The PAO family includes transport systems for amino acids that have polar or charged side chains: lysine, histidine, ornithine, arginine ( Pubmed : 7110312 ) , glutamine ( Pubmed : 3027504 ) , glutamate ( Pubmed : 1971622 ) , cystine and diaminopimelic acid ( Pubmed : 10506196 ) . Opines like octopine (N2-(1-carboxyethyl)-L-arginine) ( Pubmed : 1655707 ) and nopaline (N2-(1,3-Dicarboxypropyl)-L-arginine) ( Pubmed : 1732218 ) are transported in agrobacteria by PAO family transporters. Typical systems have in general 2 IM with the exception of the cystine- and the glutamine-specific systems that have only one IM. The BPs specific to glutamine are homologous to the extracellular portion of eukaryote ionotropic glutamate receptors. Recent studies indicated that glutamate receptors share with the bacterial PAO family BP the fundamental mechanism of amino acid recognition ( Pubmed : 9707429 ) .