The endoplasmic reticulum (ER) is a large organelle that helps process and transport proteins within cells. It has two main domains: the rough ER, where proteins are synthesized, and the smooth ER, where lipids are produced. Newly synthesized proteins enter the ER through translocon channels in the membrane. In the ER, proteins undergo modifications like folding, disulfide bond formation, glycosylation, and lipid additions to help them mature and prepare for transport to other organelles like the Golgi apparatus via transport vesicles. Integral membrane proteins are inserted into the ER membrane through translocon complexes and can span the membrane multiple times. The ER plays a key role in protein sorting and transport within eukaryotic cells

1. Cell BiologyCell Biology
S. Rahgozar,PhD
University of Isfahan
Faculty of Science
4. Protein sorting and transport
4.1. The Endoplasmic Reticulum
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2. The Endoplasmic Reticulum
 The largest organelle in most
eukaryotic cells
 ER membrane is 50% of cell
membrane
 Space enclosed by ER is 10% of the
total cell volume

3.  ER domains:
 Rough ER (+ ribosomes)
 Smooth ER where lipid metabolism
happens
 Transitional ER where vesicles exit
to the Golgi apparatus

4.  Isolation of rough ER
Rough ER
Folding
Processing
Golgi
apparatus Sorting
Processing
Transport
Secretory
vesicles
Inside
Outside

5.  Overview of protein sorting
Golgi apparatus

6.  The secretory pathway
Pancreatic cell

7.  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

8.  Cotranslational targeting of secretory proteins to the ER
SRP: signal recognition
particle
Translocon:
membrance channel

9. Structure of the SRP
Blue: SRP proteins
Orange: SRP RNA including
2 flexible hinge regions
Green: Signal sequence
The signal sequence on the nascent protein binds to a
pocket in the SRP proteins

10. 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

11.  Translocation of proteins into the ER after their translation
HSP 70
BIP acts as a ratchet to
drive the
posttranslational
translocation of proteins
into the ER.

12.  Insertion of proteins into the ER membrane
Integral membrane proteins span the membrane via α-helical regions of 20 to
25 hydrophobic amino acids, which can be inserted in a variety of orientations.

13.  Transport of integral
membranes
Proteins destined for the inner
nuclear membrane do not transfer
by vesicles, but move laterally in
the plane of the membrane.
Transport
vesicle

14.  Insertion of integral proteins
o Insertion of a membrane protein with a cleavable signal sequence
and a single stop-transfer sequence

15. o Insertion of a membrane protein with no cleavable signal sequence
?
Orientation of the
protein depends on
the orientation of the
signal sequence

16. o Insertion of a protein that spans the membrane multiple times

17.  Transport of integral
membranes
Proteins destined for the inner
nuclear membrane do not transfer
by vesicles, but move laterally in
the plane of the membrane.
Transport
vesicle
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

18.  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 in
the ER lumen.

19. o Glycosylation
14 sugar residues
3
Asn-X-Ser/Thr
Oligosaccharyl
transferase
Glycoslation
 Helps to prevent protein aggregation
 Provides signals for subsequent sorting in the secretory pathway
9

20. o Addition of glycoprotein phosphatidyl inositol (GPI) anchor
Yellow: inositol
Red: mannose
Blue: Glucosamine
Purple: N-
acetylgalactosamine