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J. Bacteriol., Nov 1995, 6118-6125, Vol 177, No. 21
GS Moeck, MJ Ratcliffe and JW Coulton
Ferrichrome-iron transport in Escherichia coli is initiated by the outer
membrane receptor FhuA. Thirty-five anti-FhuA monoclonal antibodies (MAbs)
were isolated to examine the surface accessibility of FhuA sequences and
their contribution to ligand binding. The determinants of 32 of the MAbs
were mapped to eight distinct regions in the primary sequence of FhuA by
immunoblotting against (i) five internal deletion FhuA proteins and (ii)
four FhuA peptides generated by cyanogen bromide cleavage. Two groups of
MAbs bound to FhuA in outer membrane vesicles but not to intact cells,
indicating that their determinants, located between residues 1 and 20 and
21 and 59, are exposed to the periplasm. One of the 28 strongly
immunoblot-reactive MAbs bound to FhuA on intact cells in flow cytometry,
indicating that its determinant, located between amino acids 321 and 381,
is cell surface exposed. This MAb and four others which in flow cytometry
bound to cells expressing FhuA were tested for the ability to block ligand
binding. While no MAb inhibited growth promotion by ferrichrome or cell
killing by microcin 25, some prevented killing by colicin M and were
partially able to inhibit the inactivation of T5 phage. These data provide
evidence for spatially distinct ligand binding sites on FhuA. The lack of
surface reactivity of most of the immunoblot-reactive MAbs suggests that
the majority of FhuA sequences which lie external to the outer membrane may
adopt a tightly ordered organization with little accessible linear
sequence.
Copyright © 1995, American Society for Microbiology
Topological analysis of the Escherichia coli ferrichrome-iron receptor by using monoclonal antibodies
Department of Microbiology and Immunology, McGill University, Montreal, Canada.
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