Glucose uptake protein GlcU is part of the glucose/ribose porter (GRP) family (TC 2.A.7.5), which also includes ribose transport proteins, and several putative and hypothetical membrane proteins probably involved in sugar transport across bacterial membranes. GRP family proteins have a distinctive topology: 10 putative transmembrane (TM) alpha-helical spanning domains per polypeptide chain, which apparently arose by intragenic duplication of an element encoding a primordial five-TM polypeptide. In Lactococcus lactis, GlcU was identified as the sole non-PTS (phosphoenolpyruvate:phosphotransferase systems) permease involved in the transport of glucose, which is driven by the proton-motive force. A gene from Bacillus subtilis, ycxE, that is homologous to glcU, could substitute for glcU in Escherichia coli glucose growth experiments and restored glucose repression in Staphylococcus xylosus glcU mutants. The GRP family belongs to the Drug/Metabolite Transporter (DMT) superfamily (TC 2.A.7).
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