Chaperones: Helping Protein Reach Their

Proper Folded State

Qi Bin

2005.12.11

P79

Curious finding

In fruit fly larval

When the temperature goes up from normal 25ºC to 32ºC

activate

A number of new sites on the giant chromosome

conclusion

In every type of organism, from bacteria to plants and mammals

Temperature goes up induce The expression of new genes

This response is called the heat shock response and the protein which was produced during heat shock is called Heat Shock Protein(Hsp).

PS: Hsps were found not only in heat shocked cells, but also at lower concentration in cells under normal condition

What is the function of these so-called Hsps?

•binding protein (Bip) ------- promote protein assemble

•Molecular chaperones •Hsp60

Binding protein (Bip) ------- promote protein assemble

The discovery of Bip:

A.In bacteriophage, a protein encoded by the bacterial chromosome participates in the assembly of virus particles, even though this host protein was not a component of the final virus particles. (e.g.: GroE talk about it latter )

Binding protein (Bip) ------- promote protein assemble

B. In plant, a similar assembly-promoting protein in chloroplast was found in pea plant

Rubisco is composed of 16 subunits: 8 large subunits and 8 small subunits. And the assembly-prom

oting proteins are bind to large subunit

C. In mammal, there are also some proteins which binds to the heavy chain that will assist the assembly of multisubunit proteins but is not found in final complex.

Binding protein (Bip) ------- promote protein assembly

D. Binding protein(Bip): the protein associate with newly synthesized heavy chain.and

it can promote protein assembly.

Molecular chaperone

Temperature sensitive protein:

1. Sensitive to temperature2. Small rise in temperature causing this delicate molecule to unfold3. Under heat shock, soluble protein became denatured and aggregated

Molecular chaperone:

Temperature goes up Molecular chaperone synthesize

Bind to aggregated protein

Promote disaggregation

Function: assist the assembly of proteins by preventing undesirable interaction and assist polypeptide chain folding.

Classification of molecular chaperone:

Example

Molecular chaperone

Hsp 90

Hsp 70: Bip

Hsp 60: GroE

sHsp

Hsp 60 ------ best understood chaperone

Use GroE as an example: GroE is composed of 2 separate proteins GroEl and Gro ES

A model of GroEL:• Data from electron microscope• 14 polypeptide subunits arranged in 2 stacked rings resembling a double doughnut

Reconstruction of GroEl based on High-resolution electron microscope

A BGroEL GroES bind toGroEl

Centralcavity

Hsp 60 ------ best understood chaperone

The binding of GroES to GroEL will lead to comformation

changes

The binding of GroES cap is accompanied by a 60º rotation of

apical domain 60º

Hsp 60 ------ best understood chaperone

Nonnative polypeptide enter and bind to the hydrophobic site of GroEl

Binding of GroES cap produce conformational changes and release of pol

ypeptide.

The GroES dissociate Native polypeptide is ejected.Misfold ones rebind to GroEl

chamber.

Hsp 60 ------ best understood chaperone

How it is possible for a chaperone to bind so many different polypeptide?

The binding site has structural flexibility that allows it to adjust its shape to fit the shape of the particular

polypeptide with which it has to interact.

Conclusion:

Molecular chaperones do not convey information for the folding process but instead prevent proteins from veering off their correct folding pathway and finding themselves in misfolded or aggregated

states.