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) –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 CH 3 C(CH 3 )=CHCH 2 CH 2 C(CH 3 )=CHCH CH 2 CH(CH 3 )CH 2 CH 2 OPO 3 Dolichol phosphate
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 Initial step in ER Dolichol pyrophosphoryl oligosaccharide –14 sugar moiety attached to dolichol pyrophosphate (GalNac) 2 Man 9 Glc 3 –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
O- & N-linked Oligosaccharide Chains
N-linked Glycolsylation
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
Protein Folding Occurs in ER Lumen
Misfolded Proteins Reverse Translocate to Proteasome
Unique Signaling from ER to Make More ER Pathway activated by stresses that result in more unfolded proteins –heat –unfavorable pHs
On to the Golgi
Processing in the Golgi Oligosaccharide modifications Sorting to exocytic vesicles, lysosomes and ER Proteolytic processing
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 CISTRANS
Mannose-6-Phosphate is Lysosomal Targeting Signal
Proteolytic Processing
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
Regulation of HMG-CoA Reductase