CATs, a family of three distinct mammalian cationic amino acid transporters.

Three related mammalian carrier proteins that mediate the transport of cationic amino acids through the plasma membrane have been identified in murine and human cells (CAT for cationic amino acid transporter). Models of the CAT proteins in the membrane suggest they have 12 or 14 transmembrane domains connected by short hydrophilic loops and intracellular N- and C-termini. The transport activity of the CAT proteins is sensitive to trans-stimulation and independent of the presence of sodium ions. These features agree with the behaviour of carrier proteins mediating facilitated diffusion. The three CAT proteins, CAT-1, CAT-2A and CAT-2(B) are encoded by two different genes (CAT-1 and CAT-2). CAT-1 and CAT-2(B) exhibit transport properties consistent with system y(+), the principal mechanism for cellular uptake of cationic amino acids. In contrast, CAT-2A has tenfold lower substrate affinity, greater apparent maximal velocity and it is much less sensitive to trans-stimulation. In addition to structural and functional aspects, this review discusses the role of the CAT proteins for supplying substrate to NO synthases and the property of the rodent CAT-1 proteins to function as virus receptors.