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Post-Translational Events II

Learn about protein glycosylation processes and lipid biosynthesis in the rough endoplasmic reticulum and Golgi apparatus. Explore pathways, modifications, and targeting signals involved in cellular regulation and protein folding.

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Post-Translational Events II

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  1. Post-Translational Events II ER & Golgi Processes

  2. Rough Endoplasmic Reticulum

  3. Glycolsylation • O-linked – serine, threonine, hydroxylysine (collagen) • N-linked - asparagine • The sugars • GalNac – N-acetylgalactosamine • GlcNac – N-acetylglucosamine • NANA – N-acetylneuraminic acid (sialic acid) • Gal – galactose • Glc – glucose • Man – mannose • Fuc – fucose • Dolicholpyrophosphoryl oligosaccharide • All sugars are linked to nucleoside mono- or diphosphate • UDP-gal, UDP-GalNac, UDP-GlcNac • GDP-man • CMP-NANA • Dolichol pyrophosphate

  4. Glyco-nucleoside Phosphates CH2CH(CH3)CH2CH2OPO3 CH3C(CH3)=CHCH2 CH2C(CH3)=CHCH2 15-18 Dolichol phosphate

  5. O-linked Glycosylation • Small chains of sugars • N-acetylgalactosamine (GalNac) is first sugar on S orT • GalNac transferase in ER and cis-golgi • Additional sugars added in cis, medial and trans-golgi • Gal added by galactosyl transferase • NANA added last • Each sugar addition is catalyzed by specific enzyme

  6. N-linked Glycosylation • Initial step in ER • Dolichol pyrophosphoryl oligosaccharide • 14 sugar moiety attached to dolichol pyrophosphate • (GalNac)2Man9Glc3 • Dolichol phosphate in ER membrane • Sugars added by cytostolic glycosyltransferases • Moiety flips in ER membrane to face lumen • Transferred to asparagine by oligosaccharyl transferase (oligosaccharide-protein transferase) • Target sequence N-X-S and N-X-T (X = P) • Subsequent remodeling of sugar moiety in ER & golgi • N-linked chains very long and extensively branched

  7. O- & N-linked Oligosaccharide Chains

  8. N-linked Glycolsylation

  9. GPI – Linkages • GPI – linked proteins are attached to GPI in ER • GPI in ER membrane • C-term of protein attached to sugar residue on phosphatidyl inositol • GPI linked protein will be attached to outer leaflet of plasma membrane

  10. Protein Folding Occurs in ER Lumen

  11. Misfolded Proteins Reverse Translocate to Proteasome

  12. Unique Signaling from ER to Make More ER • Pathway activated by stresses that result in more unfolded proteins • heat • unfavorable pHs

  13. On to the Golgi

  14. Processing in the Golgi • Oligosaccharide modifications • Sorting to exocytic vesicles, lysosomes and ER • Proteolytic processing

  15. Modification of N-linked Oligosaccharides • As glycosylated protein moves through ER Glc is removed • In cis Golgi, some Man is removed • In medial golgi, GlcNac is added • In trans-golgi, Gal and NANA are added CIS TRANS

  16. Mannose-6-Phosphate is Lysosomal Targeting Signal

  17. Proteolytic Processing

  18. Sterol & Lipid Biosynthesis • Phospholgycerides in ER • Sphingolipids in Golgi • Lipid profile asymmetry established via phospholipid exchange proteins and selective vesicular transport due to association between carrier proteins and certain lipid types

  19. Regulation of HMG-CoA Reductase

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