RbsC of Escherichia coli is the hydrophobic membrane component of ribose uptake system classified as the ATP-binding cassette transporter. To understand the structure and function of RbsC, its transmembrane topology was investigated by using 64 RbsC-PhoA fusions isolated either specifically or randomly. In order to confirm the cytoplasmic location of the short C-terminal region (5 amino acids), inside-out or right-side-out membrane vesicles were generated, and the C-terminal region was found to be digested by carboxypeptidase A only in inside-out vesicles. This result is consistent with the model, based on the results of alkaline phosphatase fusions, in which the protein traverses the membrane six times and the N and C termini are exposed to the cytoplasm
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