A two-lane mechanism for selective biological ammonium transport

Abstract

The transport of charged molecules across biological membranes faces the dual problem of accommodating charges in a highly hydrophobic environment while maintaining selective substrate translocation. This has been the subject of a particular controversy for the exchange of ammonium across cellular membranes, an essential process in all domains of life. Ammonium transport is mediated by the ubiquitous Amt/Mep/Rh transporters that includes the human Rhesus factors. Here, using a combination of electrophysiology, yeast functional complementation and extended molecular dynamics simulations, we reveal a unique two-lane pathway for electrogenic NH4+ transport in two archetypal members of the family, the transporters AmtB from Escherichia coli and Rh50 from Nitrosomonas europaea. The pathway underpins a mechanism by which charged H+ and neutral NH3 are carried separately across the membrane after NH4+ deprotonation. This mechanism defines a new principle of achieving transport selectivity against competing ions in a biological transport process.

Transcriptional profiling of the stringent response mutant strain E. coli SR reveals enhanced robustness to large‐scale conditions

“…Moreover, both strains depleted ammonium to comparable levels of about 1.2 mg l -1 or 67 µmol l -1 , well in line with previously reported values for E. coli K-12 strains in nitrogen limited chemostats (Hua et al, 2004). The low remaining ammonium concentration indicates that uptake in both strains is mediated actively by AmtB with K m = 0.8 mM (Williamson et al, 2020) and incorporation is accomplished by the GS-GOGAT System with GS K m = 0.1 mM (Alibhai and Villafranca, 1994). This is supported by our transcriptional data which revealed that amtB, gltB and gltD were significantly enriched for both strains over all time-points.…”

An ammonium transporter is a non-canonical olfactory receptor for ammonia

“…Ammonium transporters in other species are highly selective for ammonium, with little to no transport of larger amines [34, 35], in accordance with the narrow tuning of Amt/Rh50 + ORNs to ammonia (Figure 2A). Further, ammonium transporters are electrogenic, mediating inward, depolarizing NH 4 + influx [17, 18, 35, 36]. Given that ammonia (NH 3 ) gas is expected to form ammonium ions (NH 4 + ) in the sensillum lymph, these cations could be transported through either Amt or Rh50 and depolarize the ORNs.…”

An ammonium transporter is a non-canonical olfactory receptor for ammonia

“…The narrowly tuned ammonia response in these ORNs (Figure 2A) is consistent with the strict selectivity of ammonium transporters. 36,37 Amts also distinguish NH 4 + ions from similarly sized K + ions, 17,37,38 an advantage considering the presence of >100 mM K + in the sensillum lymph bathing ORN dendrites. We propose that a simple electrogenic influx of ammonium ions leads to ORN depolarization, but we cannot rule out an ammonium-initiated signaling cascade analogous to the mechanism for vertebrate sour sensing, in which H + influx closes acid-sensitive K + channels.…”

“…Further, ammonium transporters are electrogenic, mediating inward NH 4 + currents. 16,17,37,38 Given that NH 3 gas is expected to form ammonium ions (NH 4 + ) in the sensillum lymph, these cations could be transported through either Amt or Rh50 and depolarize the ORNs. In this manner, the transporters could act as ammonia-selective receptors.…”