May be involved in transmitting sphingosine-1 phosphate (SPP)-mediated signaling into the cell. Plays a role in the recruitment of PRDX3 to early endosomes. 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. Interacts (via PX domain) with PRDX3. 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. Due to a partial intron retention. Due to a partial intron retention. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. S6 kinase subfamily. Truncated N-terminus.