Derepression of LamB protein facilitates outer membrane permeation of carbohydrates into Escherichia coli under conditions of nutrient stress

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Abstract
The level of LamB protein in the outer membrane of Escherichia coli was derepressed in the absence of a known inducer (maltodextrins) under carbohydrate-limiting conditions in chemostats. LamB protein contributed to the ability of the bacteria to remove sugar from glucose-limited chemostats, and well-characterized lamB mutants with reduced stability constants for glucose were less growth competitive under glucose limitation than those with wild-type affinity. In turn, wild-type bacteria were less growth competitive than lamB mutants with enhanced sugar affinity. In contrast to an earlier report, we found that LamB- bacteria were less able to compete in carbohydrate-limited chemostats (with glucose, lactose, arabinose, or glycerol as the carbon and energy sources) when mixed with LamB+ bacteria. The transport Km for [14C]glucose was affected by the presence or affinity of LamB, but only in chemostat-grown bacteria, with their elevated LamB levels. The pattern of expression of LamB and the advantage it confers for growth on low concentrations of carbohydrates are consistent with a wider role in sugar permeation than simply maltosaccharide transport, and hence the well-known maltoporin activity of LamB is but one facet of its role as the general glycoporin of E. coli. A corollary of these findings is that OmpF/OmpC porins, present at high levels in carbon-limited bacteria, do not provide sufficient permeability to sugars or even glycerol to support high growth rates at low concentrations. Hence, the sugar-binding site of LamB protein is an important contributor to the permeability of the outer membrane to carbohydrates in habitats with low extracellular nutrient concentrations.