Protein Processing in the Endoplasmic Reticulum

three major protein processing events that occur in ER

folding

disulfide bond formation

glycosylation

folding

proteins are linear chains of amino acids, fold into 3D structure to perform functions

when does folding occur

can occur co translationally

one part of protein can fold as soon as it emerges from ribosome, before remainder of protein is synthesized

molecular chaperones

proteins that help other proteins fold properly

what do molecular chaperones bind to

bind to:

unfolded proteins

incorrectly folded proteins

unassembled components of protein complex

chaperone binding and unbinding of proteins

coupled to ATP hydrolysis

ER chaperones examples

BiP

calreticulun

Calnexin

disulfide bonds

covalent attachments between sulfur atoms of two cysteine amino acid side chains

contribute to protein structure

why do disulfude bonds form

form between exposed cysteine residues in ER bc environment is oxidizing

what protein forms disulfide bonds

protein disulfide isomerase

protein in ER that helps formation of correct disulfide bonds

can form between two cysteines in same protein or two cysteines in different proteins

glycosylation

most proteins synthesized at ER have covalently attached carbohydrate

attachment of carbohydrate is called glycosylation, protein in glycosylated

how does glycosylation happen

carbohydrate attached as single unit to asperigine residues in proteins

specific asperigine residues that are glycosylated usually followed by any amino acid (X) and then a serine or threonine residue

asperigine + any + serine/threonine

n linked glycosylation catalyzed by

by enzyme in ER membrane, OST

glycosylation functions

n-linked glycosylation helps proteins fold properly in ER

glycosylation and folding process

1. three triangles represent three glucose molecules at the end of a carbohydrate

2. within some period of time after carbohydrate is added to protein, 2 of the 3 glucoses removed by enzymes

3. protein containing n linked carbohydrates with a single glucose is bound by chaperone protein, calreticulin/calnexin, helps protein fold

4. protein separated from chaperone by removal of remaining glucose by enzyme glucosidase

5. protein not folded correctly, will bind to glucosyl transferase, which will add glucose back on to n linked carbohydrate

6. protein can bind to chaperones again for more help folding

7. continues until protein is properly folded

8. properly folded protein have no glucose groups remaining at end of n-linked carbohydrate

glycosylation and folding process 1-2

three triangles represent three glucose molecules at the end of a carbohydrate

within some period of time after carbohydrate is added to protein, 2 of the 3 glucoses removed by enzymes

glycosylation and folding process 3-4

protein containing n linked carbohydrates with a single glucose is bound by chaperone protein, calreticulin/calnexin, helps protein fold

protein separated from chaperone by removal of remaining glucose by enzyme glucosidase

glycosylation and folding process 5-6

protein not folded correctly, will bind to glucosyl transferase, which will add glucose back on to n linked carbohydrate

protein can bind to chaperones again for more help folding

glycosylation and folding process 7-8

continues until protein is properly folded

properly folded protein have no glucose groups remaining at end of n-linked carbohydrate