AGC kinase which plays a role in TOR complex 1 (TORC1) signaling pathway which mediates temporal control of cell growth in response to nutrients. Required for phosphorylation of ribosomal protein S6 (rps601/rps602) at 'Ser-235' and 'Ser-236'. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Phosphorylation at Tyr-102 inactivates the enzyme. Phosphorylated by ksg1 and target of rapamycin complex 1 (TORC1), affecting the kinase activity of psk1 in a nutrient-dependent manner. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family. S6 kinase subfamily.