Serine/threonine-protein kinase that acts on both serine and threonine residues and promotes apoptosis in response to stress stimuli and caspase activation. Mediates oxidative-stress-induced cell death by modulating phosphorylation of JNK1-JNK2 (MAPK8 and MAPK9), p38 (MAPK11, MAPK12, MAPK13 and MAPK14) during oxidative stress. Plays a role in a staurosporine-induced caspase-independent apoptotic pathway by regulating the nuclear translocation of AIFM1 and ENDOG and the DNase activity associated with ENDOG. Phosphorylates STK38L on 'Thr-442' and stimulates its kinase activity. In association with STK26 negatively regulates Golgi reorientation in polarized cell migration upon RHO activation. Regulates also cellular migration with alteration of PTPN12 activity and PXN phosphorylation: phosphorylates PTPN12 and inhibits its activity and may regulate PXN phosphorylation through PTPN12 (By similarity). Acts as a key regulator of axon regeneration in the adult optic nerve and radial nerve. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Monomer. Interacts with CTTNBP2NL. Interacts with RIPOR1 (via C-terminus); this interaction occurs in a PDCD10-dependent and Rho-independent manner. Interacts with PDCD10; this interaction is required for the association of STK24 with RIPOR1. The truncated form (MST3/N) translocates to the nucleus. Colocalizes with STK38L in the membrane (By similarity). Proteolytically processed by caspases during apoptosis. Proteolytic cleavage results in kinase activation, nuclear translocation of the truncated form (MST3/N) and the induction of apoptosis (By similarity). Oxidative stress induces phosphorylation. Activated by autophosphorylation at Thr-178 and phosphorylation at this site is essential for its function. Manganese, magnesium and cobalt-dependent autophosphorylation is mainly on threonine residues while zinc-dependent autophosphorylation is on both serine and threonine residues (By similarity). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.