3 | protein synthesis and transport - ER Flashcards
describe protein synthesis
mRNA acts as a template for transfer-RNA to bind to the codons and amino acids
many proteins are synthesized just by cytosolic ribosomes (those which remain in the cytosol)
what are the 2 major protein-sorting pathways?
non-secretory (Golgi, lysosomes)
secretory (plasma membrane / outside the cell)
describe the endoplasmic reticulum structure
uninterrupted membranous tubules and vesicles separated from cytoplasm
rough endoplasmic reticulum has ribosomes on the tubules (cisterna)
cisterna are stacked
endoplasmic reticulum extends from nuclear membrane
translation and translocation can occur at the same time
describe cotranslational translocation
start in the cytosol, the transcript is recognized by ribosomes and initiates synthesis of the protein.
the very beginning of the protein, the amino-terminus is recognized by a SRP, sends a signal to help bring the whole complex to the ER membrane’s SRP receptor where it binds.
to move into the ER, the protein complex goes thru the translocon – which must be opened via the hydrolysis of GTP. the hydrolysis causes the dissociation of the nascent chain, SRP, and SRP receptor. this SRP can go back and find another signal peptide…
polypeptide continues to be translated - the translocon helps push the protein pieces ahead of it further into the lumen – once moved in enough, the signal peptidase can cleave off the signal sequence – translation continues to occur and the fully synthesized protein can enter the lumen
what are the types of protein modifications that can occur in the endoplasmic reticulum?
specific proteolytic cleavage
glycosylation (added carbohydrates)
formation of disulfide bonds (stabilizes structure)
folding of the polypeptide chains (protein misfolding can occur)
describe glycosylation that occurs in the endoplasmic reticulum
enzymatic transfer of a branched 14-residue oligosaccharide precursor onto dolichol (a membrane anchored phospholipid), the oligosacharide precursor is then transferred to an asparagine (Asn) residue of a nascent polypeptide (emerging polypeptide) by oligosaccharyl transferase
several glycosidases work to subsequently modify N-glycan, removing some of the sugars..
sugars can also be added
what proteins prevent misfolding of proteins?
can be prevented by chaperone proteins
several proteins contribute to proper folding of proteins in the ER: the chaperones: BiP and the lectins, calnexin and calreticulin, and PDI
when a protein is emerging into the ER and the cell senses protein misfolding (bc of a big stretch of amino acids), the oligosaccharyl transferase will add on a big branched sugar, it’s very charged and will prevent the hydrophobic interactions from occuring
it will also act as a signal, the carbohydrates are recognized by the lectins, calnexin and calreticulin, and that will prevent further misfolding by binding to the complex sugars added by the oligosaccharyl transferase
they hold on to the structure as the protein is being made and will be released once protein is fully assembled
protein disulfide isomerase (PDI)
cysteine residues contain sulfur atoms which interact with other sulfur atoms of other cysteine residues to form a disulfide bond (strong covalent bond that stabilize structure)
for proper folding, must ensure cysteine residues interact with the cysteine residues they need. if not the right cysteine residue, PDI makes a transient disulfide bond to make sure it doesn’t interact with other cysteines - so it links to the cysteine residue preventing others from bonding to it until it’s ready to bond to the another (and the right) cysteine residue
binding of BiP, calnexin or calreticulin is believed to prevent misfolding or aggregation of nascent proteins
describe heat shock proteins
cytosolic Hsp70 prevents protein misfolding and ensures proteins stay in a liner shape pre mitochondrial matrix so it can get thru the translocon;
it also helps guide it to the mitochondria via targeting sequences
it requires energy via ATP hydrolysis
Hsp (Heat Shock Proteins) do not “fold” in heat, but rather they are produced in response to heat stress and their primary function is to help other proteins fold correctly when exposed to high temperatures by acting as molecular chaperones, preventing them from misfolding and aggregating; essentially, they help maintain protein stability under stressful conditions like heats
matrix Hsp70, found in the mitochondrial matrix, helps facilitate the process of the protein entering the mitochondrial matrix
it holds the protein as it enters, keeping it in a linear shape thru the translocon, also prevents misfolding
describe the translocons of the mitochondria
Tom (translocon of the outer membrane), proteins found embedded in the outer membrane
the hole opens up for the signal sequence allowing the protein thru
functions as a receptor too for the signal sequence
Tim (translocon of the inner membrane), often found where the cisternae is closest to the outer membrane’s and its Tom
describe the nuclear import of proteins
proteins made in the cytosol but destined for the nucleus contain a nuclear localization sequence (NLS). they’re bound to a protein called importin
importin recognizes nuclear proteins via the NLS in the cytoplasm
importin-nuclear protein complexes enter the nucleus via the nuclear pore complex (NPC) on the membrane
in the nucleoplasm, Ran-GTP binds importin and promotes decreased affinity for the NLS
releasing the protein from the importin and moves the importin back outside into the cytosol
in the nucleoplasm, Ran can also bind to GDP, so if bound to GDP, GEF will replace the GDP with GTP
Ran needs to bind to GTP for increased affinity to the importin!!
in the cytosol, Ran needs to detach from the importin. so to decrease its affinity, its GTP will be hydrolyzed to GDP - facilitated by GAP
