Analyzing Pseudophosphatase Function

Understanding Pseudophosphatase Function Through Biochemical Interactions

Pseudophosphatases have been solidified as important signaling molecules that regulate signal transduction cascades. However, their mechanisms of action remain enigmatic. Reflecting this mystery, the prototypical pseudophosphatase STYX (phospho-serine-threonine/tyrosine-binding protein) was named with allusion to the river of the dead in Greek mythology to emphasize that these molecules are "dead" phosphatases. Although proteins with STYX domains do not catalyze dephosphorylation, this does not preclude their having other functions, including as integral elements of signaling networks. Thus, understanding their roles may mark them as potential novel drug targets. This chapter outlines common strategies used to characterize the functions of pseudophosphatases, using as an example MK-STYX [MAPK (mitogen-activated protein kinase) phospho-serine-threonine/tyrosine-binding], which has been linked to tumorigenesis, hepatocellular carcinoma, glioblastoma, apoptosis, and neuronal differentiation. We start with the importance of "restoring" (when possible) phosphatase activity in a pseudophosphatase, so the active mutant may be used as a comparison control throughout immunoprecipitation and mass spectrometry analyses. To this end, we provide protocols for site-directed mutagenesis, mammalian cell transfection, co-immunoprecipitation, phosphatase activity assays, and immunoblotting that we have used to investigate MK-STYX and the active mutant MK-STYXactive. We also highlight the importance of utilizing RNA interference (RNAi) "knockdown" technology to determine a cellular phenotype in various cell lines. Therefore, we outline our protocols for introducing short hairpin RNA (shRNA) expression plasmids into mammalian cells and quantifying knockdown of gene expression with real-time quantitative PCR (qPCR). We also provide a bioinformatic approach to investigating MK-STYX and MK-STYX(active mutant). These bioinformatic approaches can stand alone experimentally but also complement and enhance "wet" bench approaches such as binding assays and/or activity assays. A combination of cellular, molecular, biochemical, proteomic, and bioinformatic techniques has been a powerful tool in identifying novel functions of MK-STYX. Likewise, the information provided here should be a helpful guide to elucidating the functions of other pseudophosphatases.

Pseudophosphatase STYXL1 depletion enhances glucocerebrosidase trafficking to lysosomes via ER stress

Pseudophosphatases are catalytically inactive but share sequence and structural similarities with classical phosphatases. STYXL1 is a pseudophosphatase that belongs to the family of dual-specificity phosphatases and is known to regulate stress granule formation, neurite formation and apoptosis in different cell types. However, the role of STYXL1 in regulating cellular trafficking or the lysosome function has not been elucidated. Here, we show that the knockdown of STYXL1 enhances the trafficking of β-glucocerebrosidase (β-GC) and its lysosomal activity in HeLa cells. Importantly, the STYXL1-depleted cells display enhanced distribution of endoplasmic reticulum (ER), late endosome and lysosome compartments. Further, knockdown of STYXL1 causes the nuclear translocation of unfolded protein response (UPR) and lysosomal biogenesis transcription factors. However, the upregulated β-GC activity in the lysosomes is independent of TFEB/TFE3 nuclear localization in STYXL1 knockdown cells. The treatment of STYXL1 knockdown cells with 4-PBA (ER stress attenuator) significantly reduces the β-GC activity equivalent to control cells but not additive with thapsigargin, an ER stress activator. Additionally, STYXL1-depleted cells show the enhanced contact of lysosomes with ER, possibly via increased UPR. The depletion of STYXL1 in human primary fibroblasts derived from Gaucher patients showed moderately enhanced lysosomal enzyme activity. Overall, these studies illustrated the unique role of pseudophosphatase STYXL1 in modulating the lysosome function both in normal and lysosome-storage disorder cell types. Thus, designing small molecules against STYXL1 possibly can restore the lysosome activity by enhancing ER stress in Gaucher disease.