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 s hock in NBL-1 cells previously induced to express system A activity by amino acid starvation. The hypertonicity-mediated effect can be block ed by cycloheximide but is insensitive to tunicamycin. The activity in duced may be inactivated irreversibly by the addition of system A subs trates, by a rapid mechanism insensitive to cycloheximide. In CHO-K1 c ells, hypertonicity increases system A activity, as has been shown in NBL-1 cells. This effect is additive to the activity produced by derep ression of system A by amino acid starvation and is insensitive to tun icamycin. Furthermore, the alanine-resistant mutant CHO-K1 ala(r)4, wh ich bears a mutation affecting the regulatory gene R1, involved in the derepression of system A activity after amino acid starvation, is sti ll able to respond to the hypertonic shock by increasing system A acti vity to a level similar to that described in hypertonicity-induced der epressed 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) t hat a regulatory protein coded by an osmotically sensitive gene is res ponsible for further activation of preexisting A carriers.