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The
Volume 270,
Number 24,
Issue of June 16, pp. 14829-14834, 1995
©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
POINT MUTANTS IN HlyB COMPENSATE FOR A DELETION IN THE
PREDICTED AMPHIPHILIC HELIX REGION OF THE HlyA SIGNAL
-hemolysin transporter of Escherichia coli, a
member of the ATP-binding cassette transporter superfamily, is
responsible for secretion of the 107-kDa protein toxin HlyA across both
membranes of the Gram-negative envelope in a single step. Secretion of
HlyA is dependent on a signal sequence, which occupies the C-terminal
50-60 amino acids of HlyA. Previously, it was shown that point
mutants in the transmembrane domain of the transporter HlyB could
partially correct the transport defect caused by a deletion of the
C-terminal 29 amino acids of HlyA. These suppressor mutations
demonstrated a direct interaction between HlyA and HlyB. They also
displayed suppressor effects on a broad spectrum of HlyA signal
mutants. In the present study, we selected HlyB alleles that
complemented an internal deletion of 29 amino acids in HlyA containing
a predicted amphiphilic helix region immediately upstream from the
previous deletion. This set of HlyB mutants identifies further sites in
HlyB that modulate substrate specificity but display allele-specific
effects on a range of HlyA signal mutants. The inability to isolate
mutations with effects restricted to either half of the signal sequence
suggests that the signal is not recognized in a modular fashion by the
transporter but rather functions as an integrated whole. We also report
the isolation of the first substrate specificity mutation, which lies
within the ATP-binding domain of HlyB. This could support a model in
which the region of the ATP-binding cassette between the two Walker
consensus motifs involved in ATP binding interacts with either the
substrate or the transmembrane domains.
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