Coexistence of ammonium transporter and channel mechanisms in Amt-Mep-Rh Twin-His variants impairs the filamentation signalling capacity of fungal Mep2 transceptors

Williamson, Gordon and Brito, Ana Sofia and Bizior, Adriana and Tamburrino, Giulia and Dias Mirandela, Gaetan and Harris, Thomas and Hoskisson, Paul A. and Zachariae, Ulrich and Marini, Anna Maria and Boeckstaens, Melanie and Javelle, Arnaud (2022) Coexistence of ammonium transporter and channel mechanisms in Amt-Mep-Rh Twin-His variants impairs the filamentation signalling capacity of fungal Mep2 transceptors. mBio, 13 (2). e02913-21. ISSN 2150-7511 (https://doi.org/10.1128/mbio.02913-21)

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Abstract

Ammonium translocation through biological membranes, by the ubiquitous Amt-Mep-Rh family of transporters, plays a key role in all domains of life. Two highly conserved histidine residues protrude into the lumen of the pore of these transporters, forming the family's characteristic Twin-His motif. It has been hypothesized that the motif is essential to confer the selectivity of the transport mechanism. Here, using a combination of in vitro electrophysiology on Escherichia coli AmtB, in silico molecular dynamics simulations, and in vivo yeast functional complementation assays, we demonstrate that variations in the Twin- His motif trigger a mechanistic switch between a specific transporter, depending on ammonium deprotonation, to an unspecific ion channel activity. We therefore propose that there is no selective filter that governs specificity in Amt-Mep-Rh transporters, but the inherent mechanism of translocation, dependent on the fragmentation of the substrate, ensures the high specificity of the translocation. We show that coexistence of both mechanisms in single Twin-His variants of yeast Mep2 transceptors disrupts the signaling function and so impairs fungal filamentation. These data support a signaling process driven by the transport mechanism of the fungal Mep2 transceptors.

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