LAUSR

Catabolite repression is a mechanism that enables bacteria to control carbon utilization. As part of this global regulatory network, components of the phosphoenolpyruvate:carbohydrate phosphotransferase system inhibit the uptake of less favorable sugars when a preferred carbon source such as glucose is available. This process is termed inducer exclusion. In bacteria belonging to the phylum Firmicutes, HPr, phosphorylated at serine 46 (P‐Ser46‐HPr) is the key player but its mode of action is elusive. To address this question at the level of purified protein components, we have chosen a homolog of the Escherichia coli maltose/maltodextrin ATP‐binding cassette transporter from Lactobacillus casei (MalE1‐MalF1G1K12) as a model system. We show that the solute binding protein, MalE1, binds linear and cyclic maltodextrins but not maltose. Crystal structures of MalE1 complexed with these sugars provide a clue why maltose is not a substrate. P‐Ser46‐HPr inhibited MalE1/maltotetraose‐stimulated ATPase activity of the transporter incorporated in proteoliposomes. Furthermore, cross‐linking experiments revealed that P‐Ser46‐HPr contacts the nucleotide‐binding subunit, MalK1, in proximity to the Walker A motif. However, P‐Ser46‐HPr did not block binding of ATP to MalK1. Together, our findings provide first biochemical evidence that P‐Ser‐HPr arrests the transport cycle by preventing ATP hydrolysis at the MalK1 subunits of the transporter.