Lecture 15 Intracellular Trafficking to ER

Majority of proteins are synthesized by

free cytosolic ribosomes

One-third of all proteins are synthesized by

ribosomes on the ER membrane

process by which newly synthesized proteins are delivered to their appropriate destination in cells

protein targeting

present within the polypeptide targets the protein to different locations

signal sequence

ER signal sequence -------> ER

Mitochondrial signal sequence

Mitochondria

Nuclear signal sequence

Nucleus

No signal sequence

Cytoplasm

inserted into the lipid bilayer

Membrane proteins

the entire protein is translocated across the membrane into the aqueous interior of the organelle

Water-soluble proteins

targeted to the endoplasmic reticulum, mitochondria, chloroplasts, peroxisomes, and nucleus through signal sequences

Proteins

NH3+ end

6-12 hydrophobic aa,1 basic aa

ER

NH3+ end

3-5 non-consecutive Arg, Lys

Mitochondria

NH3+ end
rich in Ser, Thr

Chloroplast

COO- end

Ser-Lys-Leu

Peroxisome

Internal

5 basic aa/2 small clusters of basic aa

Nucleus

Vesicle-based trafficking of Proteins (Secretory Pathway) involves transport of proteins from the ER to their destination with the help of

membrane-bound vesicles

Secretory pathway begins in

ER

A network of membranous tubules within the cytoplasm of a
eukaryotic cell

Endoplasmic Reticulum

studded with membrane bound ribosomes

Rough endoplasmic reticulum

Lacks ribosomes.
Makes phospholipids and fatty acids.

Smooth endoplasmic reticulum

synthesized on ribosomes bound to the cytosolic face of the ER membrane

Secretory proteins

The polypeptides produced by ribosomes end up within the lumen of the

endoplasmic reticulum

translocated into the ER lumen

Soluble (secretory) proteins

embedded in the ER membrane bilayer

Transmembrane proteins

located at the N-terminus of the protein

ER targeting signal sequence

Continuous stretch of 6 – 12 hydrophobic amino acids at the N-terminus

ER signal sequence

critical for interacting with the machinery that
targets the protein to the ER membrane

hydrophobic core

The transport of most secretory proteins into the ER lumen begins while the nascent protein is still bound to the ribosome

co-translational translocation

proteins in yeast that enter the ER lumen after translation is completed

Post-translational Translocation

refers to the number of times the polypeptide chain spans the membrane and the orientation of the membrane-spanning segments

Topology of membrane proteins

always faces cytoplasmic

Cytosol

face Exoplasmic side

ER Lumen

the end that faces or is inside the lumen will face

extracellular space

only 1 membrane spanning ⍺-helix

Single Pass

2 or more membrane spanning ⍺-helix

Multi Pass

N-terminal end is inside ER lumen (exoplasmic space) Nexo

C-terminal end is in the cytoplasmic side Ccyto

N-terminal cleavable ER signal sequence

TYPE I Transmembrane Protein

N-terminal end in cytoplasmic side – Ncyto
C-terminal end in ER lumen (exoplasmic space) – Cexo

TYPE II Transmembrane Protein

N-terminal end in cytoplasmic side – Ncyto
C-terminal end in ER lumen (exoplasmic space) – Cexo

single “internal” hydrophobic signal-anchor sequence (SA)

TYPE III Transmembrane Protein

cleaves off the original stop-transfer anchor sequence and transfers the luminal portion of the protein to a preformed GPI anchor in the membrane.

Transamidase