Atypical p38 kinase signaling and pathological impact on human health.

Protein phosphorylation is a fundamental mechanism regulating signal transduction, metabolic control, and cellular adaptation to environmental stress, with these processes orchestrated largely by protein kinases organized into hierarchical signaling cascades. Among these, the p38 mitogen-activated protein kinases (MAPKs) play a central role in stress and inflammatory signaling and are classically characterized by activation by upstream kinases MKK3 and MKK6. However, accumulating biochemical, structural, and cellular evidence demonstrates that p38-particularly the p38α isoform-can also be activated through atypical, MKK3/6-independent mechanisms driven by direct protein-protein interactions that promote kinase autophosphorylation. In the present review, we synthesize current insights into atypical p38 activation, comparing evidence for signaling mediated by the transforming growth factor β-activated kinase 1 binding protein 1, zeta-chain-associated protein kinase 70, the Hepatitis C virus core protein, and the Toxoplasma gondii protein GRA24. We examine the molecular and structural determinants of their interactions, the conformational changes that enable cis-autophosphorylation, and the resulting differences in regulation and substrate engagement compared with canonical p38 signaling. We further discuss evidence linking atypical p38 activation to selective pathological outcomes in models of cardiac ischemia, infection, inflammation, and vascular dysfunction. Together, these studies establish atypical p38 signaling as a distinct biochemical regulatory paradigm that expands our understanding of MAPK signaling specificity and context-dependent kinase function.