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. Highly expressed in heart, skeletal muscle, testis, liver and colon. 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.