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

CH3C(CH3)=CHCH2 CH2C(CH3)=CHCH215-18CH2CH(CH3)CH2CH2OPO3

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)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

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

CIS TRANS

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