Tumor suppressor serine/threonine-protein kinase involved in mTORC1 signaling and post-transcriptional regulation. Phosphorylates FOXO3, ERK3/MAPK6, ERK4/MAPK4, HSP27/HSPB1, p53/TP53 and RHEB. Acts as a tumor suppressor by mediating Ras-induced senescence and phosphorylating p53/TP53. Involved in post-transcriptional regulation of MYC by mediating phosphorylation of FOXO3: phosphorylation of FOXO3 leads to promote nuclear localization of FOXO3, enabling expression of miR-34b and miR-34c, 2 post-transcriptional regulators of MYC that bind to the 3'UTR of MYC transcript and prevent MYC translation. Acts as a negative regulator of mTORC1 signaling by mediating phosphorylation and inhibition of RHEB. Part of the atypical MAPK signaling via its interaction with ERK3/MAPK6 or ERK4/MAPK4: the precise role of the complex formed with ERK3/MAPK6 or ERK4/MAPK4 is still unclear, but the complex follows a complex set of phosphorylation events: upon interaction with atypical MAPK (ERK3/MAPK6 or ERK4/MAPK4), ERK3/MAPK6 (or ERK4/MAPK4) is phosphorylated and then mediates phosphorylation and activation of MAPKAPK5, which in turn phosphorylates ERK3/MAPK6 (or ERK4/MAPK4). Mediates phosphorylation of HSP27/HSPB1 in response to PKA/PRKACA stimulation, inducing F-actin rearrangement. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated following phosphorylation at Thr-182 by p38-alpha/MAPK14, p38-beta/MAPK11, ERK2/MAPK1, ERK3/MAPK6, and ERK4/MAPK4. Activated by stress-related extracellular stimuli; such as H(2)O(2), arsenite, anisomycin TNF alpha and also PMA and the calcium ionophore A23187; but to a lesser extent. In vitro, activated by SQSTM1. Inhibited by diterpenoid alkaloid noroxoaconitine. Interacts with ERK3/MAPK6 and ERK4/MAPK4 (via FRIEDE motif); the interaction is direct (By similarity). Interacts with YWHAE; the interaction prevents phosphorylation of HSP27/HSPB1 leading to disrupt F-actin polymerization. Interacts with SQSTM1. Translocates to the cytoplasm following phosphorylation and activation. Interaction with ERK3/MAPK6 or ERK4/MAPK4 and phosphorylation at Thr-182, activates the protein kinase activity, followed by translocation to the cytoplasm. Phosphorylation by PKA/PRKACA at Ser-115 also induces nuclear export. Expressed ubiquitously. Directly regulated by MYC: expression is activated by MYC, suggesting the existence of a feedback regulatory loop. Phosphorylated on Thr-182 ERK3/MAPK6 or ERK4/MAPK4; which is the regulatory phosphorylation site and is located on the T-loop/loop 12, leading to activation. Phosphorylation at Thr-182 by p38-alpha/MAPK14, p38-beta/MAPK11 is subject to debate. Phosphorylated at Ser-115 by PKA/PRKACA, leading to localization to the cytoplasm. Autophosphorylated (By similarity). Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family. The role of p38 MAPK kinases is unclear in phosphorylation and activation of MAPKAPK5. According to some reports, it interacts and is phosphorylated by p38-alpha/MAPK14 and p38-beta/MAPK11 (PubMed:9628874 and PubMed:12808055). According to other reports, it is not activated by p38-alpha/MAPK14 and p38-beta/MAPK11. An explanation for these discrepancies, might be that the interaction with p38 MAPK kinases is weak and occurs only under specific conditions.