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J. Bacteriol., Feb 1995, 953-963, Vol 177, No. 4
WB Snyder and TJ Silhavy
The wild-type LamB-LacZ hybrid protein inhibits the export machinery upon
induction when assayed by biochemical and genetic techniques, a phenotype
referred to as hybrid protein jamming. This hybrid protein also renders
cells sensitive to growth in the presence of the inducer maltose,
presumably because of the jamming. We constructed a new version of this
fusion by adding alkaline phosphatase, encoded by phoA, to the C terminus
of the LamB-LacZ hybrid protein. This tripartite protein, LamB-LacZ-PhoA,
is as toxic to cells as the hybrid LamB-LacZ; however, it does not jam at
temperatures greater than 33 degrees C. Extreme C-terminal sequences of
LacZ function as a critical folding domain and are therefore responsible
for stabilizing the LacZ structure. To determine if this region of LacZ is
important for jamming, we recombined a late nonsense mutation (X90) onto
the hybrid construct. We found the toxicity of this new hybrid,
LamB-LacZX90, to be nearly identical to that of the full-length protein,
but it also does not jam the secretion machinery. This suggests that
jamming is caused by LacZ folding. We found no inhibition of secretion in
the tripartite and X90 fusion strains at 37 degrees C, suggesting that the
toxicity of the new fusions is novel. Under these conditions, the
tripartite and X90 fusion proteins form disulfide-bonded aggregates with
high molecular weights in the periplasm. Accordingly, we believe that LacZ
disrupts some essential function(s) in the periplasm.
Copyright © 1995, American Society for Microbiology
Beta-galactosidase is inactivated by intermolecular disulfide bonds and is toxic when secreted to the periplasm of Escherichia coli
Department of Molecular Biology, Princeton University, New Jersey 08544.
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