7.10-7.20 Flashcards
what happens once hydrophobic anchor sequences emerge from the ribosome
recognized and are transferred out of the translocon into the bilayer
some hydrophobic sequences are what?
both dual purpose and anchor sequences
when is the N-terminal signal likely cleaved
after the beginning of translocation
signal anchor proteins target by what?
using an internal transmembrane domain (signal anchor)
what decides whether the N-terminal or C-terminal is translocated into the ER
distribution of charged residues on either side of the transmembrane domain
polytopic proteins
proteins that span the membrane multiple times
polytopic proteins can have transmembrane regions that integrate __________ or in _______
one at a time or in pairs
what happens if the ER-signal peptide is N-terminal?
it is almost always cleaved after it has served its purpose
signal peptidase complex composition
5 subunits: 2 with proteolytic activity and 3 regulatory
what is the exact cleavage site influenced by
aa residues in immediate vicinity of cleavage site
what is the signal peptide often processed with after removal
signal peptide petidase
IMPs
integral membrane proteins, non-removable from the membrane by a salt extraction procedure
GPI-anchoring always tethers a protein to the ___________ face of a membrane
non-c
what does GPI stand for
glycosylphosphatidylinositol
what is a glycosylphosphatidylinositol
PI that has sugars added to it
which side does GPI-anchoring begin on
c-face of ER membrane
how does initial GPI get from c-face to non-c-face
flippase/translocase moes it across to the lumen sdie
what happens once the GPI is moved to the lumen side
more sugars are added along with three phosphoethanolamines
signal for GPI-anchoring
small C-terminal hydrophobic domain of variable length
what is the GPI-anchoring signal recognized by
integral membrane complex which cuts the signal off from the protein (was in process of being translocated)
omega site
new C-terminus to which integral membrane complex is attached
what does the integral membrane complex attach omega site to
terminal phosphoethanolamine residue of GPI, enzyme complex liberates itself
- (2-step transamidation reaction)
purpose of GPI-anchoring
- way of targeting proteins in polarized cells/lipid rafts
- would give IMP more lateral mobility
- free protein from membrane association by enzymatic cleavage of attachment, passing message to interior of cell
more than half of secretory and membrane proteins in a cell are ___________
glycosylated
glycosylation in ER is termed what?
N-linked
why is glycosylation in ER is termed N-linked
sugars are attached to asparagine residues
oligosaccharyltransferase
transfers oligosaccharide en bloc onto translocating protein
where was the oligosaccharide first synthesized
pre-synthesized beginning in cytosol, with minor membrane phospholipid dolichol-phosphate
where are sugars added during glycosylation
to dolichol-P
- then flippase moves it to the lumen-face of the membrane, and more sugars are added
where do removal of some sugar residues occur after oligosaccharide addition
in ER and Golgi apparatus
most abundant protein in the ER lumen
BiP
description of BiP
chaperoning, member of hsp70 family
what does BiP bind to
exposed hydrophobic patches
where are hydrophobic patches in a protein normally found
buried within globular protein
does the cytosol have an oxidizing or reducing environment
reducing
true or false: disulfide bonds form in cytosol
false
does the ER lumen and ECM have an oxidizing or reducing environment
oxidizing
true or false: disulfide bonds form readily in the ER lumen and ECM
true
what are disulfide bonds catalyzed by
protein disulfide isomerases
what does PDI catalyze
disulfide bond rearrangements and formation
PDI is ____________ when it ___________ the cysteines in a translocating protein to help it fold
reduces, oxidizes
what does PDI oxidize
cysteines in translocating protein
how is PDI re-oxidized
ER protein EroP1 oxidizes PDI by being itself reduced
what is EroPI oxidized by
FAD
what is a lectin
protein that binds specifically to sugar residues
2 mains types of lectin
- calnexin
- calreticulin
calnexin
IMP in ER membrane
calreticulin
ER-lumenal protein
purpose of calnexin or calreticulin activity in
QC - to make sure certain proteins are properly folded before they are allowed to leave the ER
ticket to enter calnexin cycle
have to be N-glycosylated
what happens after entering calnexin cycle
2 distal glucose residues are quickly removed by glucosidase I and II
what happens after 2 distal glucose residues removed
calnexin associates and helps contact ERp57 to catalyze disulfide bond formation/rearrangment and allows folding/refolding
ERp57
chaperonin and PDI family member
what happens if a protein isn’t folded properly
glucose is re-added by UGGT, which allows it to Reb ind to calnexin and repeat the cycle
UGGT
UDP-glucose-glycoprotein-glycosyl transferase
how does UGGT know that a protein isn’t folded
exposed clusters of hydrophobic residues
proteins in the secretory pathway whose structure depends on the assembly of subunits are retained in the ER by what?
chaperone-association until that assembly is completed
oligomerization
formation of complexes
various retention mechanisms
- associating subunit with BiP
- an exposed cysteine binding to a PDI
- exposed ER retention signal that will be masked later
