Family 2.A.49 - The Ammonium Transporter Family
Family ID: 52637
The proteins of the Amt family vary in size from 391 to 622 amino
acyl residues and possess 11 (most members) or 12 (the E. coli
AmtB protein) transmembrane a-helical spanners. They occur in
Gram-negative and Gram-positive bacteria, archaea, yeast, plants
and animals. The eukaryotic proteins are, in general, larger than
the prokaryotic proteins. All functionally characterized members
of the family are ammonia or ammonium uptake transporters. Some,
but not others, also transport methylammonium.
Proteins of
the Amt family are probably ubiquitous. Homologues have been found
in archaea and animals, and many organisms from all major kingdoms
of living organisms possess multiple homologues. One of these
proteins, Mep2 of Saccharomyces cerevisiae, has been shown to
function both as a transporter and as a sensor, generating a signal
that regulates filamentous growth (pseudohyphal differentiation)
in response to ammonium starvation. This protein has an N-terminal,
asparaginyl-linked glycosylated domain where only Asn-4 is glycosylated.
Mep2, but not Mep1 or Mep3, has an extracytoplasmic N-terminus
(Marini and André, 2000). This N-terminal domain is not
required for either transport or sensing. Of the three S. cerevisiae
Amt family paralogues, Mep2 exhibits higher affinity for NH4+
(1 mM) than Mep1 (10 mM), and Mep1 exhibits higher affinity than
Mep3 (1 mM).
One plant
homologue, AtAMT2, has been reported to more closely resemble
bacterial AMT transporters than other plant proteins and to transport
ammonium in an energy-dependent fashion (Sohlenkamp et al., 2000).
Evidence for an oligomeric transporter has been presented, and
the previously characterized GMSAT1 protein, thought to be a NH4+
channel protein, is probably a regulatory protein instead of a
transporter (Marini et al., 2000). The Amt family includes the
Rhesus blood group protein of the human red blood cell, but the
function of this distant homologue is not known.
