Lecture 18: Protein processing and degradation Flashcards
Protein Export
-protein synthsis by ribosomes of rough ER
-lumen of ER like the exterior of cell
-signal peptide
Signal peptide
-usually at or near N-terminus
-positively charged N-terminal region and a core of 8-12 hydrophobic amino acids in a-helix
-followed by more polar C-terminal segement that serves as cleavage site
Protein export
- SRP recognizes signal peptide = STALLS translation
- SRP associates with SRP receptor
- ribosome associates with translocon
- translation resumes and peptides extrude into ER
-signal cleaved by peptidase
Glycoproteins
-proteins with carb chains
-many exported proteins and extracellular domains of membrane proteins
-recognition signals
glycosylation
alters stablities, solubilities, and size of proteins
N-linked glycosylation
-oligosaccharide linked to amide nitrogen of asparagine in Asn-X-Thr sequence
-oligo is synthesized as intermediate with dolichol phosphate
-then transferred to new peptide in ER (cotranslational)
tunicamycin
-inhibits step A of N-linked glycosylation
glycosidases and glycosyltransferases
may further modify oligosaccharride
O-LINKED GLYCOSYLATION
-oligo bound to hydroxyl of serine/theronine
-occurs in golgi (posttranslational)
-initiated by GalNAc-transferase
-results in heterogenous oligos
Targeting to membranes
anchoring sequence
-hydrophobic
-inserted to the membranes through TRANSLOCON
-multiple sequence = protein spans membrane multiple times
Retention of SOLUBLE proteins in ER
-C-terminal KDEL sequence
Targeting to nucleus
-made in cytosol with nuclear localization signals (clusters of amino acids)
-PKKKRKV
-KR[PAATKKAGQA]KKKK
-transported by carrier proteins to nuclear pore complexes
-needs GTPase Ran
-phosphorylation of SPS or TPT seq includes nuclear uptake
Targeting to mitochondira
-made in cytosol as PREproteins with N-terminal presequences (+ charged helices)
-recognized by mitochondrial receptor
-TOM and TIM complex
-translocation requires unfolding
-energy DEPENDENT
TOM
-translocase of outer membrane
TIM
-translocase of inner membrane
protein turnover
-constant
-regulate metabolism
-eliminate bad proteins
-break down as carbon and nitrogen sources in time of need
protein half lives
-short for regulatory
-long for housekeeping
Lyosomal protein degradation
-contain ~50 hydrolytic enzymes
-ph 5
-cathepsins
lysosomal uptake
-endocytosis for extracellular
-autophagy for intracellular (nonselective)
Protein degradation
-to amino acids
-recycled for protein synthesis or metabolized
-proteolysis
Intracellular proteolysis
-POLYUBIQUINATION marks protein for degradation
-PROTEASOMEs recognize polyubiquinated proteins
-proteins unfolded and translocated into chamber of proteasomes USING ATP
-Proteasome active site inside chamber
Ubiquitin dependent protein degradation
-highly conserved
-posttranslational mod
-E1-E3
E1
-activates ubiquitin to form thioester withits C-term
-ATP REQUIRED
