Serine/threonine-protein kinase that acts as a mediator of cell growth (PubMed:11641781, PubMed:17360971). Modulates apoptosis (PubMed:11641781, PubMed:17360971). In association with STK24 negatively regulates Golgi reorientation in polarized cell migration upon RHO activation (PubMed:27807006). Phosphorylates ATG4B at 'Ser-383', thereby increasing autophagic flux (PubMed:29232556). 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. Homodimer (PubMed:20730082). Interacts with PDCD10 (PubMed:17360971, PubMed:19370760, PubMed:20332113). Interacts with GOLGA2 (PubMed:15037601, PubMed:20332113). Interacts with CTTNBP2NL (PubMed:18782753). Interacts with RIPOR1 (via C-terminus); this interaction occurs in a PDCD10-dependent and Rho-independent manner (PubMed:27807006). Interacts with PDCD10; this interaction is required for the association of STK26 with RIPOR1 (PubMed:27807006). 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 (PubMed:27807006). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.