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BIO120H1 (1,000)
Lecture

Notes taken during lecture


Department
Biology
Course Code
BIO120H1
Professor
Jean Jiang Nash

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LECTURE
Cell is compartment and has compartments within need to regulate composition of each
compartment
Overview of protein sorting
Protein sorting to nucleus, mitochondria, peroxisomes
Protein Sorting
Movement of proteins within cell to different compartments
oout of cell
ointo cell
oExocytosis and endocytosis
In the diagram names of compartments you should know ER, peroxisomes, endosomes,
lysosomes, mitochondria, nucleus. Should know how proteins get into each compartment
what signal lets it in, and what
All proteins being life in the cytosol translation of all proteins start in the cytosol except
for the odd ones in the mitochondria and chloroplasts
Protein sorting and protein synthesis
1. Post-translational process proteins fully synthesized in cytosol before sorting
oAfter synthesis is done, then sorted
oRough ER ribosome docked on ER translating protein into lumen of ER
oAll proteins have ER signal sequence see 2
o
2. Associated with ER during protein synthesis
oNot permanently attached to ER
Gated transport through nuclear pore
Transmembrane transport embedded in membrane each protein called translocators
Vesicular transport both directions okay vesicles can move
***Only one way into green area through ER only way into this vesicle transport system
Once in ER protein can make way to Golgi
Gated transport
Selective transport of macromolecules
Free diffusion of small molecules (<5000 daltons)
oAnything bigger needs to be carried through, and requiring a nuclear
localization signal
Nuclear import from cytosol to nucleus
oOn cytosolic surface protein fibres and the fibre baskets made up of
nucleoporins rich in FG repeats (phenylalanine glycine?) makes coating
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Nuclear export from nucleus to cytosol
Nuclear import
oBrings cargo protein into nucleus with the NLS (the little arrow)
recognized by import receptor (S-shaped)
oTwo kinds of receptors
oDiff kinds of NLS exist
oBinds to NLS (rich in Lys and Arg)
oBinds to nucleoporins in NPC
Cargo
oBinds to NES
oBinds to nucleoporins in NPC
oConstantly moving in and out of the nucleus regardless of whether
bound to cargo or not
Direction of transport
oNuclear RAN-GTP causes cargo release for nuclear import********
oVERSUS Ran-GTP binds to cargo and S inside the cell
Ran GTPase enzyme binds to GTP and cleaves the third phosphate group, leaving GDP
Not efficient at hydrolyzing ran-GTP needs helper protein
GDP-bound
GTP-bound
Stimulates GTP hydrolysis by Ran
Ran-GAP only found in cytosol
Ran-GEF exchange factor
Promotes exchange of GDP for GTP by Ran
- GEF only found in nucleus so exchange occurs in nucleus exclusively
GAP in cytosol
GEF in nucleus
The Ran-GTP gradient
Critical for direction of transport
Ran GTPase- movement between the nucleus and cytosol
Notice NTF2 import factor for Ran
Brings Ran back into nucleus
Import receptor blue because has no cargo bound and interacting with Ran-
GTP (green and red thing)
oOn way back out of nucleus
Export receptor purple also bound to Ran-GTP and cargo
Nuclear import of cargo
Binds cargo in cytosol
oDo not need to worry about Ran at this stage
oHigh affinity for nucleoporin
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