[1][2][3][4] They occur in bacteria, archaea, fungi, unicellular eukaryotic protists, slime molds, plants and animals.
Most of them possess twelve transmembrane α-helical spanners but have a re-entrant loop involving TMSs 2 and 3.
Some animal proteins, for example, those in the LAT subfamily (TC# 2.A.3.8) including ASUR4 (gbY12716) and SPRM1 (gbL25068) associate with a type 1 transmembrane glycoprotein that is essential for insertion or activity of the permease and forms a disulfide bridge with it.
[11] Two APC family members, LAT1 and LAT2 (TC #2.A.3.8.7), transport a neurotoxicant, the methylmercury-L-cysteine complex, by molecular mimicry.
Subfamilies of the APC family, and the proteins in these families, can be found in the Transporter Classification Database:[6] Based on 3-D structures of APC superfamily members, Rudnick (2011) has proposed the pathway for transport and suggested a "rocking bundle" mechanism.