The HAA family is comprised of systems specific for the transport of the hydrophobic amino acids leucine, isoleucine and valine ( Pubmed : 2195019 ) . A transport system involved in the uptake of urea and short chain aliphatic amines in Methylophilus methylotrophus belongs to this family ( Pubmed : 9492267 ) . This system is homologous to Synechocystis and Anabaena systems in the uptake of neutral amino acids Ala, Val, Phe, Ile, and Leu ( Pubmed : 9006043 ) . It is therefore possible that the urea transporter of M. methylotrophus could also transport such amino acids. Systems of the HAA family have a characteristic organization made up of one or several BPs, two IMs and two ABCs. The eukaryote gamma-aminobutyric acid type B (GABA(B)) receptors and the related metabotropic glutamate receptor-like family of G-protein-coupled receptors have their extracellular domains homologous to the bacterial leucine binding protein. Furthermore, effect of point mutations can be explained by the Venus flytrap model, which proposes that the initial step in the activation of the receptor by the agonist results from the closure of the two lobes of the binding domain ( Pubmed : 10224098 ) .