It has been shown  that integral membrane proteins that mediate the intake
of a wide variety of molecules with the concomitant uptake of sodium ions
(sodium symporters) can be grouped, on the basis of sequence and functional
similarities into a number of distinct families. One of these families is
known as the sodium:galactoside symporter family (SGF) and currently consists
of the following proteins:
The melibiose carrier (gene melB) from a variety of enterobacteria. This
protein is responsible for melibiose transport and is capable of using
hydrogen, sodium, and lithium cations as coupling cations for cotransport.
The lactose permease from Lactobacillus (gene lacS or lacY). This protein
is responsible for the transport of β-galactosides into the cell, with
the concomitant export of a proton. It consists of two domains; a N-
terminal SGF domain and a C-terminal domain that resembles that of enzyme
IIA of the PEP:sugar phosphotransferase system.
The raffinose permease from Pediococcus pentosaceus. It also consists of a
N-terminal SGF domain and a C-terminal IIA domain.
The glucuronide carrier (gene gusB or uidP) from Escherichia coli.
The xylose transporter (gene xylP) from Lactobacillus pentosus.
Escherichia coli hypothetical protein yagG.
Escherichia coli hypothetical protein yicJ.
Escherichia coli hypothetical protein yihO.
Escherichia coli hypothetical protein yihP.
Bacillus subtilis hypothetical protein yjmB.
Bacillus subtilis hypothetical protein ynaJ.
Like sugar transport proteins, these integral membrane proteins are predicted
to comprise twelve membrane spanning domains. As a signature pattern we
selected a highly conserved region which is located in a cytoplasmic loop
between the second and third transmembrane regions. This region starts with
a conserved aspartate which has been shown , in melB, to be important for
the activity of the protein.
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