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J. Bacteriol., Jan 1995, 297-306, Vol 177, No. 2
JP Sarsero and AJ Pittard
The mtr gene of Escherichia coli K-12 encodes an inner membrane protein
which is responsible for the active transport of trypotophan into the cell.
It has been proposed that the Mtr permease has a novel structure consisting
of 11 hydrophobic transmembrane spans, with a cytoplasmically disposed
amino terminus and a carboxyl terminus located in the periplasmic space
(J.P. Sarsero, P. J. Wookey, P. Gollnick, C. Yanofsky, and A.J. Pittard, J.
Bacteriol. 173:3231-3234, 1991). The validity of this model was examined by
the construction of fusion proteins between the Mtr permease and alkaline
phosphatase or beta- galactosidase. In addition to the conventional
methods, in which the reporter enzyme replaces a carboxyl-terminal portion
of the membrane protein, the recently developed alkaline phosphatase
sandwich fusion technique was utilized, in which alkaline phosphatase is
inserted into an otherwise intact membrane protein. A cluster of alkaline
phosphatase fusions to the carboxyl-terminal end of the Mtr permease
exhibited high levels of alkaline phosphatase activity, giving support to
the proposition of a periplasmically located carboxyl terminus. The
majority of fusion proteins produced enzymatic activities which were in
agreement with the positions of the fusion sites on the proposed
topological model of the permease. The synthesis of a small cluster of
hybrid proteins, whose enzymatic activity did not agree with the location
of their fusion sites within putative transmembrane span VIII or the
preceding periplasmic loop, was not detected by immunological techniques
and did not necessitate modification of the proposed model in this region.
Slight alterations may need to be made in the positioning of the
carboxyl-terminal end of transmembrane span X.
Copyright © 1995, American Society for Microbiology
Membrane topology analysis of Escherichia coli K-12 Mtr permease by alkaline phosphatase and beta-galactosidase fusions
Department of Microbiology, University of Melbourne, Parkville, Victoria, Australia.
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