Tumor suppressor serine/threonine-protein kinase that controls the activity of AMP-activated protein kinase (AMPK) family members, thereby playing a role in various processes such as cell metabolism, cell polarity, apoptosis and DNA damage response. Acts by phosphorylating the T-loop of AMPK family proteins, leading to promote their activity (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Catalytic component of a trimeric complex composed of STK11/LKB1, STRAD (STRADA or STRADB) and CAB39/MO25 (CAB39/MO25alpha or CAB39L/MO25beta). Oocytes, eggs and early embryos. Its expression peaks in the oocyte and unfertilized egg, begins to decrease gradually after fertilization, and disappears during the gastrulation stage. Phosphorylated by a cAMP-dependent protein kinase. Autophosphorylated in a reaction that prefers Mn(2+) to Mg(2+). Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family. LKB1 subfamily.