Maltose and maltodextrin transport in the thermoacidophilic gram-positive bacterium Alicyclobacillus acidocaldarius is mediated by a high-affinity transport system that includes a maltose binding protein tolerant to low pH

We have studied the uptake of maltose in the thermoacidophilic gram-positiv e bacterium Alicyclobacillus acidacaldarius, which grows best at 57 degrees C and pH 3.5. Under these conditions, accumulation of [C-14] maltose was ob served in cells grown with maltose but not in those grown with glucose. At lower temperatures or higher pH values, the transport rates substantially d ecreased. Uptake of radiolabeled maltose was inhibited by maltotetraose, ac arbose, and cyclodextrins but not by lactose, sucrose, or trehalose. The ki netic parameters (K-m of 0.91 +/- 0.06 muM and V-max ranging from 0.6 to 3. 7 nmol/min/mg of protein) are consistent with a binding protein-dependent A TP binding cassette (ABC) transporter. A corresponding binding protein (Mal E) that interacts with maltose with high affinity (K-d of 1.5 muM) was puri fied from the culture supernatant of maltose-grown cells. Immunoelectron mi croscopy revealed distribution of the protein throughout the cell wall. The malE gene was cloned and sequenced. Five additional open reading frames, e ncoding components of a maltose transport system (MalF and MalG), a putativ e transcriptional regulator (MalR), a cyclodextrinase (CdaA), and an alpha -glucosidase (GlcA), were identified downstream of malE, The malE gene lack ing the DNA sequence that encodes the signal sequence was expressed in Esch erichia coli. The purified wild-type and recombinant proteins bind maltose with high affinity over a wide pH range (2.5 to 7) and up to 80 degreesC. R ecombinant MalE crossreacted with an antiserum raised against the wild-type protein, thereby indicating that the latter is the product of the malE gen e. The MalE protein might be well suited as a model to study tolerance of p roteins to low pH.