The twin-arginine translocation (Tat) pathway serves the role of transporting
folded proteins across energy-transducing membranes . Homologs of the genes
that encode the transport apparatus occur in archaea, bacteria, chloroplasts,
and plant mitochondria . In bacteria, the Tat pathway catalyzes the export
of proteins from the cytoplasm across the inner/cytoplasmic membrane. In
chloroplasts, the Tat components are found in the thylakoid membrane and
direct the import of proteins from the stroma. The Tat pathway acts separately
from the general secretory (Sec) pathway, which transports proteins in an
unfolded state .
It is generally accepted that the primary role of the Tat system is to
translocate fully folded proteins across membranes. An example of proteins
that need to be exported in their 3D conformation are redox proteins that have
acquired complex multiatom cofactors in the bacterial cytoplasm (or the
chloroplast stroma or mitochondrial matrix). They include hydrogenases,
formate dehydrogenases, nitrate reductases, trimethylamine N-oxide (TMAO)
reductases and dimethyl sulfoxide (DMSO) reductases [4,5]. The Tat system can
also export whole heteroligomeric complexes in which some proteins have no Tat
signal. This is the case of the DMSO reductase or formate dehydrogenase
complexes. But there are also other cases where the physiological rationale
for targeting a protein to the Tat signal is less obvious. Indeed, there are
examples of homologous proteins that are in some cases targeted to the Tat
pathway and in other cases to the Sec apparatus. Some examples are: copper
nitrite reductases, flavin domains of flavocytochrome c and
N-acetylmuramoyl-L-alanine amidases .
In halophilic archaea such as Halobacterium almost all secreted proteins
appear to be Tat targeted. It has been proposed to be a response to the
difficulties these organisms would otherwise face in successfully folding
proteins extracellularly at high ionic strength .
The Tat signal peptide consists of three motifs: the positively charged
N-terminal motif, the hydrophobic region and the C-terminal region that
generally ends with a consensus short motif (A-x-A) specifying cleavage by
signal peptidase. Sequence analysis revealed that signal peptides capable of
targeting the Tat protein contain the consensus sequence [ST]-R-R-x-F-L-K. The
nearly invariant twin-arginine gave rise to the pathway's name. In addition
the h-region of Tat signal peptides is typically less hydrophobic than that of
Sec-specific signal peptides [4,5].
The profile we developed recognizes the whole prokaryotic and archeal Tat
signal from the methionine to the A-x-A short motif.
May 2007 / First entry.
PROSITE method (with tools and information) covered by this documentation:
Wickner W., Schekman R.
Protein translocation across biological membranes.
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