Figure 3.
Chaperone paradigms: Trigger factor and Hsp70 system. (A) Trigger factor (TF): The left panel shows the domain structure of TF, including the FRK ribosome-binding loop in the amino-domain (PDB 1W26) (Ferbitz et al. 2004), and the right panel shows a model of the TF reaction cycle (modified from Kaiser et al. [2006]). (1) Free TF is in rapid equilibrium between monomeric and dimeric states. (2) TF monomer binds to nontranslating ribosomes with a KD of ∼1 µM and a mean residence time of approximately 10 seconds. Ribosome binding causes a conformational expansion of TF and may activate TF for interaction with nascent chains. (3) Nascent chains that interact weakly with TF may begin to compact cotranslationally in the vicinity of TF. Release of TF from nascent chain coincides with TF dissociation from the ribosome and allows completion of folding to native state (N). (4) Structurally more complex proteins may strongly interact with TF. TF remains bound to nascent chain after dissociating from the ribosome and a new TF molecule can enter at the ribosome. Eventual chain dissociation from TF facilitates transfer to DnaK or folding. (5) Released TF enters the monomer-dimer pool. (B) Hsp70 system. Left panel: Structures of the ATPase domain (PDB 1DKG) and the peptide-binding domain (PDB 1DKZ) (Zhu et al. 1996) of Hsp70 shown representatively for Escherichia coli DnaK. The α-helical lid of the peptide binding domain is shown in yellow and the extended peptide substrate as a ball-and-stick model in pink. ATP indicates the position of the nucleotide binding site. The amino acid sequence of the peptide is indicated in single-letter code. The interaction of prokaryotic and eukaryotic cofactors with Hsp70 is shown schematically. Residue numbers refer to human Hsp70. Only the Hsp70 proteins of the eukaryotic cytosol have the COOH-terminal sequence EEVD that is involved in binding of tetratricopeptide repeat (TPR) cofactors (Scheufler et al. 2000). Right panel: Hsp70 reaction cycle. NEF, nucleotide exchange factor (GrpE in case of E. coli DnaK; Bag, HspBP1, and Hsp110 in case of eukaryotic cytosolic Hsp70). N, native protein. (A,B: Modified with permission from Hartl and Hayer-Hartl 2009.)