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Post-Translational Events II. ER & Golgi Processes. Rough Endoplasmic Reticulum. Glycolsylation. O-linked – serine, threonine, hydroxylysine (collagen) N-linked - asparagine The sugars GalNac – N-acetylgalactosamine GlcNac – N-acetylglucosamine NANA – N-acetylneuraminic acid (sialic acid)

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post translational events ii

Post-Translational Events II

ER & Golgi Processes

glycolsylation
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
glyco nucleoside phosphates
Glyco-nucleoside Phosphates

CH2CH(CH3)CH2CH2OPO3

CH3C(CH3)=CHCH2

CH2C(CH3)=CHCH2

15-18

Dolichol phosphate

o linked glycosylation
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
n linked glycosylation
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
gpi linkages
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
unique signaling from er to make more er
Unique Signaling from ER to Make More ER
  • Pathway activated by stresses that result in more unfolded proteins
    • heat
    • unfavorable pHs
processing in the golgi
Processing in the Golgi
  • Oligosaccharide modifications
  • Sorting to exocytic vesicles, lysosomes and ER
  • Proteolytic processing
modification of n linked oligosaccharides
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

sterol lipid biosynthesis
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