DNA sequence analysis of the lamB gene from Klebsiella pneumoniae: implications for the topology and the pore functions in maltoporin

We have determined the sequence of the lamB gene from Klebsiella pneumoniae. It encodes the precursor to the LamB protein, a 429 amino acid polypeptide with maltoporin function. Comparison with the Escherichia coli LamB protein reveals a high degree of homology, with 325 residues strictly identical. The N-terminal third of the protein is the most conserved part of the molecule (1 change in the signal sequence, and 13 changes up to residue 146 of the mature protein). Differences between the two mature proteins are clustered mainly in six regions comprising residues 145–167, 173–187, 197–226, 237–300, 311–329, and 367–387 (K. pneumoniae LamB sequence). The most important changes were found in regions predicted by the two-dimensional model of LamB folding to form loops on the cell surface. In vivo maltose and maltodextrin transport properties of E. coli K 12 and K. pneumoniae strains were identical. However, none of the E. coli K12 LamB-specific phages was able to plaque onto K. pneumoniae. Native K. pneumoniae LamB protein forms highly stable trimers. The protein could be purified by affinity chromatography on starch-Sepharose as efficiently as the E. coli K12 LamB protein, indicating a conservation of the binding site for dextrins. However, none of the monoclonal antibodies directed against native E. coli K12 LamB protein recognized native purified K. pneumoniae LamB protein. These data indicate that most of the variability occurs within exposed regions of the protein and provide additional support for the proposed model of LamB folding. The fact that the N-terminal third of the protein is highly conserved is in agreement with the idea that it is part of, or constitutes, the pore domain located within the transmembranous channel and that it is not accessible from the cell surface.