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 [2] is known as the sodium:dicarboxylate symporter family (SDF) and currently consists of the following proteins:

• Escherichia coli proton-glutamate symport protein (glutamate-aspartate carrier) (gene gltP). GltP is a sodium-independent carrier for glutamate and aspartate.
• Bacillus proton/sodium-glutamate symport protein (gene gltT or gltP). GltT is a sodium-dependent carrier for glutamate and aspartate.
• Rhizobium and Escherichia coli C4-dicarboxylate carrier (gene dctA), which is responsible for the transport of dicarboxylates such as succinate, fumarate, and malate. In Rhizobium this transport system plays an important role in the energy supply of the legume symbionts.
• Three different mammalian sodium-dependent L-glutamate and aspartate high- affinity transporters (also known as excitatory amino acid transporters (EAAT)) [3,4,5]. These brain proteins are essential for terminating the postsynaptic action of glutamate by rapidly removing released glutamate from the synaptic cleft.
• EAAT-4, a mammalian chloride-dependent L-glutamate and aspartate high- affinity transporter [6].
• Mammalian neutral amino acid transporter (SATT), a brain protein that plays a role in the transport of alanine, cysteine, serine and threonine [7].
• Mammalian insulin-activated amino acid transporter (AAAT), an adipose tissue protein that transports serine and, to a lesser extent, alanine and glutamate [8].
• Hypothetical protein ygjU from Escherichia coli and HI1545, the corresponding Haemophilus influenzae protein.

These transporters are proteins of from 420 to 573 amino acids which are highly hydrophobic and which probably contain about 10 transmembrane regions. As signature patterns, we selected two conserved regions. The first pattern is located in the N-terminal section and seems to include a hydrophilic region between two transmembrane regions. The second pattern is located in the C-terminal section.