Protein Targeting and Glycosylation

Question 1

targeting signals

Answer 1

present in the precursors of all organellar or secreted proteins

Question 2

post-translational protein targeting

Answer 2

for proteins destined for cytoplasm, nucleus, mitochondria, or peroxisomes; interact with carrier proteins

Question 3

co-translational protein targeting

Answer 3

proteins destined for ER, golgi, or lysosome are synthesized in ER and transferred in vesicles to appropriate destination

Question 4

all co-translational targeting begins with _

Answer 4

cytosolic ribosomes

Question 5

steps of vesicular transport

Answer 5

cargo recruitment –> vesicle budding –> targeting –> fusion

Question 6

ER association begins with _

Answer 6

recognition of an N-terminal signal sequence on the nascent protein by the SRP

Question 7

SRP

Answer 7

signal recognition particle

Question 8

steps in co-translational targeting

Answer 8

GTP-SRP binds signal sequence –> protein synthesis is paused while SRP binds to receptor –> forms a complex that associates with the pore –> GTP hydrolysis results in ribosome release –> ribosome goes through translocon –> synthesis of protein continues into ER lumen

Question 9

SRP structure

Answer 9

contains hydrophobic pocket for signal sequence and GTPase for binding to signal sequence and ribosome

Question 10

ER insertion/translocation

Answer 10

nascent protein inserted into ER lumen through translocon –> signal sequence cleaved –> protein folds

Question 11

transmembrane proteins

Answer 11

have signal anchor or stop-transfer sequences to prevent entire protein from entering ER lumen and allow transmembrane domains

Question 12

transmembrane proteins

Answer 12

have signal anchor or stop-transfer sequences to prevent entire protein from entering ER lumen and allow transmembrane domains

Question 13

heat shock protein (Hsp) family

Answer 13

bind to exposed hydrophobic patches to aid in protein folding (chaperones); have a slow ATPase activity

Question 14

BiP (immunoglobulin binding protein)

Answer 14

member of Hsp family; possess ATP binding domain and peptide binding domain

Question 15

BiP peptide binding domain

Answer 15

bind exposed hydrophobic sites (exposed hydrophobic sites indicate a misfolded protein)

Question 16

BiP mechanism

Answer 16

BiP binds peptide, stimulating ATPase –> ATP hydrolysis locks BiP on protein –> ATP exchange for peptide release –> cycle repeats until proper folding

Question 17

ERAD

Answer 17

terminally misfolded proteins sent to ER for degradation

Question 18

example of ERAD protein

Answer 18

deltaF508 in cystic fibrosis

Question 19

N-linked glycosylation is _

Answer 19

co-translational

Question 20

N-glycosylation function

Answer 20

aids folding, promotes stability, targets proteins to compartments within secretory pathway

Question 21

N-linked glycans attach to _

Answer 21

the nitrogen of Asn that is present as part of a Asn-X-Ser/Thr consensus sequence

Question 22

glycan attached in N-glycosylation

Answer 22

GlcNAc

Question 23

biosynthesis of N-linked glycans

Answer 23

synthesis of dolichol core –> transfer of precursor oligosaccharide to protein –> processing of oligosaccharide

Question 24

glycosyl transferases

Answer 24

add sugars

Question 25

glycosidases

Answer 25

trim sugars

Question 26

dolichol phosphate

Answer 26

embedded in ER membrane

Question 27

terminal Glc on N-linked glycan

Answer 27

indicator of need for chaperone function –> proteins will be glucosylated

Question 28

calnexin and calreticulin

Answer 28

chaperones that bind glucosylated proteins and prevent ER exit –> once folded properly, Glc is trimmed by glucosidase

Question 29

glucose present

Answer 29

ER retention

Question 30

glucose absent

Answer 30

ER exit and modification of oligosaccharide in Golgi

Question 31

targeting to lysosome requires _

Answer 31

mannose-6-P on N-linked glycan

Question 32

I cell disease

Answer 32

results from a Man6P deficiency; inactive phosphotransferase prevents formation of mannose-6-P –> lysosomal hydrolases will not make it to lysosome –> lysosome can’t degrade results in in undigested product accumulation (inclusions)

Question 33

similarities in N and O-linked glycosylation

Answer 33

both processed in Golgi, sugars added to both are donated from nucleotides, contain similar sugars, serve recognition functions

Question 34

How is O-glycosylation different?

Answer 34

initiation is post-translational, attachment sites have no consensus sequence, no characteristic final structure types

Question 35

How is N-glycosylation different?

Answer 35

initiation is co-translational (ER), attaches to consensus sequence, added as core structure and then modified over time, characteristic final structure, can function in protein folding

Question 36

example of O-linked glycosylation

Answer 36

ABO blood type