The endomembrane system I

Endoplasmic reticulum (ER)

continuous network of flattened sacs, tubules, and vesicles through the cytoplasm of a eukaryotic cell

ER functions

1. Synthesis of proteins for (a) incorporation into the plasmA membrane, (b) organelles of the endomembrane system and (c) export from the cell

2. Synthesis of lipids

How to distinguish rough ER from smooth ER

• Rough ER membranes form large flattened sheets

• Smooth ER membranes form tubular structures

• Cells involved in synthesis of secretory proteins have prominent rough ER networks

• Cells producing steroid hormones tend to have extensive networks of smooth ER

Rough ER functions

• synthesizes membrane bounded and soluble proteins for the endomembrane system

• The initial steps of addition of carbohydrates to glycoproteins

• The folding of polypeptides

• Recognition and removal of misfolded proteins

• Assembly of multimeric proteins

Smooth ER functions

• drug detoxification

• Carbohydrate metabolism

• Calcium storage

• Steroid biosynthesis

Glucose-6-phosphate

hydrolyzes the phosphate from glucose-6-phosphate to form free glucose (associated with smooth ER)

Golgi complex

• Central role in membrane and protein trafficking in eukaryotic cells

• series of flattened membrane-bounded cisternae

• Glycoproteins and membrane lipids from the ER are further processed and sorted and packaged for transport

Transport vesicles

carry lipids and proteins from the ER to the Golgi complex and to various destinations in the cell

Each Golgi stack has two distinct faces

1. cis-Golgi network (CGN) oriented towards the ER

2. trans-Golgi network (TGN) oriented away from the ER

Medial cisternae

• between the TGN and CGN

• Most protein processing occurs

How do lipids and proteins flow through the Golgi?

1. Stationary cisternae model

2. Cisternal maturation model

• both models are not mutually exclusive, both involve formation of transport vesicles

Stationary cisternae model

• Each cisterna is a stable structure

• Transport of materials from one cisterna to another is mediated by shuttle vesicles

• bud off from one cisterna and fuse with the next cisterna in a cis-to-trans sequence

Cisternal maturation model

• Golgi cisternae are transient compartments

• Enzymes not needed in later compartments are returned to earlier compartments in vesicles

Anterograde transport

• Movement of material toward the plasma membrane

• As a secretory granule fuses with the plasma membrane and discharges its contents (exocytosis), a bit of ER membrane becomes part of the plasma membrane

Retrograde transport

• flow of vesicles from Golgi cisternae back to the ER

• Balance the flow of lipids toward the plasma membrane

• Ensures a supply of materials for forming new vesicles

N-linked glycosylation

addition of an oligosaccharide to the nitrogen atom of certain asparagines

O-linked glycosylation

addition of the oligosaccharide to the oxygen atom on the hydroxyl group of certain serines or threonines

Monoglucosylated glycoproteins can form a complex with:

• calnexin (CNX) or calreticulin (CRT)

• ERp57 thiol oxidoreductase (promote disulfide bond formation)