APC Superfamily
| Identifiers | |
|---|---|
| Symbol | APC |
| TCDB | 2.A.3 |
| OPM superfamily | 67 |
The Amino acid-Polyamine-organoCation (APC) superfamily is the second largest superfamily of secondary carrier proteins currently known. Originally, the APC superfamily consisted of subfamilies under the transporter classification number (TC #) 2.A.3. This superfamily has since been expanded to include eighteen different families.
The most recent families added include the PAAP (Putative Amino Acid Permease), LIVCS (Branched Chain Amino Acid:Cation Symporter), NRAMP (Natural Resistance-Associated Macrophage Protein), CstA (Carbon starvation A protein), KUP (K⁺ Uptake Permease), BenE (Benzoate:H⁺ Virginia Symporter), and AE (Anion Exchanger). Bioinformatic and phylogenetic analysis is used to continually expand currently existing families and superfamilies.
Other constituents of the APC superfamily are the AAAP family (TC# 2.A.18), the HAAAP family (TC# 2.A.42) and the LCT family (TC# 2.A.43). Some of these proteins exhibit 11 TMSs. Eukaryotic members of this superfamily have been reviewed by Wipf et al. (2002) and Fischer et al. (1998).
Currently recognized families within the APC Superfamily (with TC numbers in blue) include:
The topology of the well-characterized human Anion Exchanger 1 (AE1) conforms to a UraA-like topology of 14 TMSs (12 α-helical TMSs and 2 mixed coil/helical TMSs). All functionally characterized members of the APC superfamily use cation symport for substrate accumulation except for some members of the AE family which frequently use anion:anion exchange. All new entries contain the two 5 or 7 TMS repeat units characteristic of the APC superfamily, sometimes with extra TMSs at the ends likely the result of an addition prior to duplication. The CstA family contains the greatest variation in TMSs. New functionally characterized,members transport amino acids, peptides, and inorganic anions or cations. Except for anions, these are typical substrates of established APC superfamily members. Active site TMSs are rich in glycyl residues in variable but conserved arrangements.
In CadB of E. coli (2.A.3.2.2), amino acid residues involved in both uptake and excretion, or solely in excretion are located in the cytoplasmic loops and the cytoplasmic side of transmembrane segments, whereas residues involved in uptake are located in the periplasmic loops and the transmembrane segments. A hydrophilic cavity is proposed to be formed by the transmembrane segments II, III, IV, VI, VII, X, XI, and XII. Based on 3-D structures of APC superfamily members, Rudnick (2011) has proposed the pathway for transport and suggested a "rocking bundle" mechanism.
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