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
What are topogenic sequences?
N-terminal signal sequence,internal stop-transfer anchor sequences and internal signal anchor sequence → direct insertion of nascent proteins into the ER membrane
Classes of ER membrane and proteins
Type l
Type 1l
Etc.
What is type 1 single- pas?
Step 1,2,3
Step 1: translocation initiation and ss cleavage by some mechanism as for soluble secretary proteins
Step 2:nascent peptide elongates
Step 3: elongation continues until a hydrophobicstop- transfer anchor sequence enters the transloca preventus the nascent chair from extruding father into the lumen ER
What is type 1 single pass?
Step 5,6,7
Stop transfer anchor sequence mores laterally through a hydrophobic cleft between translocon submits and becomes anchored in the phospholipid bylayer →translocon closes
Step 5 → synthesis continues elongating chain loops into the cytosol through a small space between the ribosome and translocation
Step6 → synthesis completes at stop codon→
Ribosomal subunits are released into the cutosol
Freeing nascent protein→ diffuse laterally in ER membrane
What is hydropathy profile?
Predicts likely topogenic sequences in integral membrane proteins
Plot of total hydrophobicity of each 20 AA segment long the length
Lysosomes and endocytosis pathway
Major function → degrade
Extracellular material take up by via endocytosis/ phagocytosis
Material needs to reach the lumen, where low pH & strongly hydrolytic enzyme are active
