Covalent Modifications Involved in the Secretory Pathway

trimming of glucose residues from core oligosaccharides

occurs in: ER targets to: ?

disulfide bond formation by protein disulfide isomerase (PDI)

occurs in: ER targets to: ??

GPI (glycosylphosphatidylinositol) anchor addition

occurs in: ER targets to: cell membrane (GPI is critical addition for anchoring proteins on cell surface)

N-linked core oligosaccharide attachment (or N-linked glycosylation)

occurs in: ER only on NXT sequences (Asn (N) - Any - Thr (T)) of amino acids targets to: ??

signal sequence cleavage

occurs in: ER or mitochondria targets to: usually occurs after the protein has reached its destination

addition of outer-chain oligosaccharide to N-linked core (like GlcNac)

occurs in: Golgi targets to: lysosome

phosphorylation to produce mannose-6-phosphate (M6P)

occurs in: Golgi targets to: lysosome (mostly targets proteins to lysosome) --> modification alters the N-linked core oligosaccharide

proteolytic processing (pro-enzyme conversion)

occurs in: Golgi/ endosomes (?) targets to: lysosomes --> pro enzyme conversion ensures that the enzymes are inactive until they reach their destination to prevent destruction of necessary proteins elsewhere