Family 2.A.2 - The Glycoside-Pentoside-Hexuronide Cation Symporter
Family
Family ID: 52613
GPH:cation symporters catalyze uptake of
sugars in symport with a monovalent cation (H+ or Na+). Mutants
of two groups of these symporters (the melibiose permeases of
enteric bacteria, and the lactose permease of Streptococcus thermophilus)
have been isolated in which altered cation specificity is observed
or in which sugar transport is uncoupled from cation symport (i.e.,
uniport is catalyzed). The various members of the family can use
Na+, H+ or Li, Na+ or Li+, H+ or Li+, or only H+ as the symported
cation. All sequenced members of the family are from bacteria
except the distantly related sucrose:H+ symporters of plants and
a yeast maltose/sucrose:H+ symporter of S. pombe. This yeast protein
is about 24% identical to the plant sucrose:H+ symporters and
is more distantly related to the bacterial members of the GPH
family (Reinders and Ward, 2001).
Proteins of the GHP family are generally about 500 amino acids
in length, although the Gram-positive bacterial lactose permeases
are larger, due to a C-terminal hydrophilic domain that is involved
in regulation by the phosphotransferase system (TC #4.A.1). All
of these proteins possess twelve putative transmembrane a-helical
spanners. Limited sequence similarity of some of these proteins
with members of the major facilitator superfamily (MFS, TC #2.A.1)
has been observed, but this degree of similarity is insufficient
to establish homology. PSI-BLAST results substantiate the conclusion
that the GPH family is a member of the MFS.
