The amino acid-polyamine-organocation superfamily

The amino acid-polyamine-organocation (APC) superfamily has been shown to include five recognized families, four of which are specific for amino acids and their derivatives. Recent high-resolution X-ray crystallographic data have shown that four additional transporter families (BCCT, TC No. 2.A.15; SSS, 2.A.21; NSS, 2.A.22; and NCS1, 2.A.39), transporting a wide range of solutes, exhibit sufficiently similar folds to suggest a common evolutionary origin. We have used established statistical methods, based on sequence similarity, to show that these families are, in fact, members of the APC superfamily. We also identify two additional families (NCS2, 2.A.40; SulP, 2.A.53) as being members of this superfamily. Repeat sequences, each having five transmembrane α-helical segments and arising via ancient intragenic duplications, are demonstrated for all of these families, further strengthening the conclusion of homology. The APC superfamily appears to be the second largest superfamily of secondary carriers, the largest being the major facilitator superfamily (MFS). Although the topology of the members of the APC superfamily differs from that of the MFS, both families appear to have arisen from a common ancestral 2 TMS hairpin structure that underwent intragenic triplication followed by loss of a TMS in the APC family, to give the repeat units that are characteristic of these two superfamilies.