The twin arginine consensus motif of Tat signal peptides is involved in Sec-independent protein targeting in Escherichia coli.
In Escherichia coli a subset of periplasmic proteins is exported through the Tat pathway to which substrates are directed by an NH(2)-terminal signal peptide containing a consensus SRRXFLK "twin arginine" motif. The importance of the individual amino acids of the consensus motif for in vivo Tat transport has been assessed by site-directed mutagenesis of the signal peptide of the Tat substrate pre-SufI. Although the invariant arginine residues are crucial for efficient export, we find that slow transport of SufI is still possible if a single arginine is conservatively substituted by a lysine residue. Thus, in at least one signal peptide context there is no absolute dependence of Tat transport on the arginine pair. The consensus phenylalanine residue was found to be a critical determinant for efficient export but could be functionally substituted by leucine, another amino acid with a highly hydrophobic side chain. Unexpectedly, the consensus lysine residue was found to retard Tat transport. These observations and others suggest that the sequence conservation of the Tat consensus motif is a reflection of the functional importance of the consensus residues. Tat signal peptides characteristically have positively charged carboxyl-terminal regions. However, changing the sign of this charge does not affect export of SufI.
Stanley
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NR
Centre for Metalloprotein Spectroscopy and Biology, School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom.
Palmer
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T
Berks
B C
BC
D019943:Amino Acid Substitution; D001120:Arginine / Q000032:analysis* / Q000235:genetics; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics*; D002352:Carrier Proteins / Q000737:chemistry* / Q000235:genetics*; D016384:Consensus Sequence; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins; D008239:Lysine / Q000032:analysis / Q000235:genetics; D026901:Membrane Transport Proteins; D016297:Mutagenesis, Site-Directed; D012694:Serine / Q000032:analysis / Q000235:genetics
D001426:Bacterial Proteins; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; C408585:twin-arginine translocase complex, E coli; D012694:Serine; D001120:Arginine; D008239:Lysine