Protein Transport → Weeklo Flashcards
Overview of major protein- sorting pathways in Euks
Step 1→synthesis of proteins lacking on e.r signal (targeting) sequence is completed on free ribosomes.
Step 2→ proteins that contain no targeting sequence remains in the cytosol.
Step 3-6 → proteins with an organelle specific targeting sequence - imported from the cytosol into mitochondria - chloroplast, perioxsomes inucleus
Transport into and out of the nucleus.
Nuclear envelope contains nuclear pore complex larger complex structures →multiple copies of nuclear porch’s.
Proteins imported/exported from nucleus contain specific AA sequence functioning as a nuclear-localisation signal ( nls) / nuclear export signal (NES)
What are nuclear -localisation signal?
Targets protein for import through nuclear pores into the nucleus,
Experiments → pyruvate kinase
What is the nuclear envelope?
2 membrane system → separates nuclear from cyproplasm
Nuclear pores → spans both membranes 4 transport between cytoplasm &nucleus.
What is the nuclear pore complex model ?
Major structural features - formed by membranes nuclearporins, structural nuclear porins and FG nucleoporins
What is the nuclear pore structural scaffold?
16 copies of the y complex (8 associated with each membrane) - form a major part of the complex
What are FG - nuclearporens?
Extended disordered structures?
What is ran dependent mechanism for nuclear import of proteins?
Ran → small monomer G proteins existing in GTP or GDP bound conformations.
Ran cycles between the cyproplasm nucleus leading to GTP hydrolysis which provides energy 4 unidirectional transport of molecules across nuclear membranes
GTP hydrolysis mediated byGTPase activating protein, ran - gap
Cargo → transported morales
Cargo receptor and vehicles →importin
What is the ran cycle?
Part 1
Cytoplasm
Importin (soluble nuclear transport receptor)- binds an NLS of a cargo protein to form an importin - cargo complex. (ICC)
ICC diffuses through NPC by transiently interacting with FG-nucleoporins
Ran-gdp diffuses into nucleu s
The ran cycle?
Part 2
Nucleoplasm
Ran-gdp activated by guanine exchange factor →
Releases GDP, binds GTP
Ran - GTP binding to the importin causes importin confrontational change that releases the NLS - cargo protein.
The ran cycle?
System recycling
Importin -ran-GTP complex - transported back to cytoplasm
A GTPase - activating proteins (gap) associated with cytoplasmic filaments of the NPC → stimulates ran hydrolysis of its bound GTP
Ran-gdp conformational change
Co-translation translocation?
Step 1,2,3
Step 1:N-terminal ER signal sequence emerges from the ribosome first during the nascent protein synthesis
Step 2: signal recognition particle (srp) binds SS _ arrests protein synthesis
Step 3: SRP - nascent polypeptide chain _ribosome - complex
→ binds to the SRP receptor in the ER membrane, interaction strengthened by GTP binding both SRP and receptor
Co-translation translocation?
Step 4
Transfer of the nascent polypeptide ribosome to the translocon -
→ open translocation channeladmit growing polypeptide
A signal sequence → transferred to a hydrophobic binding site next to the central pore.
→ SRP and receptor - hydrolyse bound GTP dissociate from ribosome Te receptor ( restarting protein synthesis)
Co translation translocation
Step 5?
Elongating polypeptide chain
Passes through the translocon channel into the ER lumen
Signal sequence - cleaved by signal peptidase and rapidly degraded
Co translation translocation?
Step 6,7,8
Growing peptide chain → continues extrusion through the translocan into the ER as the mRNA translated through 3’ end
Step 7 -: translation completes at mRNA stop codon_ ribosome released
Step 8: nascent proteins where remainder drawn into the ER
lumen
