) ingested via large phagosomes (pseudopods-actin)
- only used by specialized phagoctytic cells
- it’s a triggered process
- fuses with lysosome and material is degraded
receptor mediated endocytosis
same process as budding off from golgi (clathrin/adaptin etc)
-specific for a target solute
(cholesterol)
lysosomes
membrane enclosed and contain enzymes for intracellular digestion
-enzymes are tagged with mannose-6-P (target them to lysosomes)
3 pathways for material to get to lysosomes
- phagocytosis
- endocytosis
- autophagy
what is SRP
signal recognition particle
