Cell Biology

S. Rahgozar,PhD

University of Isfahan

Faculty of Science

4. Protein sorting and transport

4.3. Lysosomes and Vesicular transport

1392-93

Vesicular transport plays a central role in the traffic of molecules

Why is it important?

The selectivity of such transport is key to maintaining the functional

organization of the cell

Experimental approaches?

Isolation of yeast mutants defective in protein transport and sorting

Reconstitution of vesicular transport in cell-free systems

Biochemical analysis of synaptic vesicles

Tracing the path of specific GFP fusion proteins through the

secretory network

Proteomic analysis of specific secretory compartments

between membrane-enclosed compartments

from the cell surface

Formation and fusion of a

transport vesicles

Cargo selection,

Coat proteins,

Vesicle budding

The coats

assemble as the secretory

protein-containing vesicle

buds off the donor membrane

are removed from the vesicle

in the cytosol before it reaches

its target.

Three important aspects in cargo selection and transport

Small GTP-binding proteins (transferred from cytosol)

ADP-ribosylation factors

o ARFs 1-3

o Sar1

Rab proteins (for docking of transport vesicles)

Adaptor proteins

Coat proteins

o Clathrin

o COPI

o COPII

(for vesicular budding)

Regulation of monomeric GTPase

RGS

RGS : Regulators of G protein

signaling protein. Also

called GAP (GTPase-

activating proteins)

GEF: Guanine nucleotide

exchange factor

Transport by coated vesicles

COP: coat protein

Golgi apparatus

Golgi

stack

Golgi in a fibroblast

Initiation of a clathrin-coated

vesicle by ARF1

GGA: adaptor protein

AP1: adaptor protein

Incorporation of lysosomal proteins into

clathrin-coated vesicles

Delivery of Rab to a membrane

GDI: GDP-dissociation inhibitor

Rab is carried in the cytosol bound to

a GDI, keeping it in the Rab/GDP

state.

At a membrane, a non-specific GDI-

displacement factor can remove the

Rab/GDP from the GDI and insert it

into the membrane.

If a specific Rab-GEF is present, the

GDP on Rab will be exchanged for

GTP and the active Rab/GTP can

interact with effector proteins. If not,

the Rab/GDP will be removed by a

GDI and carried to another

membrane.

Vesicle fusion

Vesicle fusion is initiated by Rab/GTP, and

continues with SNARE-SNARE interaction.

Interaction provides energy to bring the vesicle

and target membranes into close proximity.

This proximity destabilizes the lipid bilayers,

and the vesicle and target membrane fuse.

Fusion recruit NSF and SNAP.

The NSF/SNAP complex disassemble the

SNARE complex using energy from the

hydrolysis of ATP.

GDI: GDP-dissociation inhibitor

Lysosomes

Electron micrograph of lysosomes and mitochondria in a mammalian cell

Lysosomes are membrane-enclosed organelles that contain an array of enzymes

capable of breaking down all types of biological polymers, proteins, nucleic acids,

carbohydrates, and lipids.

Yeasts and plant cells lack

lysosomes.

Vacuole assume the functions of

lysosomes in these cells as well

as performing other tasks.

In contrast to lysosomal

targeting, proteins are directed to

vacuoles by short peptide

sequences instead of by

carbohydrate markers.

Plant cell vacuole functions as lysosome in

addition to

Storing nutrients

Maintaining osmotic balance

Most lysosomal enzymes are acid

hydrolases, which are active at the acidic

pH.

The acidic internal pH results from the

action of a proton pump in the lysosomal

membrane, which imports protons from the

cytosol coupled to ATP hydrolysis.

Lysosomal storage diseases

Gaucher disease

Defficient in enzyme required

for the breakdown of glycolipids

I-cell disease

Defficient in enzyme that tags

lysosomal enzymes with mannose-6-

phosphate in Golgi apparatus

Exocytosis

Exocysts are complexes of 8 different proteins

formed during exocytosis.

Tethering and docking at exocysts results in

normal SNARE-mediated vesicle fusion.

Endocytosis

The acid hydrolases dissociate

from the mannose-6-phosphate

receptors when the transport

vesicles fuse with late

endosomes, and the mannose-6-

phosphate receptors are recycled

back to the trans Golgi network.

Phagocytosis and Autophagy

Autophagy is activated during

Nutrient starvation of the cell

It allows cells to degrade nonessential proteins and organelles so that

their components can be reutilized.

Developmental processes of multicellular organisms

For example: insect metamorphosis, which involves extensive tissue

remodeling and degradation of cellular components.

Programmed cell death

Defects in autophagy have been linked to several human diseases, in this

regards, including neurodegenerative diseases and cancer.