Pseudokinase which, in complex with CAB39/MO25 (CAB39/MO25alpha or CAB39L/MO25beta), binds to and activates STK11/LKB1. Adopts a closed conformation typical of active protein kinases and binds STK11/LKB1 as a pseudosubstrate, promoting conformational change of STK11/LKB1 in an active conformation (By similarity). Component of a trimeric complex composed of STK11/LKB1, STRAD (STRADA or STRADB) and CAB39/MO25 (CAB39/MO25alpha or CAB39L/MO25beta): the complex tethers STK11/LKB1 in the cytoplasm and stimulates its catalytic activity. Interacts with BIRC4/XIAP. These two proteins are likely to coexist in a complex with TAK1, TRAF6, TAB1 and TAB2 (By similarity). The protein kinase domain is predicted to be catalytically inactive. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily. Ser-184 is present instead of the conserved Asp which is expected to be an active site residue.