Topic 8 Flashcards
ribosomes are located in…
the cytosol; either free or on membrane bound organelles
synthesis of ALL proteins start on…
free ribosomes; then can continue membrane bound
two major branches of protein sorting
cytosolic and secretory
targets of cytosolic pathways
nucleus, mitochondria, chloroplasts, peroxisome
in cytosolic pathway, protein synthesis starts in ________ and finishes in _________.
cytosol, cytosol
targets of secretory pathway
nuclear membrane proteins, ER proteins, golgi proteins, endoscopes, lysosomes, secretory vesicles, plasma membrane
secretory pathway protein synthesis starts in ________ and finishes in ________.
cytosol, ER
cytosolic pathways moved to final destination through recognition of…
specific targeting amino acid sequences (signal sequences) within the protein
most cellular compartment have their own….
chaperones
What replaces cytosolic chaperones?
mitochondrial chaperones
endoplasmic reticulum with ribosomes
rough ER
rough ER
involved in protein metabolism
smooth ER
involved in lipid metabolism and not associated with ribosomes
The ________ is the entry point for most proteins into the secretory pathway.
rough ER
co-translational transport
when majority of proteins destined for secretory pathway are translocated into ER during synthesis on membrane-bound ribosomes
how does co-translational transport occur?
signal hypothesis
C terminus
carboxyl terminus; end of peptide
N terminus
amino terminus; start of peptide
signal recognition particles, SRPs
recognize and bind to signal sequences in the growing protein while it’s still being translated
signal sequences
specific amino acid sequences
SRPs are composed of..
small, cytoplasmic non-coding RNA and proteins
SRP receptors
proteins on the membrane of the ER that bind the SRP
translocon
a membrane channel through which polypeptide chains with a signal sequence are transported into the ER
Why is the term “secretory pathway” misleading?
It’s not just for secretion of proteins, but carries out sorting of both free and membrane-bound proteins to some intracellular destinations
Players in “Signal Hypothesis”
- signal sequence
- SRP
- signal recognition particle receptor
- translocon
- signal peptidase enzyme (cleave off signal sequence)
When ribosome sits over translocon channel…
protein is synthesized into the channel
After proteins are translocated to ER as unfolded polypeptide chains,….
they fold into their 3D conformations
passage into ER lumen and ER membranes involve….
chaperones (BiP is one in the ER)
Intergral membrane proteins must be inserted in the correct orientation in the ER because….
they cannot be flipped within the membrane
transmembrane sequence
stops the polypeptide from entering the lumen of ER and changes conformation of translocon channel so that it opens and releases polypeptide chain to ER membrane
Describe Smooth ER and membrane assembly
due to hydrophobicity… membrane lipids are synthesized with already existing cellular membranes of the smooth ER
flippases
enzymes that facilitate translocation of newly synthesized phospholipids to opposite side of bilayer
cis face of Golgi
closest to ER where proteins enter
trans face of the Golgi
from ER where proteins exit
Sorting off proteins starts in the Golgi… why?
ER proteins (BiP) first go to Golgi before they are sent back to the ER
mannose-6-phosphate
specialized form of oligosaccharide †ha† target proteins for lysosomes
_______ of vesicular transport is key to maintaining the functional organization of the cell
selectivity
2 steps to ensure correct proteins get to their correct final destination
- get correct protein into correct vesicle
- get vesicle to correct final destination
Most transport vesicles that carry secretory proteins from the ER to the Golgi and from the Golgi to other targets are coated with cytosolic coat proteins and are called..
coated vesicles
coated vesicles
play a key role in ensuring the vesicles reach their correct targets within the cell following exit from Golgi
different kinds of protein coats
cathrin, COPI, COPII
lysosomes
membrane-enclosed organelles that contain enzymes capable of breaking down all types of polymers at acidic pH
cathrin-coated vesicle
sort through lysosomal proteins
most lysosomal enzymes are…
acid hydrolases
acid hydrolases
active at acidic pH that is maintain within lysosomes, but not at neutral pH in rest of cytoplasm
proton pumps
maintain acidity of lysosome
snare hypothesis of vesicle fusion
proposal for mechanism of fusion for transport and secretion
two stages of snare hypothesis
- transport vesicle must recognize the correct target
- vesicle and target must fuse for purpose of delivering the cargo
tethering factors
stimulate formation of SNARE complexes between vesicles and target membrane, beginning process of membrane fusion.
SNARE-SNARE paring
provides energy to drive membrane fusion
Rab protein
recognize and bind tethering factors on target; forming initial interactions between vesicle and target membrane
Nuclear pore complex consists of …
a structure with eight-fold symmetry organized around a large central channel
nuclear pore complex is composed of ……. different nucleoporins
30-50
Depending on their size, small polar molecules, ions, and macromolecules can travel through the nuclear pore complex by…
passive and energy dependent
nuclear localization signals
specific amino acid sequences that are recognized by transport receptors and direct the transport of proteins from the cytoplasm through the nuclear pore complex into the nucleus.
nuclear transport receptors
proteins that recognize nuclear localization signals and mediate transport across the nuclear envelope
karyopherins
family of nuclear transport receptors
importins
karypherins that direct nuclear import
Ran
a GTP-binding protein that regulates movement of importing/cargo complex into nucleus
What drives importing movement across th pore?
unequal distribution of Ran/GTP
nuclear export signals (NES)
specific amino acid sequences that target proteins for export from nucleus
exportins
member of karyopherin family that interact with NES; similar mechnanism as importing
RNA transport across nuclear envelope as..
ribonucleoprotein complexes (RNPs)
rRNAs transport across nuclear envelope as..
ribosomal subunits
mRNAs transport across nuclear envelope as..
in association with two proteins known as mRNA exporter complex
tRNAs transport across nuclear envelope as..
associated with exporting-t
snRNAs transport across nuclear envelope as..
assembled into snRNPs and then back into nucleus as splice some complex
