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Figure 5.
FIGURE 5. Tentative model of colicin M uptake across the
outer membrane (OM) of E. coli. I, colicin M (crystal structure)
binds to the FhuA protein, whose crystal structure is shown. II,
colicin M partially unfolds while bound to FhuA. III, N-terminal
domain (red) with the TonB box (yellow) enters the pore in FhuA,
which is formed by interaction of FhuA with energized TonB
(green), leading to movement of the globular domain (cork) out
of FhuA. IV, TonB box of colicin M interacts with TonB, and
colicin M unfolds further and enters the periplasm through the
FhuA pore. VI, in the periplasm, colicin M is refolded with the
help of the FkpA chaperon and cleaves the pyrophosphate bond
between C55 isoprenoid and the murein precursor. IM, inner
membrane;  , membrane potential.
For the sake of clarity, the ExbB and ExbD proteins associated
with TonB and required for TonB activity are not shown.
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