From the Cradle to the From the Cradle to the grave: molecular grave: molecular

chaperones that may chaperones that may choose between folding choose between folding

and degradationand degradation

By: Erica ZakhemBy: Erica Zakhem

ProteinsProteins

The function of a protein is determined by The function of a protein is determined by the 3D structure of the Amino Acid chainthe 3D structure of the Amino Acid chain

This 3D structure is produced after This 3D structure is produced after translationtranslation

Proteins are constantly under threat of Proteins are constantly under threat of unfolding due to chemical and cellular unfolding due to chemical and cellular stressstress

Hydrogen bonds may be disrupted by Hydrogen bonds may be disrupted by change in temperature and varying pH change in temperature and varying pH levelslevels

Protein structureProtein structure

Protein structure and quality is Protein structure and quality is mediated by two systemsmediated by two systems

Molecular chaperonesMolecular chaperones Energy-dependent proteasesEnergy-dependent proteases

Molecular chaperonesMolecular chaperones

They aid in the binding of non-They aid in the binding of non-native proteins, and inhibit protein native proteins, and inhibit protein aggregation aggregation

Work with regulatory co-Work with regulatory co-chaperones to facilitate the protein chaperones to facilitate the protein folding processfolding process

Energy-dependent Energy-dependent proteasesproteases

Eliminate proteins that do not fold Eliminate proteins that do not fold according to their native 3D according to their native 3D structurestructure

Misfolded proteins are sent to the Misfolded proteins are sent to the cell’s degradation machinery for cell’s degradation machinery for destruction destruction

Hsp70 and Hsp90Hsp70 and Hsp90

The major chaperones found in The major chaperones found in mammals are the Heat shock proteins mammals are the Heat shock proteins 70 and 9070 and 90

Hsp70 is involved in the folding of Hsp70 is involved in the folding of newly synthesized proteins, and in the newly synthesized proteins, and in the protection of proteins during cellular protection of proteins during cellular stress and protein traffickingstress and protein trafficking

Hsp90 functioning is restricted, but it Hsp90 functioning is restricted, but it plays a role in stress protectionplays a role in stress protection

Both these types of chaperones are Both these types of chaperones are associated with non-native protein associated with non-native protein substrates through the hydrophobic substrates through the hydrophobic portions in the native 3D structureportions in the native 3D structure

They control conformational regulation of They control conformational regulation of proteins involved in signal transduction, proteins involved in signal transduction, cell proliferation, and apoptosiscell proliferation, and apoptosis

They cooperate with the degradation They cooperate with the degradation machinery in the cellmachinery in the cell

Hsp70 and Hsp90 have been shown to Hsp70 and Hsp90 have been shown to require several co-chaperones in order to require several co-chaperones in order to act act

Co-chaperones Co-chaperones They can take either one of two options They can take either one of two options

when regulating chaperone functioningwhen regulating chaperone functioning They can regulate the ATPase cycle of They can regulate the ATPase cycle of

the chaperone, thus influencing its the chaperone, thus influencing its affinity for the protein substratesaffinity for the protein substrates

Or they can recruit the chaperones to Or they can recruit the chaperones to specific proteins or protein complexesspecific proteins or protein complexes

Many co-chaperones exhibit both Many co-chaperones exhibit both chaperone-binding and chaperone-chaperone-binding and chaperone-regulating motifsregulating motifs

Chaperone-binding Chaperone-binding motifsmotifs

Chaperone-binding motifs found in Chaperone-binding motifs found in Hsp70 and Hsp90 co-chaperones consist Hsp70 and Hsp90 co-chaperones consist of a tandem arrangement of three of a tandem arrangement of three degenerate 34 amino acid repeats-degenerate 34 amino acid repeats-tetratricopeptide repeats, TPRstetratricopeptide repeats, TPRs

The Hsp70/Hsp90 organizing protein The Hsp70/Hsp90 organizing protein Hop contains multiple TPRs which allow Hop contains multiple TPRs which allow it to bind to Hsp70 and Hsp90, and to it to bind to Hsp70 and Hsp90, and to promote the regulation of signal promote the regulation of signal transduction pathwaystransduction pathways

Ubiquitin/proteosome Ubiquitin/proteosome systemsystem

Major degradation pathway in Major degradation pathway in eukaryotic cellseukaryotic cells

Proteins that are to be degraded are Proteins that are to be degraded are labeled with a multi-ubiquitin chainlabeled with a multi-ubiquitin chain

The proteins are then taken to 26S The proteins are then taken to 26S proteosome which consists of proteases proteosome which consists of proteases which degrade the proteins.which degrade the proteins.

Ubiquitylation is mediated by an enzyme Ubiquitylation is mediated by an enzyme complex comprising of E1, E2 and E3complex comprising of E1, E2 and E3

Carboxyl terminus of Carboxyl terminus of Hsp70 Interacting Hsp70 Interacting

ProteinProtein-CHIP-CHIP CHIP is a co-chaperone that interacts CHIP is a co-chaperone that interacts

with Hsc70, and it accelerates with Hsc70, and it accelerates ubiquitin-dependent degradation of ubiquitin-dependent degradation of chaperone substrateschaperone substrates

The N-terminal contains three tandem The N-terminal contains three tandem TPRs adjacent to a highly charged TPRs adjacent to a highly charged αα--helix which form a chaperone adaptorhelix which form a chaperone adaptor

The C-terminal contains a U-box The C-terminal contains a U-box comprising of a ubiquitin activator comprising of a ubiquitin activator (E1), a ubiquitin conjugating enzyme (E1), a ubiquitin conjugating enzyme (E2) and a ubiquitin ligase enzyme (E3)(E2) and a ubiquitin ligase enzyme (E3)

The structure of CHIP enables the co-The structure of CHIP enables the co-chaperone to link molecular chaperones chaperone to link molecular chaperones to the degradation machineryto the degradation machinery

CHIP targets substrates to the CHIP targets substrates to the ubiquitin/proteasome system and ubiquitin/proteasome system and controls the balance between protein controls the balance between protein folding and protein degradationfolding and protein degradation

CHIP plays an integral role in the CHIP plays an integral role in the ubiqutin ligase complex ubiqutin ligase complex

Its ability to recognize non-native Its ability to recognize non-native proteins and specific proteins allows proteins and specific proteins allows it to select substrates for CHIP-it to select substrates for CHIP-mediated ubiquitylationmediated ubiquitylation

Association with Hsp70 and Hsp90 Association with Hsp70 and Hsp90 promotes the ubiquitylation process promotes the ubiquitylation process of various substratesof various substrates

Chaperone machinesChaperone machines

Upon association with CHIP chaperones Upon association with CHIP chaperones Hsp70 and Hsp90 are turned into degradation Hsp70 and Hsp90 are turned into degradation factorsfactors

Regulation of protein quality occurs when Regulation of protein quality occurs when CHIP occupies co-chaperone sites on Hsp70 CHIP occupies co-chaperone sites on Hsp70 and Hsp90 it competes with other co-and Hsp90 it competes with other co-chaperoneschaperones

Hop is a co-chaperone that is a CHIP Hop is a co-chaperone that is a CHIP antagonistantagonist

Like CHIP, Hop associates with Hsp70 via the Like CHIP, Hop associates with Hsp70 via the TPR adapter TPR adapter

As opposed to As opposed to CHIP, Hop assists CHIP, Hop assists in chaperone-in chaperone-mediated protein mediated protein foldingfolding

Competing co-Competing co-chaperones chaperones determine whether determine whether a chaperone a chaperone machine is involved machine is involved in protein folding or in protein folding or protein degradationprotein degradation

BAG-1BAG-1

The Hsp70 co-chaperone BAG-1 acts as The Hsp70 co-chaperone BAG-1 acts as a link between molecular chaperones a link between molecular chaperones and the ubiquitin/proteasome systemand the ubiquitin/proteasome system

This co-chaperone uses its ubiquitin This co-chaperone uses its ubiquitin domain for binding to the proteasome, domain for binding to the proteasome, which promotes association between which promotes association between Hsp70 and the proteolytic complexHsp70 and the proteolytic complex

Increased BAG-1 levels is insufficient to Increased BAG-1 levels is insufficient to promote degradationpromote degradation

A co-chaperone complex that includes A co-chaperone complex that includes both CHIP and BAG-1 has been shown to both CHIP and BAG-1 has been shown to regulate proteasomal sorting of regulate proteasomal sorting of chaperone substrateschaperone substrates

CHIP binds to the C-terminal while BAG-1 CHIP binds to the C-terminal while BAG-1 binds to the N-terminal of the chaperonebinds to the N-terminal of the chaperone

BAG-1 competes with Hip, which is a BAG-1 competes with Hip, which is a folding-stimulating co0chaperone in the folding-stimulating co0chaperone in the binding to the ATPase of Hsp70binding to the ATPase of Hsp70

Again, this illustrates that competing Again, this illustrates that competing factors determine the function of factors determine the function of molecular chaperonesmolecular chaperones

Hip and BAG-1 compete for Hip and BAG-1 compete for binding to the ATPase, while CHIP binding to the ATPase, while CHIP and Hop compete for association and Hop compete for association

with the C-terminalwith the C-terminal

ConclusionsConclusions

Hsp70 and Hsp90 interact with Hsp70 and Hsp90 interact with many co-chaperones that determine many co-chaperones that determine their fatetheir fate

Co-chaperones have been used Co-chaperones have been used elucidate the roles of chaperones in elucidate the roles of chaperones in protein folding and degradationprotein folding and degradation

Our knowledge of chaperones and Our knowledge of chaperones and co-chaperone mechanisms are still co-chaperone mechanisms are still limitedlimited