The Dynamics of Pseudophosphatase MK‐STYX and HDAC6 Ubiquitination

Abstract

MK‐STYX [mitogen‐activated protein kinase phospho‐serine/threonine/tyrosine binding protein] is a pseudophosphatase and a member of the MAP kinase phosphatase family. Due to the absence of cysteine and histidine residues in the signature active site motif (HCX5R), MK‐STYX is catalytically inactive. Even so, it maintains the ability to bind phosphorylated residues. MK‐STYX is an important molecule in various signaling pathways, including the stress response pathway. Our previous research showed that the expression of MK‐STYX decreases stress granules [SG], which are aggregates of untranslated mRNA and misfolded proteins caused by cellular stress. However, the molecular mechanism by which MK‐STYX decreases SG remains elusive. Histone deacetylase isoform 6 (HDAC6) serves as a major component of SG. Therefore, we investigated whether MK‐STYX affects the dynamics of HDAC6. Our findings showed that MK‐STYX alters the subcellular localization of HDC6, which became nuclear, and decreased and altered the location of HDAC6‐aggregates. Because HDAC6 contains a ubiquitin binding domain, we investigated whether MK‐STYX affects the ubiquitination of HDAC6 and/or the ubiquitin proteasome system. Ubiquitin assays in stressed and unstressed cells showed that ubiquitinated proteins increased in the presence of MK‐STYX. Immunoprecipitation of HDAC6 and ubiquitin assays show that HDAC6‐ubiquitin interactions decreased in the presence of MK‐STYX, when HEK‐293 cells were exposed to MG132, whereas HDAC6‐ubiquitin interaction increased in cells expressing the active mutant of MK‐STYX (MK‐STYX(active)), which dephosphorylates phosphorylated residues. Overall, our research suggests that MK‐STYX affects ubiquitination; however, further studies are important to determine whether the effects of MK‐STYX on HDAC6 are linked to the decrease in stress granules.Support or Funding InformationDepartment of Biology, Integrated Science Center, William and Mary, Williamsburg, VA 23187