Phosphorylates RELL1, RELL2, RELT and PAK1. Phosphorylates PLSCR1 in the presence of RELT. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] By autophosphorylation on threonine. Interacts with PAK1 (By similarity). Interacts with chloride channel proteins SLC12A6 isoform 2, SLC12A1 and SLC12A2 but not with SLC12A4 and SLC12A7, possibly establishing sensor/signaling modules that initiate the cellular response to environmental stress (PubMed:12386165). Interacts with RELL1, RELL2 and RELT (By similarity). Interacts with PLSCR1 in the presence of RELT (By similarity). Ubiquitously expressed in all tissues examined, except thymus. Autophosphorylated. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.