Serine/threonine kinase which acts as a negative regulator of Ras-related Rap2-mediated signal transduction to control neuronal structure and AMPA receptor trafficking (PubMed:10708748, PubMed:16337592). Required for normal synaptic density, dendrite complexity, as well as surface AMPA receptor expression in hippocampal neurons (By similarity). 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 (PubMed:18930710, PubMed:21690388). Has an essential function in negative selection of thymocytes, perhaps by coupling NCK1 to activation of JNK1 (By similarity). Activator of the Hippo signaling pathway which plays a pivotal role in organ size control and tumor suppression by restricting proliferation and promoting apoptosis. MAP4Ks act in parallel to and are partially redundant with STK3/MST2 and STK4/MST2 in the phosphorylation and activation of LATS1/2, and establish MAP4Ks as components of the expanded Hippo pathway (PubMed:26437443). ; Isoform 4 can activate the JNK pathway. Involved in the regulation of actin cytoskeleton reorganization, cell-matrix adhesion, cell-cell adhesion and cell migration.
