lec 7- protein synthesis

Question 1

what was done to see the path of proteins from synthesis to discharge?

Answer 1

a radioactively labelled amino acid (pulse) was added to cells, and was used in protein synthesis, then it was washed out and a solution with no label was added to examine where the label is. (pulse-chase)

Question 2

what system has been used to identify roles of proteins involved in membrane trafficking?

Answer 2

cell-free systems

Question 3

what are liposomes?

Answer 3

vesicles with a phospholipid bilayer

Question 4

when are buds and vesicles produced?

Answer 4

They are formed after liposomes are incubated with purified proteins normally on coats on the cytosolic surface of transport vesicles in the cell

Question 5

what has a network of membranes that penetrate much of the cytoplasm?

Answer 5

the ER

Question 6

what part of the ER has ribosomes and doesn’t have ribosomes?

Answer 6

rough ER has and smooth ER doesn’t

Question 7

what is continuous with the outer membrane of the nuclear envelope?

Answer 7

The ER

Question 8

what are the two locations where proteins are synthesized?

Answer 8

one third at the RER and the rest are synthesized at ribosomes in cytosol

Question 9

where does protein synthesis begin and where do they end up?

Answer 9

begins on ribosomes in the cytosol and ends up in an organelle, in the plasma membrane, be secreted, remain in the cytoplasm

Question 10

what is co-translational translocation and post-translational translocation?

Answer 10

co-translational translocation- transport of most secretory proteins into the ER lumen begins while the protein is still being synthesized on the ribosome
post-translational translocation- translocation that takes place after protein is synthesized

Question 11

what do the proteins in translocation have?

Answer 11

have a signal sequence

Question 12

what is SRP?

Answer 12

a signal recognition particle that binds to signal sequence on nascent proteins and large ribosomal units

Question 13

what happens after ribosome binds to SRP?

Answer 13

it is handed to the translocon (a protein channel embedded in the ER membrane). Upon attachment, the signal sequence is recognized and the polypeptide is inserted into the translocon, the signal sequence is cleaved off by signal peptidase and degraded, translocation occurs until protein is released into lumen, proteins are being modified by folding, disulfide bond formation and glycosylation

Question 14

what part of the amino acid sequence stops the translocation (movement of protein across membrane)?

Answer 14

the stop-transfer sequence, translocation stops but translation continues

Question 15

what can the orientation of a single-spanning membrane proteins be?

Answer 15

it can have the N-terminus facing towards the lumen of the ER or the cytosol

Question 16

what do proteins produced in membrane-bound ribosomes become?

Answer 16

glycoproteins

Question 17

how do the carbohydrate groups help proteins?

Answer 17

act as macromolecule binding sites and aid in protein folding and stabilization

Question 18

how is glycosylation of proteins done?

Answer 18

sugars are added to an oligosaccharide which are then catalyzed by glycotransferases, each transfers a specific monosaccharide to the growing end of the carbohydrate chain until a 14 unit “oligosaccharide precursor” is generated and attached to the amide nitrogen of an asparagine residue

Question 19

how is the oligosaccharide precursor formed?

Answer 19

2 GlcNAc and 5 mannose residues are added to the cytosolic face, sugars are found in the cytosol. The 7 residue oligosaccharide is then flipped to the luminal side, the sugars are then transferred to luminal side by dolichol phosphate

Question 20

where is the 14 residue oligosaccharide transferred to?

Answer 20

an asparagine residue on the nascent polypeptide in a tripetide sequence of Asn-X-Ser/Thr and further modified

X is any amino acid apart from proline

Question 21

what happens to misfolded proteins in the ER?

Answer 21

they need to be degraded, degradation generally takes place in the cytosol

Question 22

what is Dislocation or retrotranslocation?

Answer 22

the movement of misfolded proteins from the ER to the cytosol

Question 23

what is ERAD proteins?

Answer 23

ER associated degredation proteins that export improperly folded proteins from the ER to the cytosol for the degredation by proteosomes

Question 24

which proteins are allowed to leave the ER?

Answer 24

only properly folded proteins, misfolded stay in ER

Question 25

what is the unfolded protein response (UPR)?

Answer 25

a response where if too many misfolded proteins accumulate in the ER, the cell will make more ER, this mechanism ensures a sufficient amount of properly folded proteins entering the secretory pathway but if too much ER is made apoptosis is initiated