Cell BiologyCell Biology

S. Rahgozar,PhD

University of IsfahanFaculty of Science

4. Protein sorting and transport

4.1. The Endoplasmic Reticulum

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The Endoplasmic Reticulum

The largest organelle in mosteukaryotic cells

ER membrane is 50% of cellmembrane

Space enclosed by ER is 10% of thetotal cell volume

ER domains:

Rough ER (+ ribosomes)

Smooth ER where lipid metabolismhappens

Transitional ER where vesicles exitto the Golgi apparatus

Isolation of rough ER

Rough ERFolding

ProcessingGolgi

apparatus Sorting

Processing

Transport

Secretoryvesicles

Inside

Outside

Overview of protein sorting

Golgi apparatus

The secretory pathway

Pancreatic cell

Incorporation of secretory proteins into cytoplasm/microsomes

The signal sequence of growth hormone

20 hydrophobic amino acid proceeded by a basic aa (ie; Arg)

N

Cotranslational targeting of secretory proteins to the ER

SRP: signal recognitionparticle

Translocon:membrance channel

Structure of the SRP

Blue: SRP proteins

Orange: SRP RNA including2 flexible hinge regions

Green: Signal sequence

The signal sequence on the nascent protein binds to apocket in the SRP proteins

Translocons are

Complexes of 3 membrane proteins called the Sec61 proteins

Conserved among prokaryotic and eukaryotic cells

Signal sequence Short hydrophobic side chains in translocon neck

Opening of the translocon channel

Transfer of the growing polypeptide chain through the translocon

Translocation of proteins into the ER after their translation

HSP 70

BIP acts as a ratchet todrive theposttranslationaltranslocation of proteinsinto the ER.

Insertion of proteins into the ER membrane

Integral membrane proteins span the membrane via α-helical regions of 20 to25 hydrophobic amino acids, which can be inserted in a variety of orientations.

Transport of integralmembranes

Proteins destined for the innernuclear membrane do not transferby vesicles, but move laterally inthe plane of the membrane.

Transportvesicle

Insertion of integral proteins

o Insertion of a membrane protein with a cleavable signal sequenceand a single stop-transfer sequence

o Insertion of a membrane protein with no cleavable signal sequence

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Orientation of theprotein depends onthe orientation of thesignal sequence

o Insertion of a protein that spans the membrane multiple times

Transport of integralmembranes

Proteins destined for the innernuclear membrane do not transferby vesicles, but move laterally inthe plane of the membrane.

Transportvesicle

Specific tansmembrane sequences located

in the inner nuclear membrane proteins

may alter their interaction with the

translocon and signal their transport to the

inner nuclear membrane where they are

retained by interactions with nuclear

components such as lamins or chromatin

Types of post translational modification of proteins in the ER

o Protein folding in the ER

o Formation of disulfide bonds (S-S)

Formation of S-S bonds is facilitated by PDI , the enzyme which is located inthe ER lumen.

o Glycosylation

14 sugar residues

3

Asn-X-Ser/ThrOligosaccharyltransferase

Glycoslation

Helps to prevent protein aggregation

Provides signals for subsequent sorting in the secretory pathway

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o Addition of glycoprotein phosphatidyl inositol (GPI) anchor

Yellow: inositol

Red: mannose

Blue: Glucosamine

Purple: N-acetylgalactosamine