Protein Targetting, Processing, Degradation Flashcards
What are the steps of secreted protein translation, starting from a free ribosome?
- Signal sequence emerges from ribosome.
- SRP binds it and temporarily stops translation.
- Complex attaches to SRP docking protein on ER.
- GTP hydrolysis moves the complex to a translocon, SRP is released.
- Translation resumes, once in the ER the signal sequence is removed by peptidase.
- Translocon closes once translation is complete.
What are ER resident protein signals?
Membrane retention: KKXX, XRRX
Lumen retention: C-terminal KDEL (retrieval signal as well).
How are proteins retained in the ER membrane during cotranslation?
A stop transfer, membrane spanning alpha-helix sequence causes translation to stop and protein to move laterally out of the translocon.
Describe movement of lysosomal proteins from ER to golgi.
- Protein is glycosylated in the ER lumen.
- Mannose glycosylation is phosphorylated to create M6P.
- M6P protein binds specific receptors of lysosomal sorting vesicles in the golgi.
- Low pH of sorting endosome causes dissociation of M6P protein from receptor, phosphate is removed via a phosphatase, vesicle buds from sorting endosome and fuses with lysosome.
Describe I-cell disease.
Autosomal recessive.
Acid hydrolases lack M6P signal due to deficient phosphotransferase activity, causes secretion of them from cell. This leads to lysosomal inclusions forming.
Phenotype is skeletal abnormalities, psychomotor impairment, and death due to cardiorespiratory issues.
Describe function of Ran in nuclear localization.
Ran:GDP moves into the nucleus from cytoplasm along with protein:importin.
Once in nucleus, Ran:GDP interacts with GEF to become Ran:GTP.
Ran:GTP reaction with protein:importin causes protein release.
Ran:GTP:Importin moves back to cytosol
Ran:GTP:Exportin:protein also leaves nucleus, hydrolysis of GTP then causes dissociation.
Describe Sweyer syndrome.
Due to a defect in the NLS of SRY protein, movement into nucleus does not occur and testis do not develop. This leads to XY individuals born as phenotypic females.
What causes human deafness dystonia?
X-recessive.
Defect of DDP section of the TIM complex.
Phenotype is deafness, blindness, and dysphagia.
What is Zellweger’s syndrome?
Autosomal recessive.
Proteins are not targeted to peroxisome correctly, leads to dysfunctional peroxisomes.
Neurological impairment and early death occurs.
Describe the HSP response.
HSPs are chaperones expressed during adverse conditions to inhibit premature folding or facilitate proper folding.
Important classes are hsp 60 and 70 which bind to hydrophobic sticky patches.
Often create disulfide bonds within the protein to assist in folding.
What are the unfolded protein sensing factors within the ER, and their function once activated?
PERK- phosphorylates eIF-2a.
ATF6 and IRE1- Activates TFs needed in chaperone production.
What are major steps of the unfolded protein response pathway?
- Transcription of chaperone mRNAs is increased so ER folding capacity increases.
- General translation is inhibited to decrease new proteins entering ER.
- GRP78 recognizes unfolded proteins via interaction with exposed hydrophobic patches and redirects them to glycosylation pathways in an attempt to refold it correctly.
What are the sequential steps if futile folding occurs during the ERAD pathway?
- Ejection into the cytosol.
- Removal of remaining glycosylation residues.
- Protein degradation.
Describe the proteasome-mediated pathway.
A minimum of a 4-ubiquitin long chain is attached to internal degradation sequences on the protein, like PEST. They are shipped to the proteasome for ATP dependent degradation (two regulatory caps recognize the tag, catalytic core splices the peptide).
