Lecture 11 - Endomembranes & ER Flashcards
def: sites for protein and lipid synthesis, processing and sorting
- rough and smooth ER
- golgi complex
def: carry and sort material bought into the cell
endosomes
def: digest ingested material and unneeded cellular components
lysosomes
def: move molecules between the compartments and plasma membrane
transition, transport and secretory vesicles
def: house hydrogen peroxide generating reactions
peroxisomes
def: store ions, sugars, amino acids, and toxic compounds
vacuoles
from the protein synthesis perspective, what is the start of the endomembrane system?
the rough endoplasmic reticulum
the rough ER consists of tubular membranes and flattened sacs called _______
cisternae
about ___ of the proteins are synthesized in the RER
1/3
where are these proteins produced in the cell:
- secreted proteins
- transmembrane proteins
- soluble proteins that reside in the ER, Golgi complex, lysosomes, endosomes vesicles, and vacuoles
rough ER
where are these proteins synthesized:
- ones destined to remain in the cytosol
- peripheral proteins of the cystolic surface membranes
- proteins that are transported to the nucleus
- proteins that are incorporated into peroxisome, chloroplasts and mitochondria
on “free” cytosolic ribsomes
where do all proteins BEGIN synthesis?
on ribosomes in the cytosol
how do proteins know where they need to go?
information is coded in the primary sequence: ER signal sequence or sorting signal
how can the compartments of the eukaryotic cell be divided?
- the endomembrane system
- the cytosol
- the mitochondria, chloroplasts, peroxisomes, and the interior of the nucleus
what are the 2 pathways for routing the protein products?
- free ribosomes
- ER docked ribosomes
most membrane proteins take which pathway of protein synthesis?
ER-docked ribosomes
what are the 2 pathways of protein sorting?
- co-translational import
- post-translational import
def: protein sorting - proteins carrying an ER signal sequences direct the ribosomes-polypeptide complex to the rough ER, translation is completed on the rough ER and then process and sorted in the ER and Golgi
co-translation import
def: protein sorting - proteins lacking an ER signal sequence complete their synthesis on ribosomes that are free in the cytoplasm, proteins are released into the cytoplasm and the ones with an organelle-specify sorting signal are imposed into the organelle
post-translational import
do cytoplasmic proteins have sorting signals
no they remain in the cytoplasm
co-translational import is considered a _________ pathway while post-translational import is a _____________ pathway
secretory, non-secretory
def: multiple ribosomes synthesizing the same mRNA
polysome
sorting signals direct proteins to _______ __________-
their compartments
an ER signal sequence directed the polypeptide/ribosme to the __ __________
ER membrane
co-translational translocation of _______ ______ deposits the polypeptide into the ER lumen by a ribosome that is attached to the ER membrane
soluble proteins
def: proposes that intrinsic molecular signals determine the localizations of some polypeptides
signal hypothesis
what happens if a sorting signal is lost?
targeting of the protein is lost
what happens if a sorting signal is transplanted to another protein?
targeting is imparted to the recipient protein
ER signal sequences usually have _________ ________ region and a _______ region near where the cleavage from the mature protein will take place
hydrophobic N-terminal region, polar region
def: particle that bind the Ribosome-mRNA-Polypeptide complex to the ER protein
signal recognition particle (SRP)
contact with the ER is mediated by __, which binds the __ _____ _______ of the newly forming polypeptide
SRP, ER signal sequence
what does the SRP contain
proteins and RNA
polypeptide synthesis proceeds until the ___ ______ sequence has been formed
ER signal sequecne
what happens when the SRP binds to the signal sequence?
blocks further translation
where does the SRP bind to on the ER membrane?
to the SRP receptor, which is an integral membrane protein
the ER signal sequence is inserted into the _________, a channel protein in the membrane once the SRP has been released
translocon
what happens once the ER signal sequence makes contact with the interior of the translocon?
the plug is displaced and the channel opens to the ER lumen
def: cleaves ER signal sequence during polypeptide elongation
signal peptidase
def: bind the nascent peptide to facilitate folding
chaperones
G proteins can acts as ______ ______ as seen in the SRP and SRP receptor proteins
molecular switches
in GTP-bound form, SRP and SRP receptor have _____ affinity for each other
high
when hydrolysis of GTP occurs, affinity for SRP to SRP receptor _______ causing their release
decreases
the orientation of multi-pass protein integral membrane protein is determined by _____ _____ of the first transmembrane domain
charge/orientation
what are the two kinds of transmembrane proteins?
single pass or multi-pass
when is transmembrane protein orientation established?
during their synthesis
__________ chains are always present on the non-cytosolic side
oligosaccharide
asymmetry of transmembrane proteins reflects their ______
function
hydrophobic transmembrane domains can ________ into the lipid bilayer through a seam along one side of the translocon
dissolve
each subsequent transmembrane domain must have _________ ________-
opposite charge/orientation
what can help but in a subsequent transmembrane domain
ribosomes
in terms of protein synthesis, what are 3 specific functions of the rough ER?
- glycolysation
- folding of polypeptide chains and subunit assembly
- disulphide bond formation
in terms of recognition, what are 2 specific functions of the rough ER?
- ER associated degradation (ERAD) proteins that are incorrectly folded, modified, or assembled are exported
- degradation occurs in cytosolic proteasome
most proteins synthesized in the rough ER are _________ on the amide nitrogen of an asparagine rescue by the addition of a pre-formed oligosacharride. what is this process called?
glycosylated, N-linked glycosylation
where are O-linked sugars added ?
in the Golgi
what are the 3 functions of carbohydrate groups?
- macromolecule binding sites
- aid in protein folding
- increase stability
def: when oligosaccharide is added to the recipient protein as the polypeptide is being synthesized
co-translational glycosylation
what do the lectins Calnexin and Clareticulin do ?
bind to N-linked oligosaccharides preventing aggregation and promoting proper folding
calnexin and clareticulin recognize a _______ _____ glucose and therefor bind only ______ the oligosaccharide precursor has begun
single terminal, after
when can the protein dissociate from its chaperone and leave the ER
when the third glucose is removed
binding of calnexin and calreticulin _______ _______ and drives ______ ______ formation
prevents aggregation and drives disulphide bond formation
def: abundant chaperone in the ER lumen, member of Hsp70 chaperones
BiP (binding proteins)
what does BiP do?
binds to hydrophobic regions of polypeptide chains and prevents aggregation of polypeptides with similar regions
what does BiP prevent?
interaction between hydrophobic regions of different proteins
if the protein folds correctly, the hydrophobic region is where?
buried in the interior of the molecule
if the protein does not fold correctly, what happens?
it will have to interact with BiP again
def: enzyme that catalyzes disulfide bond formation in the lumen of the ER
protein disulfide isomerase (PDI)
disulfide bond in the active site of PDI is __________ to a protein
transferred
where are disulfide bonds only formed in eukaryotic cells?
only in the ER
def: a cellular stress response pathway in which sensor molecules in the ER detect the misfiled proteins
unfolded protein respones (UPR)
what do phosphorylate translation factors do?
inhibit protein synthesis to decrease the flow of proteins to the ER
what do UPR’s up regulate?
- ER based chaperones
- transport that move proteins out of the ER
- protein degradation machinery
def: component of the UPR that recognizes misfolded or unassembled proteins
ER-associated degradation (ERAD)
what degrades misfiled proteins ?
proteasomes
def: large protein degrading structures, predominant proteases of the cytosol
proteasomes
what do proteasome bind to?
ubiquitin-labeled proteins
def: small protein containing 76 amino acids
ubiquitin
ubiquitin is joined to target proteins by a process involving 3 enzymes:
- ubiquitin activating enzyme
- ubiquitin conjugating enzyme
- substrate recognition protein, or ubiquitin ligase
ER is the primary source of ________ _______
membrane lipids
fatty acids for membrane phospholipids are synthesized in the _________ and incorporated into the ER membrane on the cytosolic side
cytosol
what transfers fatty acids for membrane phospholipids to the lumen side of the bilayer of the ER?
phospholipid translocators (flippases)
T or F: the ER cannot form contact sites with other organelles
false, it can
def: proteins that exchange lipids between the compartments of the ER
lipid transfer proteins
