Serine/threonine-protein kinase that acts as a mediator of cell growth. Modulates apoptosis. In association with STK24 negatively regulates Golgi reorientation in polarized cell migration upon RHO activation. Phosphorylates ATG4B at 'Ser-383', thereby increasing autophagic flux. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interaction with Golgi matrix protein GOLGA2 leads to autophosphorylation on Thr-178, possibly as a consequence of stabilization of dimer formation. May also be activated by C-terminal cleavage (By similarity). Homodimer. Interacts with PDCD10. Interacts with GOLGA2. 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 STK26 with RIPOR1. Colocalized with RIPOR1 in the Golgi of serum-starved cells and relocated to cytoplasmic punctae, probably vesicular compartments, along with RIPOR1 upon serum stimulation in a Rho- and PDCD10-dependent manner. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.