Figure 3.

ATN6 mediated the regulation of ATC3 by PII, regulated the basal AT activity of ATC3, and mediated inhibition of the ATC3 AT activity by PII-UMP. (A) ATN6 mediates PII regulation of ATC3 AT activity. The initial rate of GS adenylylation was measured as described in , with GS at 2.5 μM, α-ketoglutarate at 0.05 mM, [α-³²P]ATP at 0.5 mM, pyruvate kinase at 0.022 unit/μL, PEP at 3 mM, bovine serum albumin at 0.3 mg/mL, and ATC3 at 0.25 μM. (B) ATN6 inhibits the basal AT activity of ATC3 and mediates PII activation of the ATC3 AT activity. Conditions were as described in and for panel A, except that the concentration of the enzyme ATC3 was increased to 0.8 μM to facilitate examination of the basal AT activity. (C) ATN6 mediates PII-UMP inhibition of the AT activity of ATC3 when glutamine was at the K_act. Conditions were as described in , with GS at 2.5 μM, ATP at 0.5 mM, α-ketoglutarate at 1 mM, ATC3 at 0.025 μM, glutamine at 1.5 mM, pyruvate kinase at 0.022 unit/μL, and PEP at 5 mM. (D) ATN6 mediates PII-UMP inhibition of the AT activity of ATC3 when glutamine was at a saturating level. Conditions were as described in and for panel C, except that glutamine was present at a concentration of 20 mM and ATC3 was present at a concentration of 0.125 μM.