Family 2.A.69 - The Auxin Efflux Carrier Family
Family ID: 52644
Plants possess tissue-specific, pmf-driven, cellular, auxin efflux
systems. These carriers are saturable, auxin-specific, and localized
to the basal ends of auxin transport-competent cells. They may
be found in various plant tissues including vascular tissues and
roots. They are responsible for the polar (downwards) transport
of auxins from the leaves to the roots. They also function in
gravitropism. A single plant such as Arabidopsis thaliana possesses
at least six such systems. Two isoforms, one in vascular tissue
(PIN1) and one in roots (REH1) have been functionally characterized
as has a homologue from Oryza satira. These plant proteins are
600-700 amino acyl residues long and exhibit 8-12 transmembrane
spanners.
Homologues of the AEC family are found in bacteria
(E. coli, Klebsiella pneumoniae, Synechocystis, Aquifex aeolicus,
Bacillus subtilis and Rickettsia prowazekii) as well as in archaea
(Methanococcus jannaschii and Methanobacterium thermoautotrophicum).
The K. pneumoniae homologues (MdeF, 319 aas) has been implicated
in malonate uptake. The bacterial proteins are 300-400 aas in
length.
Yeast also
possess homologues of the AEC family. Saccharomyces cerevisiae
has three functionally uncharacterized AEC members (YL52, spP54072,
64.0 kDa; YNJ5, spP53930, 71.2 kDa; and YB8B, spP38355, 47.5 kDa),
and Schizosaccharomyces pombe also has a sequenced homologue.
It is thus clear that members of the AEC family are widespread,
being found in Gram-negative, Gram-positive and cyanobacteria,
in archaea, and in both fungi and plants. C. elegans, however,
appears to lack identifiable homologues of the AEC family. Based
on PSI-BLAST results, the AEC family may be distantly related
to the bile acid:Na+ symporter (BASS) family (TC #2.A.28) and
the divalent anion:Na+ symporter (DASS) family which include members
found in animals. It is therefore a constituent of the ion transporter
(IT) superfamily.
