Protein Transport (Lecture 12) Flashcards
______ is where most phospholipids are synthesized.
SER (smooth endoplasmic reticulum)
______ is where most membrane proteins are assemble.
RER (rough endoplasmic reticulum)
Translocation through the translocon is driven by ______.
Translation elongation
How does cotranslational translocation work?
Signal sequence of protein (NH3+) recruits a signal recognition particle (SRP)
SRP binds to the signal sequence arresting translation
SRP binds to a SRP receptor in the ER membrane
SRP is released and translocon opens, translation resumes
Signal peptidase cleaves signal sequence
Translation then continues until protein complete inside ER
Mammalian translocon consists of three proteins, collectively called
the ______ and n bacteria & archaea this is called ______
Sec61 complex, SecYEG
Sec61 i made up of ______, an integral membrane protein with ______ membrane-spanning α helices and two smaller proteins, termed ______ and ______.
Sec61α, 10
Sec61β, Sec61γ
What are the two reasons that other molecules don’t leak through the translocons?
Hydrophobic hour glass-shaped channel has a helical plug that seals the translocon in absence of protein cargo
Ring of hydrophobic Ile
residues at constriction of pore form a gasket, preventing passage of polar molecules
What is the purpose of the lateral exit in the translocon?
Allows a nascent protein hydrophobic transmembrane domain out of translocon and into the ER membrane lipid bilayer
The ______ recognizes a sequence on the C-terminal end of the hydrophobic core of the signal peptide and cleaves the chain specifically at this sequence once it has emerged into the luminal space of the ER.
Signal peptidase
True or False: In most eukaryotes, most secretory proteins enter the ER by cotranslational translocation
True
In yeast, some secretory proteins enter ER only ______ translation is complete.
After
______ tetrameric complex in ER membrane near translocon in post translational translocation.
Sec63
True or False: SRP and SRP receptor not involved in post-translational translocation.
True
Molecular chaperone ______ is in ER lumen and has a peptide-binding domain and a ATPase domain
BiP
______ is required for BiP to function.
ATP hydrolysis
True or False: Successive BiP binding acts as ‘ratchet’ to pull protein into ER.
True
Post-translocation is common for translocation into organelles like ______ & ______.
Mitochondria and chloroplasts
______ membrane proteins contain one or more hydrophobic membrane-spanning domains
Intergral
______ proteins have a polypeptide covalently to one or more lipid molecules.
Lipid-anchored
______ proteins are bound to the side of the membrane by by interactions with integral or lipid- anchored membrane proteins, or with lipid head groups
Peripheral
Orientation of membrane proteins is known as?
Topology
True or False: Membrane proteins do NOT flip-flop across the membrane.
True
Topogenic sequences can be known as?
N- terminal signal sequences,
internal stop-transfer anchor sequences,
internal signal- anchor sequences
______ direct the insertion of nascent proteins into the ER membrane.
Topogenic sequences
What are the five types of transmembrane proteins?
Type I, II, III, IV and tail-anchored proteins
What are the key features of Type I single-pass transmembrane proteins?
N-terminus on exoplasmic face, C-terminus on cytoplasmic face
How are Type I transmembrane proteins made?
Signal peptidase cleaves signal sequence (lateral exit)
N-terminal grows into exoplasmic face
Stop-transfer anchor sequence halts translocon (lateral exit)
Polypeptide chain grows with C-terminus on cytosol face
How are Type II transmembrane proteins made?
Same as type 1, but signal-anchor sequence instead of signal sequence
signal anchor sequence moved laterally out of translocon
N-terminus on cytoplasmic face, C- terminus on exoplasmic face
No cleavage
How does Type III transmembrane proteins work?
Same as type II except:
N-terminus on exoplasmic face, C- terminus on cytosolic face
How does insertion of tail-anchored proteins work?
Hydrophobic C-terminal tail binds to Get3
Get3 bind to Get1 and Get2
ATP hydrolysis inserts the C-terminal tail into the ER membrane
ADP is released and protein is moved laterally
______ lipid anchor protein is attached to N-terminal Gly
Acylation
______ lipid anchor protein is attached to C-terminal Cys
Prenylation
______ lipid anchor protein is attached to C-terminus
GPI anchor
______ cleaves the precursor protein within the exoplasmic-facing domain, near the stop-transfer anchor sequence (red)
GPI transamidase
True or False: The GPI transamidase covalently links new C- terminus to the terminal amino group of a preformed GPI anchor
True
What are the four main modifications in the ER.
- Covalent addition and processing of carbohydrates
(glycosylation) in the ER and Golgi complex - Formation of disulfide bonds in the ER
- Proper folding of polypeptide chains and assembly of multisubunit proteins in the ER
- Specific proteolytic cleavages in the ER, Golgi complex, and secretory vesicles.
What are the three different sugars coming from the oligosaccharide pre made in glycosylation?
Mannose
Glucose
N-acetylglucosamine (GlcNAc)
True or False: In glycosylation, Full 14-residue precursor is transferred to certain Asn-X- Ser/Thr of a polypeptide as it emerges into the ER lumen
True
What are the 3 reasons for oligosaccharide attachment?
Required in some cases for proper protein folding
Confer stability on many secreted glycoproteins
Play a role in cell-to-cell adhesion and immune response
______ help stabilize the tertiary and quaternary structure of many proteins
Difulside bonds
True or False: Disulfide bond formation formed by the oxidative linkage of sulfhydryl groups (–SH, thiol groups) on two cysteine residues in the same or different polypeptide chains
True
True or False: The disulfide bond formation can proceed only when a suitable oxidant is present
True
In eukaryotic cells, where are disulfide bonds only formed?
Lumen of rough ER
What if proteins are not properly folded?
Presence of unfolded proteins in the rough ER increases transcription of genes that encode ER chaperones and other folding catalysts
What happens in the unfolded protein response?
BiP attaches to unfolded proteins
Ire1 forms dimer, endonuclease cuts Hac1 in mRNA
WHat are the 4 steps in misfolded protein degradation.
Modifications of N-linked oligosaccharides are used to monitor folding and for quality control
Misfolded proteins are targeted for transport from ER lumen back to the cytosol; called dislocation
ERAD (ER-associated degradation) complex enables dislocation of misfolded proteins through the membrane
Specific ubiquitin ligase enzymes that are components of the ERAD complex add ubiquitin residues to the dislocated peptides
True or False: Proteins acquire oligosaccharides and disulfide bonds via ER enzymes
True
______ and ______ work to ensure proper folding of new proteins or transport of unassembled/misfolded proteins back to the cytosol, where they are degraded in the ubiquitin-proteasome pathway
ER chaperones and lectins
True or False: Only properly folded proteins and assembled subunits are transported from the rough ER to the Golgi complex in vesicles
True
