protein sorting mechanisms Flashcards
A protein must be converted from a linear chain of amino acids to a specific 3D shape (conformation) to gain its ???
function.
where do most proteins begin their synthesis?
in the cytosol
do proteins remain in the cytosol once synthesised?
not necessarily, they can be transported to nucleus, mitochondria, chloroplasts or peroxisomes OR will enter the ER membrane or even be transported out of cell
what are the three distinct routes of protein importation INTO organelles?
through nuclear piores
across membranes
via vesicles
TRUE or FALSE: importation of proteins can occur co-translationally OR post-translationally
true
the fate of a protein depends on ??? which are stored in amino acid sequence and can be built in via a signal peptide or a signal patch
sorting signals
??? were first discovered in proteins imported into the rER. Two reactions – the one with rough microsomes from the ER → size difference = short N‐terminal sequence.
sorting signals
Continuous bidirectional transport
across channels in the nuclear
envelope is SELECTIVE or NONSELECTIVE?
selective
transport of proteins between the nucleus and the cytosol: Histones, DNA and RNA polymerases, gene regulatory proteins, and RNA processing proteins all require import from the ??? where they are made
cytosol
transport of proteins between the nucleus and the cytosol: tRNA’s and mRNA’s synthesised in the nucleus are transported into the cytosol where they participate in ???
translation
the lumen of the nuclear envelope is continuous with the ER lumen. Bidirectional traffic of proteins between the nucleus and cytosol occurs through the ???
nuclear pore complexes
nuclear pore complexes are freely permeable to small water soluble molecules, proteins with a nuclear localisation signal are ??? by nuclear import receptors
recognised
RNA and new ribosomal subunits have nuclear IMPORT or EXPORT (?) signals recognised by nuclear export receptors. this allows proteins to cross from cytosol to nucleus
export
Nuclear localisation signals direct proteins to the ???
nucleus
TRUE or FALSE: no nuclear localisation signals are specifically recognised by nuclear import receptors
FALSE most are recognised
nuclear import receptors are SOLUBLE or INSOLUBLE (?) cytosolic proteins that bind to both the NLS on the protein being transported and to NPC proteins
soluble
Interaction between FG repeats on fibrils/filaments and binding sites on ??? enables transport across the nuclear pore complex
nuclear import receptors
what enables protein transport across the nuclear pore complex into nucleus?
interaction between FG repeats on fibrils/filaments and binding sites on nuclear import receptors
what imposes directionality on nuclear import through NPCs?
Ran GTPase
The critical difference between Ran‐mediated nuclear import and nuclear export is the nature of cargo binding by the cargo receptor. In nuclear IMPORT, cargo
binding is ??? of Ran‐GTP; in nuclear EXPORT, cargo binding requires Ran‐GTP
mutually exclusive
translocation of proteins into mitochondria depends on ??? and protein translocators
signal sequences
Mitochondrial proteins are imported post‐translationally OR co-translationally (?) as unfolded polypeptide chains
post-translationally
Protein import into ??? is powered by ATP hydrolysis, a membrane potential,
and redox potential
mitochondria and chloroplasts
Multi‐subunit protein complexes that function as protein translocators mediate movement across ??? membranes
mitochondrial
importing proteins into the inner mitochondrial membrane requires a combination of translocator protein complexes, signal sequences and sometimes ??? to mediate movement
chaperones
chloroplasts have 6 compartments:
* inner and outer envelope
membrane
* intervening membrane space
* stroma
* thylakoid membrane
* ???
lumen
Translocation into chloroplasts similar to mitochondrial transport
→ Occurs post‐translationally OR co-translationally (?)
→ Uses separate translocator complexes in each membrane (TOC and TIC)
→ requires energy
→ Uses N‐terminal signal sequences that are cleaved after use
post-translationally
Hydrolysis of ATP and GTP drives import of proteins into ??? inner membrane
chloroplast
three pathways of protein import into chloroplasts:
- Sec pathway
- ??? pathway
- TAT pathway
OXA pathway
peroxisomes Oxidise organic molecules by using O2 to remove H2 =H202 which it degrades. ALSO breaks down ???
Fatty Acids
peroxisomes acquire most proteins by selective import from the cytosol but some enter the via the ???
ER
??? direct the import of proteins into peroxisomes
short signal sequences
Soluble Peroxisomal Targeting Signal
(PTS) receptors and docking receptors on the cytosolic surface help import proteins into ??? organelles
peroxisome
import of proteins innto peroxisomes is driven by ???
ATP hydrolysis
import into peroxisomes: A complex of at least 6 different proteins forms a translocator. Imported in native confirmation aided by at least one ??? import receptor, Pex5. Pex5 carries its cargo all the way into the peroxisome before being recycled
soluble
RER has bound ribosomes on the ??? face
cytosolic
the RER or SER (?) synthesises proteins destined for endomembrane system or export. Also adds simple oligosaccharides to proteins
RER
the RER or SER has NO bound ribosomes and is in charge of lipid metabolism, synthesis of cholesterol and steroid hormones, and detoxification of foreign substances
SER
where is the site of production of most lipid classes?
ER membrane
what establishes polarity of membranes?
ER membrane
TRUE or FALSE: Most membrane lipid bilayers are assembled in the ER. Transmembrane proteins inserted in predictable orientation as determined by amino acid sequence
TRUE
ER as the site of lipid biosynthesis: lipids are inserted into ??? face of bilayer and are bidirectionally moved to reach asymmetry by scramblases
cytosolic
Flipases or scramblases move lipids into other half of bilayer to produce asymmetric distribution?
flipases
flipases or scramblases move lipids inserted into cytosolic face of bilayer to reach assymetry?
scramblases
TRUE or FALSE: membranes grow as newly synthesised proteins and lipids are inserted into existing ER membrane
TRUE
The golgi and other cell compartments receive their lipids from ER vesicles that have assymetric phospholipid composition maintained by ???
flipases
maintenance of membrane asymmetry by scramblases is NON-SPECIFIC or SPECIFIC (?) and bidirectional- is energy independent
non-specific
maintenance of membrane asymmetry by flipases is NON-SPECIFIC or SPECIFIC (?), unidirectional, and requires ATP
specific
Polysomes (AKA polyribosomes) occur when multiple ribosomes (80 – 100 nucleotides apart) are translating
proteins simultaneously on a single ???molecule
mRNA
are polyribosomes/polysomes found free in cytosol or attached to the ER?
both
Transport of soluble proteins in the ER is achieved by ??? chaperones bound to the polypeptide. this chaperone detach as polypeptide is passed through the membrane
Hsp70
Transport of Soluble Proteins in the ER requires Sec62 and Sec63 complex attached to the Sec61 translocator and positions Bip molecules to bind the polypeptide in the ???. Bip chaperones assist pulling the polypeptide into the ER lumen using a ratchet like mechanism
ER lumen
Co‐translational import of proteins in the ER: ribosome is brought to membrane by SRP + SRP receptor and then engages with the Sec61 translocator. The growing polypeptide chain is threaded across the membrane ONCE it is made or AS it is made?
AS it is made
TRUE or FALSE: no additional energy is needed for co-translational import of proteins in the ER as the only path available to the growing chain is to cross the membrane
TRUE
The proteins are never released into the cytosol and there is no danger of the protein folding before reaching the
membrane‐associated translocator in CO-TRANSLATIONAL or POST-TRANSLATIONAL import of proteins into the ER?
co-translational
TRUE or FALSE: co-translational import of proteins into ER involves a signal recognition particle
true
A signal‐recognition particle (SRP) directs ER signal peptides to a specific receptor in the RER membrane. As a signal sequence emerges from the ribosome and binds to the SRP, a ??? change in the SRP exposes a binding site for the SRP receptor
conformational
Transport of Single‐pass Transmembrane Proteins in the ER: an N-terminal signal sequences initiates ??? and an ADDITIONAL hydrophobic segment acts as a stop-transfer signal to anchor the protein in the membrane
translocation
Transmembrane proteins contain ??? segments that are recognised like signal sequences and remain in the lipid bilayer as a membrane‐spanning alpha‐helix
hydrophobic
Transport of multi‐pass integral membrane proteins in the ER: Combinations of start‐ & stop‐ transfer signals determine the topology of multipass integral membrane proteins. Each successive transmembrane segment is inserted into the membrane via the ??? gate in an orientation opposite to that of the transmembrane segment that immediately preceded it.
lateral gate
Folding and assembly of translocated proteins is assisted by ER resident proteins: ER retention signal at C-terminus is responsible for retaining the protein in the ???
ER
Folding and assembly of translocated proteins is assisted by ER resident chaperones, e.g. Bip, which pulls proteins ??? into the ER. Can recognise misfolded proteins as well as protein sub-units that have not assembled into oligomeric form
TRANSLATIONALLY
Folding and assembly of translocated proteins is assisted by ER resident proteins: protein disulfide isomerase catalyses the ??? of free sulfhydryl groups on cysteine to form disulphide bonds (S-S)
oxidation
Most proteins synthesised in the rough ER become glycoproteins including those destined for the golgi, lysosomes, plasma membrane or extracellular space. this is largely achieved through addition of common ???
N‐linked oligosaccharides
Key functions of carbohydrate groups on glycoproteins via glycosyltransferases during glycosylation:
- Can be important in binding interactions with other macromolecules
- Aids ???
- Can be essential for secretion
correct folding
glycosylation in RER: a single N-acetylglucosamine group is added to a serine and ??? residue of the protein
threonine
Transmembrane proteins are packaged into budding transport vesicles through
interactions of exit signals on their cytosolic tails with adaptor proteins of the inner COPII coat. Some of these transmembrane proteins function as cargo receptors, binding soluble proteins in the ER lumen and helping to package them
into vesicles. These vesicles are sent off to the ???
golgi apparatus
??? ensure proteins are
properly folded before they leave
the ER
Chaperones
glycosylation can help identify proteins that are destined for ??? by recognition of phosphorylated N-linked carbohydrates
lysosomes
Trafficking through the Endomembrane System:
Step 1. Ribosomes of the RER synthesise proteins; proteins enter ER lumen; initial ??? occurs
glycosylation
Trafficking through the Endomembrane System:
Step 2. ??? vesicles carry proteins and lipids to cis‐Golgi network (CGN).
Transition
Trafficking through the Endomembrane System:
Step 3. Lipids and proteins move through the ??? of the Golgi.
cisternae
Trafficking through the Endomembrane System:
Step 4. At the trans‐Golgi network (i) some
vesicles form secretory vesicles that release contents by exocytosis, (ii) others form endosomes and then ???
lysosomes
