Upon serum stimulation, phosphorylates CDKN1B/p27Kip1, thus controlling CDKN1B subcellular location and cell cycle progression in G1 phase. May be involved in trafficking and/or processing of RNA (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with stathmin, PAM and CDKN1B/p27Kip1. Mostly nuclear. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.