title: Protein phosphatase 4 is a positive regulator of hematopoietic progenitor kinase 1.
abstract: Hematopoietic progenitor kinase 1 (HPK1) is a hematopoietic specific mammalian Ste20-like protein kinase and has been implicated in many cellular signaling pathways including T cell receptor (TCR) signaling. However, little is known about the in vivo regulation of HPK1. We present evidence that HPK1 is positively regulated by protein phosphatase 4 (PP4; also called PPX and PPP4), a serine/threonine phosphatase. We found that PP4 interacted with HPK1 and that the proline-rich region of HPK1 was necessary and sufficient for this interaction. We also found that PP4 had phosphatase activity toward HPK1 in vivo and that co-transfection of PP4 with HPK1 resulted in specific kinase activation of HPK1. Moreover, we found that the PP4-induced HPK1 kinase activation was accompanied by an increase in protein expression of HPK1. Pulse-chase analysis showed that PP4 increased the half-life of HPK1. Further studies showed that HPK1 was subject to regulation by ubiquitination and ubiquitin-targeted degradation and that PP4 inhibited HPK1 ubiquitination. In addition, we found that TCR stimulation enhanced the PP4-HPK1 interaction and that wild-type PP4 enhanced, whereas a phosphatase-dead PP4 mutant inhibited, TCR-induced activation of HPK1 in Jurkat T cells. Combined with the observation that PP4 enhanced HPK1-induced JNK activation, our studies identify PP4 as a positive regulator for HPK1 and the HPK1-JNK signaling pathway.
authors.affiliation: Department of Immunology, Baylor College of Medicine, Houston, Texas 77030, USA.
authors.forename: Guisheng
authors.initials: G
authors.lastname: Zhou
authors.forename: Jonathan S
authors.initials: JS
authors.lastname: Boomer
authors.forename: Tse-Hua
authors.initials: TH
authors.lastname: Tan
mesh_terms: D020816:Amino Acid Motifs; D015153:Blotting, Western; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D047468:Immunoprecipitation; D019169:Jurkat Cells; D009154:Mutation; D010749:Phosphoprotein Phosphatases / Q000502:physiology*; D010766:Phosphorylation; D011392:Proline / Q000737:chemistry; D011485:Protein Binding; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D015398:Signal Transduction; D013997:Time Factors; D014162:Transfection; D025801:Ubiquitin / Q000378:metabolism
chemical_list: D025801:Ubiquitin; D011392:Proline; C102102:hematopoietic progenitor kinase 1; D017346:Protein Serine-Threonine Kinases; D010749:Phosphoprotein Phosphatases; C080406:protein phosphatase 4