Evidence for a regulatory protein involved in the increased activity of system A for neutral amino acid transport in osmotically stressed mammalian cells.

System A for neutral amino acid transport is increased by hypertonic shock in NBL-1 cells previously induced to express system A activity by amino acid starvation. The hypertonicity-mediated effect can be blocked by cycloheximide but is insensitive to tunicamycin. The activity induced may be inactivated irreversibly by the addition of system A substrates, by a rapid mechanism insensitive to cycloheximide. In CHO-K1 cells, hypertonicity increases system A activity, as has been shown in NBL-1 cells. This effect is additive to the activity produced by derepression of system A by amino acid starvation and is insensitive to tunicamycin. Furthermore, the alanine-resistant mutant CHO-K1 alar4, which bears a mutation affecting the regulatory gene R1, involved in the derepression of system A activity after amino acid starvation, is still able to respond to the hypertonic shock by increasing system A activity to a level similar to that described in hypertonicity-induced derepressed CHO-K1 (wild type) cells. These results suggest (i) that the hypertonicity-mediated increase of system A activity occurs through a mechanism other than that involved in system A derepression and (ii) that a regulatory protein coded by an osmotically sensitive gene is responsible for further activation of preexisting A carriers.