INCREASED CAT3-MEDIATED CATIONIC AMINO-ACID-TRANSPORT FUNCTIONALLY COMPENSATES IN CAT1 KNOCKOUT CELL-LINES

Arginine transport is important for a number of biological processes i n vertebrates, and its transport may be rate-limiting for the producti on of nitric oxide. The majority of L-Arg transport is mediated by Sys tem y(+), although several other carriers have been kinetically define d. System y(+) cationic amino acid transport is mediated by proteins e ncoded by a family of genes, Cat1, Cat2, and Cat3, High affinity L-arg inine transport was investigated in embryonic fibroblast cells derived from Cat1 knockout mice that lack functional Cat1. Both wild type and knockout cells transport arginine with comparable K-m and V-max. Howe ver, the apparent affinity for lysine transport was 2.4 times lower in Cat1(-/-) cells when compared with wild type cells, a property charac teristic of Cat3-mediated transport. Northern analysis-documented Cat2 mRNA increased 2-fold, whereas Cat3 mRNA levels increased 11-fold in Cat1(-/-) relative to Cat1(+/+) cells. The low affinity Cat2a mRNA was not detectably expressed in these cells. Even though Cat3 expression is normally limited to adult brain, there was a large increase in the amount of Cats protein present at the plasma membrane of Cat1(-/-) emb ryonic fibroblast cells. These results suggest that Cat3 compensates f or the loss of functional Cat1 in cells derived from Cat1 knockout mic e and mediates the majority of high affinity arginine transport.