Stress-activated, pro-apoptotic kinase which, following caspase-cleavage, enters the nucleus and induces chromatin condensation followed by internucleosomal DNA fragmentation. Key component of the Hippo signaling pathway which plays a pivotal role in organ size control and tumor suppression by restricting proliferation and promoting apoptosis. The core of this pathway is composed of a kinase cascade wherein stk3/mst2 and stk4/mst1, in complex with its regulatory protein sav1, phosphorylates and activates lats1/2 in complex with its regulatory protein mob1, which in turn phosphorylates and inactivates yap1 oncoprotein and wwtr1/taz. Phosphorylation of yap1 by lats2 inhibits its translocation into the nucleus to regulate cellular genes important for cell proliferation, cell death, and cell migration. Phosphorylates 'Ser-14' of histone H2B (H2BS14ph) during apoptosis. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Inhibited by the C-terminal non-catalytic region. Activated by caspase-cleavage. Full activation also requires homodimerization and autophosphorylation of Thr-185 (By similarity). Homodimer; mediated via the coiled-coil region. The caspase-cleaved form cycles between nucleus and cytoplasm. Proteolytically cleaved by caspase-3 during apoptosis at Asp-328 resulting in a 37 kDa form. Proteolytic cleavage results in kinase activation and nuclear translocation of the truncated form (MST1/N) (By similarity). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.