Cell Biology

S. Rahgozar,PhD

University of Isfahan

Faculty of Science

4. Protein sorting and transport

4.2. ER and Golgi apparatus

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Glycoprotein folding by calreticulin

Quality control in ER

The process involves BIP, other

chaperons, PDI, …

The sensor assesses the

extent of folding by

monitoring exposed

hydrophobic regions. If

none are found the GP

passes into the transitional

ER. =glucosyltransferase

Mannose residues are

removed

Unfolded protein response

Heat, chemical insult, viral infection

Bip’s roles:

Protein import

Protein folding

Inactivation of

signaling molecules

The smooth ER

3 main types of lipids:

Glycolipids

Cholestrol

Synthesis of phospholipids

Phospholipids

SER is abundant in

Steroid producing cells

liver (containing

detoxifying enzymes

inactivating lipid-soluble

compounds)

Translocation of

phospholipids across the

ER membrane

There are several families of

these enzymes, some of which

are specific for particular

phospholipids.

Structure of cholesterol and ceramide

Glocolipiods

Sphingomyelin

Golgi

apparatus

Vesicular transport from the ER to the Golgi

ERGIC: ER-Golgi

intermediate compartment

export signals

import (retrieval)

signals

Code:

KDEL Lys-Asp-Glu-Leu

KKXX

The Golgi apparatus

Proteins and lipids from the ER enter the

Golgi apparatus at its cis face and exit

near its trans face

Regions of the Golgi

apparatus

Protein glycosylation

o Processing of N-Linked oligosaccharides for proteins destined for secretion

or for the plasma membrane

Glycosyltransferase Glycosidase

o Targeting of lysosomal proteins by phosphorylation of mannose residues

The recognition determinants that lead to mannose phosphorylation are signal patches

in contrast to linear signals.

This recognition depends on the three-dimensional conformation of the folded protein.

Synthesis of sphingomyelin and glycolipids

Transport from

the Golgi apparatus

Proteins can be

transported to the plasma

membrane either directly

or via recycling

endosomes.

Transport to the plasma membrane of

polarized cells