PROSITE documentation PDOC00680 [for PROSITE entry PS00872]

Sodium:galactoside symporter family signature

It has been shown [1] 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 [2], in melB, to be important for the activity of the protein.