Topological analyses of the l‐fucose‐H+ symport protein, FucP, from Escherichia coli

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Abstract
The transport of l‐fucose into Escherichia coli is mediated by the l‐fucose‐H+ symport protein (FucP). The fucP gene has been sequenced and encodes a hydrophobic protein that contains 438 amino acid residues, with a predicted Mr of 47773. The hydropathic profile of FucP indicates 10 to 12 hydrophobic regions that could span the membrane as α‐helices. A 12‐helix model with the N‐ and C‐termini located in the cytoplasm was derived from the hydropathic profile and from application of the ‘positive inside’ rule. This model was tested using β‐lactamase fusion technology. Analyses of 62 different FucP‐β‐lactamase fusions suggested that the FucP protein crosses the cytoplasmic membrane of E. coli 12 times, with the N‐ and C‐termini in the cytoplasm. From measurements of [14C]‐l‐fucose uptake, it was deduced that the last putative transmembrane region must be complete for transport activity to be retained and that the four C‐terminal residues were unnecessary for transport activity. Fourier transform analyses show that all the predicted helices contain a periodicity that enables hydrophobic/hydrophilic faces to be identified; these were particularly evident in putative helices 1, 3, 4, 5, 6, 10 and 11.

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