Serine/threonine kinase which acts as a negative regulator of Ras-related Rap2-mediated signal transduction to control neuronal structure and AMPA receptor trafficking. Required for normal synaptic density, dendrite complexity, as well as surface AMPA receptor expression in hippocampal neurons. Can activate the JNK and MAPK14/p38 pathways and mediates stimulation of the stress-activated protein kinase MAPK14/p38 MAPK downstream of the Raf/ERK pathway. Phosphorylates: TANC1 upon stimulation by RAP2A, MBP and SMAD1. Has an essential function in negative selection of thymocytes, perhaps by coupling NCK1 to activation of JNK1. Isoform 4 can activate the JNK pathway. Involved in the regulation of actin cytoskeleton reorganization, cell-matrix adhesion, cell-cell adhesion and cell migration. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with TANC1 (By similarity). Interacts with RAP2A. Isoform 4 interacts with NCK1. Expressed in the brain, isoform 2 is more abundant than isoform 1. Isoform 3 is ubiquitously expressed. Isoform 1 is most abundant in the skeletal muscle. Isoform 4 is ubiquitously expressed with relative high levels in brain, skeletal muscle, pancreas and testis. Activated after Ras induction via a mechanism involving reactive oxygen species. Autophosphorylated. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.