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Journal of Bacteriology, January 2001, p. 604-610, Vol. 183, No. 2
Institut für Biotechnologie 1,
Forschungszentrum Jülich GmbH, D-52425
Jülich,1 and Lehrstuhl
für Genetik, Universität Bayreuth, D-95440
Bayreuth,2 Germany
Received 24 August 2000/Accepted 25 October 2000
The bacterial twin arginine translocation (Tat) pathway
translocates across the cytoplasmic membrane folded proteins
which, in most cases, contain a tightly bound cofactor. Specific
amino-terminal signal peptides that exhibit a conserved amino acid
consensus motif, S/T-R-R-X-F-L-K, direct these proteins to the Tat
translocon. The glucose-fructose oxidoreductase (GFOR) of
Zymomonas mobilis is a periplasmic enzyme with tightly
bound NADP as a cofactor. It is synthesized as a cytoplasmic precursor
with an amino-terminal signal peptide that shows all of the
characteristics of a typical twin arginine signal peptide. However,
GFOR is not exported to the periplasm when expressed in the
heterologous host Escherichia coli, and enzymatically
active pre-GFOR is found in the cytoplasm. A precise replacement of the
pre-GFOR signal peptide by an authentic E. coli Tat signal
peptide, which is derived from pre-trimethylamine N-oxide
(TMAO) reductase (TorA), allowed export of GFOR, together with its
bound cofactor, to the E. coli periplasm. This export was
inhibited by carbonyl cyanide m-chlorophenylhydrazone, but not by sodium azide, and was blocked in E. coli tatC and
tatAE mutant strains, showing that membrane translocation
of the TorA-GFOR fusion protein occurred via the Tat pathway and not
via the Sec pathway. Furthermore, tight cofactor binding (and therefore
correct folding) was found to be a prerequisite for proper
translocation of the fusion protein. These results strongly suggest
that Tat signal peptides are not universally recognized by different
Tat translocases, implying that the signal peptides of
Tat-dependent precursor proteins are optimally adapted only to
their cognate export apparatus. Such a situation is in marked contrast
to the situation that is known to exist for Sec-dependent protein translocation.
0021-9193/01/$04.00+0 DOI: 10.1128/JB.183.2.604-610.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Specificity of Signal Peptide Recognition in
Tat-Dependent Bacterial Protein Translocation
*
Corresponding author. Mailing address: Lehrstuhl
für Genetik, Universität Bayreuth,
Universitätsstrasse 30, D-95440 Bayreuth, Germany. Phone:
49-921-552724. Fax: 49-921-552710. E-mail:
thomas.wiegert{at}uni-bayreuth.de.
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