Membrane Topology of the Mannose Transporter of Escherichia coli K12

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Abstract
The mannose transporter of the bacterial phosphotransferase system mediates carbohydrate transport across the cytoplasmic membrane concomitant with carbohydrate phosphorylation. It also functions as a receptor for bacterial chemotaxis [Adler, J. & Epstein, W. (1974) Proc. Natl Acad. Sci. USA 71, 2895–2899] and is required for infection of the cell by bacteriophage λ where it most likely functions as a pore for penetration of phage DNA [Elliott, J. & Arber, W. (1978) Mol. & Gen. Genet. 161, 1–8]. The transporter consists of two transmembrane subunits (27‐kDa IICMan and 31‐kDa IIDMan) and a hydrophilic subunit (35‐kDa IIABMan). Protein fusions of IICMan and IIDMan with β‐galactosidase (LacZ) and with alkaline phosphatase (PhoA) were analyzed to determine the membrane topology of the two proteins. Protein fusions were obtained by progressively deleting the manY and manZ genes from their 3′ ends and ligating them to lacZ and ṕhoA that lack promotor and leader sequences. Based on the analysis of 30 IICMan‐PhoA, 10 IICMan‐LacZ, 12 IIDMan‐PhoA, and 30 IIDMan‐LacZ fusions, it is predicted that IICMan has six membrane‐spanning segments with the N‐ and C‐termini on the cytoplasmic face of the membrane. IIDMan is anchored in the membrane by a single membrane‐spanning segment at the end of the C‐terminus, while most of the protein (250 residues) protrudes into the cytoplasm.

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