Protein Targeting and Glycosylation Flashcards
targeting signals
present in the precursors of all organellar or secreted proteins
post-translational protein targeting
for proteins destined for cytoplasm, nucleus, mitochondria, or peroxisomes; interact with carrier proteins
co-translational protein targeting
proteins destined for ER, golgi, or lysosome are synthesized in ER and transferred in vesicles to appropriate destination
all co-translational targeting begins with _
cytosolic ribosomes
steps of vesicular transport
cargo recruitment –> vesicle budding –> targeting –> fusion
ER association begins with _
recognition of an N-terminal signal sequence on the nascent protein by the SRP
SRP
signal recognition particle
steps in co-translational targeting
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
SRP structure
contains hydrophobic pocket for signal sequence and GTPase for binding to signal sequence and ribosome
ER insertion/translocation
nascent protein inserted into ER lumen through translocon –> signal sequence cleaved –> protein folds
transmembrane proteins
have signal anchor or stop-transfer sequences to prevent entire protein from entering ER lumen and allow transmembrane domains
transmembrane proteins
have signal anchor or stop-transfer sequences to prevent entire protein from entering ER lumen and allow transmembrane domains
heat shock protein (Hsp) family
bind to exposed hydrophobic patches to aid in protein folding (chaperones); have a slow ATPase activity
BiP (immunoglobulin binding protein)
member of Hsp family; possess ATP binding domain and peptide binding domain
BiP peptide binding domain
bind exposed hydrophobic sites (exposed hydrophobic sites indicate a misfolded protein)
BiP mechanism
BiP binds peptide, stimulating ATPase –> ATP hydrolysis locks BiP on protein –> ATP exchange for peptide release –> cycle repeats until proper folding
ERAD
terminally misfolded proteins sent to ER for degradation
example of ERAD protein
deltaF508 in cystic fibrosis
N-linked glycosylation is _
co-translational
N-glycosylation function
aids folding, promotes stability, targets proteins to compartments within secretory pathway
N-linked glycans attach to _
the nitrogen of Asn that is present as part of a Asn-X-Ser/Thr consensus sequence
glycan attached in N-glycosylation
GlcNAc
biosynthesis of N-linked glycans
synthesis of dolichol core –> transfer of precursor oligosaccharide to protein –> processing of oligosaccharide
glycosyl transferases
add sugars
glycosidases
trim sugars
dolichol phosphate
embedded in ER membrane
terminal Glc on N-linked glycan
indicator of need for chaperone function –> proteins will be glucosylated
calnexin and calreticulin
chaperones that bind glucosylated proteins and prevent ER exit –> once folded properly, Glc is trimmed by glucosidase
glucose present
ER retention
glucose absent
ER exit and modification of oligosaccharide in Golgi
targeting to lysosome requires _
mannose-6-P on N-linked glycan
I cell disease
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)
similarities in N and O-linked glycosylation
both processed in Golgi, sugars added to both are donated from nucleotides, contain similar sugars, serve recognition functions
How is O-glycosylation different?
initiation is post-translational, attachment sites have no consensus sequence, no characteristic final structure types
How is N-glycosylation different?
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
example of O-linked glycosylation
ABO blood type
