Mediates the transport of melibiose and other galactosides by a symport mechanism (PubMed:45782, PubMed:3311166, PubMed:3316227, PubMed:2185831, PubMed:7703254). Can use sodium, lithium and protons as coupling cations, depending on the sugar substrate and the cationic environment (PubMed:45782, PubMed:3311166, PubMed:3316227, PubMed:7703254). Alpha-galactosides (melibiose, raffinose, p-nitrophenyl-alpha-galactoside or methyl-alpha-galactoside) are cotransported with either Na(+) or H(+), whereas beta-galactosides (lactose, L-arabinose-beta-D-galactoside, D-fructose-beta-D-galactoside, methyl-beta-galactoside or p-nitrophenyl-beta-galactoside) are cotransported with Na(+) or Li(+) but not H(+) (PubMed:45782, PubMed:3311166). The monosaccharide D-galactose can use either Na(+) or H(+) for cotransport whereas D-fucose, L-arabinose and D-galactosamine can use only Na(+) (PubMed:3311166). melibiose(in) + Na(+)(in) = melibiose(out) + Na(+)(out) Li(+)(in) + melibiose(in) = Li(+)(out) + melibiose(out) H(+)(in) + melibiose(in) = H(+)(out) + melibiose(out) Monomer and homodimer. Consists of two pseudosymmetrical 6-helix bundles lining an internal hydrophilic cavity, which faces the cytoplasmic side of the membrane and probably contains the binding sites for both cosubstrates (PubMed:12110569, PubMed:19706416). The cytoplasmic loop connecting the membrane-spanning segments 4 and 5 is close to the sugar binding site and may participate directly in cosubstrate translocation by MelB (PubMed:12421811). The charged residue Arg-141 is probably involved in the reaction of cosubstrate translocation or substrate release in the inner compartment (PubMed:12421811). Belongs to the sodium:galactoside symporter (TC 2.A.2) family. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus.