Signal peptide–chaperone interactions on the twin-arginine protein transport pathway

Abstract

The twin-arginine transport (Tat) system is a protein-targeting pathway of prokaryotes and chloroplasts. Most Escherichia coli Tat substrates are complex metalloenzymes that must be correctly folded and assembled before transport, and a preexport chaperone-mediated ''proofreading'' process is therefore in operation. The paradigm proofreading chaperone is TorD, which coordinates maturation and export of the key respiratory enzyme trimethylamine N-oxide reductase (TorA). It is demonstrated here that purified TorD binds tightly and with exquisite specificity to the TorA twin-arginine signal peptide in vitro. It is also reported that the TorD family constitutes a hitherto unexpected class of nucleotide-binding proteins. The affinity of TorD for GTP is enhanced by initial signal peptide binding, and it is proposed that GTP governs signal peptide binding-and-release cycles during Tat proofreading.ligand binding ͉ molecular chaperone

Structural diversity in twin-arginine signal peptide-binding proteins

“…For instance, the E. coli trimethylamine N-oxide reductase TorA is subject to a chaperone-mediated quality control process known as ''Tat proofreading,'' which prevents premature targeting of TorA until all biosynthetic processes are complete (11). The TorD chaperone recognizes the TorA signal peptide and binds directly to it, thus shielding it from the Tat transporter (11,12). E. coli TorD is a member of a family of peptide-binding proteins that share a common fold comprised almost entirely of ␣-helices and completely devoid of ␤-strand (13,14).…”

Assembly pathway of a bacterial complex iron sulfur molybdoenzyme

“…GTP binding and hydrolysis are hypothesized to govern the activity of CISM maturation during the final stages of protein folding (33). It has been suggested that GTP binding by REMP proteins themselves may play a role in the REMP/substrate maturation process (33).…”

“…It has been suggested that GTP binding by REMP proteins themselves may play a role in the REMP/substrate maturation process (33). Accordingly, the domain-swapper dimer of TorD, showed an increase in GTP affinity following TorA ligand binding (27,117,118), and it was suggested to in fact be binding to the mature molybdopterin-guanine dinucleotide (MGD) form of Moco as a step in the cofactor insertion event (49). Recent investigation has illustrated strong cooperativity in the DmsD binding to both GTP and the RR-leader, suggesting that GTP binding at one site on DmsD alters affinity at additional binding sites (other protomers of a multimeric state of DmsD).…”

“…Rather, they 'escort' their CISM substrates though the entire maturation process as implied by their interactome (30) (Figure 1). Accordingly, many potential roles for REMPs have been proposed, including functioning as: (i) foldases to ensure correct secondary and tertiary structure (25,31); (ii) unfoldases to correct folding mistakes (25); (iii) avoidance chaperones to prevent incorrect membrane targeting during folding and assembly (32); (iv) cofactor-assembly chaperones to maintain apoenzymes in a cofactor-binding competent conformation (30,31); (v) cofactor-binding proteins, which bind the cofactor prior to its transfer to the apoenzyme (30,33); (vi) targeting proteins directing substrates to specific cellular locations (34,35); (vii) escort chaperones to promote transmembrane transport of enzyme complexes (30,31,34); (viii) proofreading chaperones to suppress transport until essential prior steps in the assembly process are complete (33,36,37); and (ix) protease protection chaperones to prevent degradation during assembly (38). Yet despite these proposed roles, a detailed path of actions for REMPs has yet to be defined.…”

Structural Analysis of a Monomeric Form of the Twin-Arginine Leader Peptide Binding Chaperone Escherichia coli DmsD

“…6 The REMPs and their corresponding Tat leader peptides appear to form tight associations. 7,8 A number of structures for the REMP proteins are available in the Protein Data Bank (PDB). These include Shewanella massilia TorD (SmTorD; PDB ID 1N1C), 9 Salmonella typhimurium DmsD (StDmsD; PDB ID 1S9U), 10 Archaeoglobus fulgidus AF0173 (PDB ID 2O9X), 11 A. fulgidus AF0160 (PDB ID 2IDG), and, finally, the NMR structure of the E. coli NapD protein (PDB ID 2JSX).…”