Coexistence of Ammonium Transporter and Channel Mechanisms in Amt-Mep-Rh Twin-His Variants Impairs the Filamentation Signaling Capacity of Fungal Mep2 Transceptors

TY - JOUR

T1 - Coexistence of Ammonium Transporter and Channel Mechanisms in Amt-Mep-Rh Twin-His Variants Impairs the Filamentation Signaling Capacity of Fungal Mep2 Transceptors

AU - Williamson, Gordon

AU - Brito, Ana Sofia

AU - Bizior, Adriana

AU - Tamburrino, Giulia

AU - Dias Mirandela, Gaëtan

AU - Harris, Thomas

AU - Hoskisson, Paul A.

AU - Zachariae, Ulrich

AU - Marini, Anna Maria

AU - Boeckstaens, Mélanie

AU - Javelle, Arnaud

N1 - A.B., G.W., and G.D.M. are supported by Ph.D. studentships from the University of Strathclyde. A.J. is supported by a Chancellor’s Fellowship from the University of Strathclyde and Tenovus Scotland (S17-07). G.T. and U.Z. are supported by the Scottish Universities’ Physics Alliance (SUPA). P.A.H. is supported by the Natural Environment Research Council (NE/M001415/1). A.S.B. is a research fellow of the Fund for Scientific Research (F.R.S.-FNRS), A.M.M. is a senior research associate of the F.R.S.-FNRS and a WELBIO investigator, M.B. is a scientific research worker supported by WELBIO. A.M.M. received support for this work from F.R.S.-FNRS (CDR J017617F, PDR T011515F, PDR 33658167), the Fédération Wallonie-Bruxelles (Action de Recherche Concertée), WELBIO, and the Brachet Funds.

PY - 2022/4

Y1 - 2022/4

N2 - 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. IMPORTANCE Fungal infections represent a significant threat to human health and cause huge damage to crop yields worldwide. The dimorphic switch between yeast and filamentous growth is associated with the virulence of pathogenic fungi. Of note, fungal Mep2 proteins of the conserved Amt-Mep-Rh family play a transceptor role in the induction of filamentation; however, the signaling mechanism remains largely unknown. Amt-Mep-Rh proteins ensure the specific scavenging of NH4+ through a mechanism relying on substrate deprotonation, thereby preventing competition and translocation of similar-sized K+. Our multidisciplinary approaches using E. coli AmtB, Saccharomyces cerevisiae, and Candida albicans Mep2 show that double variation of the family-defining Twin-His motif triggers a mechanistic switch from a specific transporter to an unspecific ion channel with both mechanisms still coexisting in single variants. Moreover, we show that this mechanistic alteration is associated with loss of signaling ability of Mep2, supporting a transport mechanism-driven process in filamentation induction.

AB - 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. IMPORTANCE Fungal infections represent a significant threat to human health and cause huge damage to crop yields worldwide. The dimorphic switch between yeast and filamentous growth is associated with the virulence of pathogenic fungi. Of note, fungal Mep2 proteins of the conserved Amt-Mep-Rh family play a transceptor role in the induction of filamentation; however, the signaling mechanism remains largely unknown. Amt-Mep-Rh proteins ensure the specific scavenging of NH4+ through a mechanism relying on substrate deprotonation, thereby preventing competition and translocation of similar-sized K+. Our multidisciplinary approaches using E. coli AmtB, Saccharomyces cerevisiae, and Candida albicans Mep2 show that double variation of the family-defining Twin-His motif triggers a mechanistic switch from a specific transporter to an unspecific ion channel with both mechanisms still coexisting in single variants. Moreover, we show that this mechanistic alteration is associated with loss of signaling ability of Mep2, supporting a transport mechanism-driven process in filamentation induction.

KW - Candida albicans

KW - Escherichia coli

KW - Saccharomyces cerevisiae

KW - ammonium assimilation

KW - fungal filamentation

KW - secondary transporter mechanism

UR - https://www.scopus.com/pages/publications/85129086126

U2 - 10.1128/mbio.02913-21

DO - 10.1128/mbio.02913-21

M3 - Article

C2 - 35196127

SN - 2150-7511

VL - 13

JO - MBio

JF - MBio

IS - 2

M1 - e0291321

ER -