The twin arginine consensus motif of Tat signal peptides is involved in Sec-independent protein targeting in Escherichia coli

In Escherichia coil a subset of periplasmic proteins is exported through th e Tat pathway to which substrates are directed by an NH2-terminal signal pe ptide containing a consensus SRRXFLK "twin arginine" motif. The importance of the individual amino acids of the consensus motif for in vivo Tat transp ort has been assessed by site-directed mutagenesis of the signal peptide of the Tat substrate pre-SufI. Although the invariant arginine residues are c rucial for efficient export, we find that slow transport of SufI is still p ossible if a single arginine is conservatively substituted by a lysine resi due. Thus, in at least one signal peptide context there is no absolute depe ndence of Tat transport on the arginine pair. The consensus phenylalanine r esidue was found to be a critical determinant for efficient export but coul d be functionally substituted by leucine, another amino acid with a highly hydrophobic side chain. Unexpectedly, the consensus lysine residue was foun d to retard Tat transport. These observations and others suggest that the s equence conservation of the Tat consensus motif is a reflection of the func tional importance of the consensus residues. Tat signal peptides characteri stically have positively charged carboxyl-terminal regions. However, changi ng the sign of this charge does not affect export of SufI.