Characterization of Ammonium Transporter SmATM1;1 from Selaginella moellendorffii
As autotrophic organisms, plants are able to take up inorganic forms of nitrogen and carbon from the environment and synthesize organic compounds. The main forms of nitrogen that plants take up are nitrate (NO3-) and ammonium (NH4+). Plants take up ammonium using transporter proteins in the AMT/MEP/Rh family. In this study we analyzed the transport function of a previously uncharacterized AMT, which we termed SmAMT1. This transporter is from the lycophyte Selaginella moellendorffii. We chose to work on SmAMT1 because Selaginella is a basal land plant that had a common ancestor with flowering plants around 400 million years ago. We expressed SmAMT1 in Xenopus oocytes and studied its function using electrophysiology. The results showed that SmAMT1 is an electrogenic ammonium transporter with a fairly low affinity for ammonium (Km = 0.085 mM ± 0.022 mM at pH 5.6). The differences in SmAMT1 sequence and function may be useful in understanding the structure/function relationship for the AMT/MEP/Rh family.