The pseudophosphatase MK ‐ STYX inhibits stress granule assembly independently of Ser149 phosphorylation of G3 BP ‐1

The pseudophosphatase MK ‐ STYX (mitogen‐activated protein kinase phosphoserine/threonine/tyrosine‐binding protein) has been implicated in the stress response pathway. The expression of MK ‐ STYX inhibits the assembly of stress granules, which are cytoplasmic storage sites for mRNA that form as a protective mechanism against stressors such as heat shock, UV irradiation and hypoxia. Furthermore, MK ‐ STYX interacts with a key component of stress granules: G 3 BP ‐1 ( R as‐ GTP ase activating protein SH 3 domain binding protein‐1). Because G 3 BP ‐1 dephosphorylation at S er149 induces stress granule assembly, we initially hypothesized that the inhibition of stress granules by MK ‐ STYX was G 3 BP ‐1 phosphorylation‐dependent. However, in the present study, using MK ‐ STYX constructs and G 3 BP ‐1 phosphomimetic or nonphosphorylatable mutants, we show that MK ‐ STYX inhibits stress granule formation independently of G 3 BP ‐1 phosphorylation at Ser149. The introduction of point mutations at the ‘active site’ of MK ‐ STYX that convert serine and phenylalanine to histidine and cysteine, respectively, is sufficient to generate an active enzyme. In separate experiments, we show that this active mutant, MK ‐ STYX active , has opposite effects to wild‐type MK ‐ STYK . Not only does MK ‐ STYX active induce stress granules, but also it has the capacity to dephosphorylate G 3 BP ‐1. Taken together, these results provide evidence that the pseudophosphatase MK ‐ STYX plays a key role in the cellular response to stress.