Transporters of the cationic amino acid transporter (CAT) family form a class of proteins that occur in mammalian cells as a subfamily of the solute carrier family 7 (SLC7). Expressed nearly ubiquitously in the body, they catalyze the bidirectional transport of cationic amino acids through the cell membrane including the essential amino acids lysine and arginine [1]. In several studies it was shown that this transporter is necessary for basic cell functions such as protein synthesis, nitric oxide

synthesis and interorgan amino acid transport. Additionally, it plays a key role in recovery after cell stress as it transports essential amino acids into the cell as soon as they become available again. The importance of the CAT family becomes evident from knockout studies in mice. Deletion of the mouse CAT-1 gene (mCAT-1, SLC7A1) leads to an early death of the animals at the first day after birth [2].

The mCAT-1 has 14 putative transmembrane segments with intracellular N- and C-termini. Within the length of 622 amino acids, the third extracellular loop of the receptor is most interesting. This site serves as the entry point for the Murine Leukemia Virus (MuLV) and is highly variable between different species. Even close relatives to mice like rats or hamsters exhibit a different CAT-1 that cannot be used by the MuLV as an entry point. A reason for this variability of CAT-1 among different species could be a co-evolution of virus (MuLV) and host (mouse). Changes in the region of the mouse genome coding for the third extracellular loop lead to a different structure of the receptor [4]. This change of the viral entry site prohibited viral infection of the mouse, forcing the virus to adapt to these changes of CAT-1. As a consequence, the number of hosts of the virus decreased until different mouse species remained [5].