Nutrient-regulated protein kinase that promotes the activity of at least 6 distinct transport systems for nitrogenous nutrients under conditions of nitrogen catabolite derepression. Under poor nitrogen growth conditions, required for post-Golgi sorting of the general amino acid permease GAP1 and the three known ammonia permeases, MEP1/2/3, to the plasma membrane. Contributes also to the stability and the retention of GAP1 at the plasma membrane. Inversely, promotes the degradation of tryptophan permease TAT2 under the same conditions. Activity is regulated by the TOR signaling pathway via phosphatase SIT4. Although thought to be involved in regulation of GLN3-dependent transcription by nitrogen catabolite repression, this seems to be an indirect effect from the reduced uptake of the nitrogen-repressing compound. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Dephosphorylation by SIT4 activates NPR1 kinase activity. Interacts with TIP41. Appears to be more concentrated in punctate structures reminiscent of the Golgi or of an endosomal compartment. Hyperphosphorylated in nitrogen-rich growth medium. Nitrogen limitation (or rapamycin treatment) leads to substantial, though not complete dephosphorylation. Autophosphorylation plays only a minor role and seems not to be regulated by the quality of the nitrogen source. Present with 284 molecules/cell in log phase SD medium. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.