May be involved in transmitting sphingosine-1 phosphate (SPP)-mediated signaling into the cell (PubMed:12077123). Plays a role in the recruitment of PRDX3 to early endosomes (PubMed:15750338). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with SPHK1 and phosphatidylinositol 3-phosphate (PubMed:12077123). Interacts (via PX domain) with PRDX3 (PubMed:15750338). Highly expressed in testis, skeletal muscle, brain, heart, placenta, kidney and liver and weakly expressed in thymus, small intestine, lung and colon. The PX domain is essential for its localization to the early endosomes. The first protein kinase domain appears to be a pseudokinase domain as it does not contain the classical characteristics, such as the ATP-binding motif, ATP-binding site and active site. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. S6 kinase subfamily. Instead of Lys-820, Arg-820 is found at the binding site.