Pseudophosphatase MK-STYX: the atypical member of the MAP kinase phosphatases

The regulation of the phosphorylation of mitogen-activated protein kinases (MAPKs) is essential for cellular processes such as proliferation, differentiation, survival, and death. Mutations within the MAPK signaling cascades are implicated in diseases such as cancer, neurodegenerative disorders, arthritis, obesity, and diabetes. MAPK phosphorylation is controlled by an intricate balance between MAPK kinases (enzymes that add phosphate groups) and MAPK phosphatases (MKPs) (enzymes that remove phosphate groups). MKPs are complex negative regulators of the MAPK pathway that control the amplitude and spatiotemporal regulation of MAPKs. MK-STYX (MAPK phosphoserine/threonine/tyrosine-binding protein) is a member of the MKP subfamily, which lacks the critical histidine and nucleophilic cysteine residues in the active site required for catalysis. MK-STYX does not influence the phosphorylation status of MAPK, but even so it adds to the complexity of signal transduction cascades as a signaling regulator. This review highlights the function of MK-STYX, providing insight into MK-STYX as a signal regulating molecule in the stress response, HDAC 6 dynamics, apoptosis, and neurite differentiation.