The solute transport profile of two Aza-guanine transporters from the Honey bee pathogen Paenibacillus larvae

Abstract

Two nucleobase transporters encoded in the genome of the Honey bee bacterial pathogen Paenibacillus larvae belong to the azaguanine-like transporters and are referred to as PlAzg1 and PlAzg2. PlAzg1 & 2 display significant amino acid sequence similarity, share predicted secondary structures and functional sequence motifs with two Escherichia coli nucleobase cation symporter 2 (NCS2) members; adenine permease (EcAdeP) and guanine-hypoxanthine permease EcGhxP. However, similarity does not define function. Heterologous complementation and functional analysis using nucleobase transporter deficient Saccharomyces cerevisiae strains revealed that PlAzg1 transports, adenine, hypoxanthine, xanthine and uracil, while PlAzg2 transports adenine, guanine, hypoxanthine, xanthine, cytosine and uracil. Both PlAzg1 & 2 display high affinity for adenine with K_m of 2.95 ± 0.22 μM and 1.92 ± 0.22 μM, respectively. These broad nucleobase transport profiles are in stark contrast to the narrow transport range observed for EcAdeP (adenine) and EcGhxP (guanine and hypoxanthine). PlAzg1 & 2 are similar to eukaryotic Azg-like transporters in that they share a broad solute transport profile, particularly the fungal Aspergillus nidulans AzgA (that transports adenine, guanine and hypoxanthine) and plant AzgA transporters from Arabidopsis thaliana and Zea mays (that collectively move adenine, guanine, hypoxanthine, xanthine, cytosine and uracil).