ABSTRACT Metformin is a first-line antidiabetic medication for type 2 diabetes and a widely dispersed emerging pollutant in aquatic systems. During medication, its strong positive charge (pKa = 12.4) under physiological conditions necessitates cellular uptake via non-specific cationic transporters, such as organic cation transporter 1 (OCT1) in humans. During bacterial biodegradation of metformin, it was also proposed that the transport of metformin into bacterial cells is a prerequisite. Despite progress in elucidating the biodegradation mechanism of metformin, the bacterial membrane transport systems involved remain largely unexplored. Here, we characterize a metformin transporter (MetT) from the metformin utilizer Aminobacter sp. strain NyZ550. Gene knockout and complementation experiments demonstrate that metT is essential for the growth of strain NyZ550 on metformin. Efficient bacterial degradation of metformin necessitates the presence of both the membrane transporter and the downstream catabolic enzyme. Phylogenetic analysis revealed that MetT is a member of the nucleobase cation symporter 1 (NCS1) family, but forms a separate clade distinct from previously characterized NCS1 members. It exhibits distinct polar localization within the cytoplasmic membrane, as evidenced by a MetT-green fluorescent protein fusion construct. Radiolabeled uptake assays using 14 C-metformin revealed a substrate transport affinity ( K m ) of 15.90 ± 1.75 µM, which starkly contrasts with the millimolar-range K m of human OCT1. Structural modeling and site-directed mutagenesis revealed a substrate-binding cavity consisting of aromatic residues, likely facilitating substrate recognition through cation–π interactions. Overall, this study characterizes a functional bacterial metformin transporter and a new member from the emerging NCS1 transporter family. IMPORTANCE The increasing global consumption of pharmaceuticals worldwide is contributing to the release of drugs and their catabolic byproducts into ecological systems through various routes, posing significant threats to environmental stability and public health. Environmental microbes can metabolize many of these molecules by evolving specific enzymes; however, efficient degradation necessitates their transport across the cell membrane, a process that remains poorly understood. In this study, we identified metformin transporter (MetT), a bacterial metformin transporter, from the metformin-utilizing strain NyZ550. MetT exhibits distinct specificity and transport affinity compared to human metformin transporters. This discovery offers new insights into the transport of metformin in prokaryotic cells and opens new avenues for exploring drug–microbe interactions and their implications for environmental biodegradation and gut biology.
Xu et al. (Fri,) studied this question.