molecular chaperones that may choose between folding and degradation
They bind nonnative proteins and orchestrate the folding process in conjunction with regulatory cofactors that modulate the affinity of the chaperone for its substrate. However, not every attempt to fold a protein is successful and Swarovski Outlet chaperones can direct misfolded proteins to the cellular degradation machinery for destruction. Protein quality control t Swarovski Outlet hus appears to involve close cooperation between molecular chaperones and energydependent proteases. Molecular mechanisms underlying this interplay have been largely enigmatic so far. The native structure is attained following translation or translocation and is under constant threat of unfolding as a consequence of chemical equilibrium and cellular stress. Systems that actively maintain and control protein structure are thus a prerequisite for cell survival, and involve both molecular chaperones and energydependent proteases (Wickner et al., 1999). Molecular chaperones bind nonnative proteins, inhibit protein aggregation and promote folding to the native state ( 1996; Bukau and 1998). Energydependent proteases, on the other hand, eliminate proteins that fail to attain their native conformation (Wickner et al., 1999). Thus, chaperones and proteases appear to form a cellular surveillance system that monitors protein quality. Recent findings provide insight into molecular mechanisms that underlie the interplay of chaperones and proteases.
The Hsp70 and Hsp90 chaperones
Major chaperones in the mammalian cytosol and nucleus are the 70 and 90 kDa heat shock proteins (Hsp70 and Hsp90). Hsp70 participates in the folding of newly synthesized proteins, the protection of proteins during cellular stress and intracellular protein trafficking ( 1996; Frydman and 1997; Bukau and 1998). Hsp90 function appears to be more restricted but, again, a role in stress protection has been demonstrated ( 1999; 1999). Both classes of chaperones seem to associate with nonnative protein substrates through recognition of hydrophobic patches ultimately buried in the native structure. In addition, they mediate the conformational regulation of a wide range of client proteins involved in signal transduction, cell proliferation and apoptosis (Frydman and 1997; 1999; Pandey et al, 2000; Beere and 2001). Interaction with nonnative protein substrates and client proteins is highly dynamic and coupled to cycles of ATP binding and ATP hydrolysis by the chaperones (Frydman and 1997; Prodromou et al, 1999; Young and 2000). Upon release, further protein folding or biogenesis can occur. This may involve rebinding to the same or another chaperone, or transfer to other cellular machines. Hsp70 and Hsp90 have been shown to cooperate with the degradation machinery (Schneider et al., 1996; Bercovich et al., 1997; Dul et al., 2001). Of particular interest is the pharmacological shifting of Hsp90 function from protein folding to protein degradation, induced by antitumor agents like geldanamycin (Schneider et al., 1996; Whitesell and 1996). However, the underlying molecular mechanisms remain largely elusive.
It is now widely recognized that Hsp70 and Hsp90 do not act on their own, but cooperate with several ancillary proteins, socalled chaperone cofactors or cochaperones (Frydman and 1997; 1999; 1999). In principle, chaperone cofactors have two options for modulating chaperone function. They can either regulate the ATPase cycle of the chaperone to influence its affinity for protein substrates, or recruit the chaperones to specific proteins, protein complexes and subcellular compartments. Many cofactors combine both activities and, consequently, exhibit a Swarovski Outlet modular structure comprising chaperonebinding/chaperoneregulating motifs plus other functional domains. A chaperonebinding motif found in several Hsp70 and Hsp90 cofactors is characterized by a tandem arrangement of three degenerate 34 amino acid repeats (tetratricopeptide repeats, TPRs; Frydman and 1997). The Hsp70/Hsp90organizing protein Hop possesses multiple TPR domains, enabling it to simultaneously bind Hsp70 and Hsp90, and to promote chaperone cooperation during the regulation of signal transduction pathways (Frydman and 1997; 1999; 1999). In each domain, three tandem TPRs align with an adjacent helix to form a groove that accommodates conserved peptide motifs present at the Cterminus of Hsp70 and Hsp90 (Scheufler et al., 2000). Although the presence of TPRs is not restricted to chaperone cofactors, they appear to form a stably folded adaptor well suited to mediate chaperone/cofactor contacts, even under conditions of cellular stress.
CHIP links chaperones to the degradation machinery
CHIP was initially identified in a screen for human TPRcontaining proteins (Ballinger et al., 1999). At its Nterminus three tandem TPRs are located, which, together with an adjacent, highly charged helix, form a chaperone adaptor (Figure 1). CHIP utilizes this single adaptor to contact either Hsp70 or Hsp90 (Ballinger et al., 1999; Connell et al., 2001). Association with Hsp70 blocks the ATPase cycle of the chaperone and inhibits its ability to refold nonnative proteins in vitro (Ballinger et al., 1999). Similarly, binding of CHIP to Hsp90 prevents the cooperation of the chaperone with other cofactors required for productive chaperone function (Connell et al., 2001). Cofactors that appear to link molecular chaperones to the ubiquitin/proteasome system. CHIP possesses an Nterminal chaperone binding motif formed by three TPRs and an adjacent highly charged region. A Ubox Swarovski Outlet required for ubiquitin ligase activity is present at the Cterminus. The BAG1 isoforms share a ubiquitinlike domain involved in proteasome binding and a BAG domain that mediates interaction with Hsp70. Like the BAG1 proteins, Scythe/Bat3 possesses a ubiquitinlike domain that may be used for proteasome association and a BAG domain used for binding and regulation of Hsp70. Chap1/PLIC2 combines a ubiquitinlike domain and a Uba domain, the latter of which is found in several proteins involved in ubiquitin conjugation. In addition, regions structurally related to the chaperone cofactor Hop are present in Chap1/PLIC2.
Interestingly, the Cterminus of CHIP displays structural similarities to components of the ubiquitin/proteasome system, a major protein degradation pathway in eukaryotic cells (Figure 1). Proteins destined for degradation are labelled with a multiubiquitin chain and then targeted to a large heterooligomeric protease, the 26S proteasome ( 1997; Baumeister et al., 1998). Ubiquitylation is mediated by a complex cellular machinery comprising a ubiquitin activator (the E1 enzyme), a ubiquitin conjugating enzyme (E2), and a ubiquitin ligase (E3). E2 and E3 enzymes are recruited from large protein families and the broad repertoire of distinct E2/E3 pairs is likely to ensure specific recognition of diverse substrate proteins. Additional proteins, including the yeast ubiquitylation factor Ufd2, cooperate with the E2/E3 machinery (Koegl et al., 1999). Interestingly, Ufd2 and CHIP both possess a Ubox domain shown to participate in ubiquitin conjugation. The modular structure of CHIP may thus enable the cofactor to directly link molecular chaperones to the degradation machinery.