Activation of Hematopoietic Progenitor Kinase 1 Involves Relocation, Autophosphorylation, and Transphosphorylation by Protein Kinase D1

Abstract

Innate and adaptive immune functions are governed by instructive signals from receptors that sense pathogens and toxins. Ligation of lymphocyte antigen receptors triggers a series of characteristic events that is initiated by activation of Src family tyrosine kinases followed by recruitment and activation of Syk and BTK family nonreceptor tyrosine kinases (18). The ensuing tyrosine phosphorylation provides docking sites for SH2 and PTB domains on adaptor proteins and enzymes, which assemble into a spatially highly defined complex that will ultimately cause lymphocyte activation. For instance, in T cells, the adaptor proteins SLP-76 and Gads (Grb2-related adaptor downstream of Shc) are recruited to the transmembranous adaptor LAT, forming a central scaffold for the activation of several effector molecules, such as phospholipase C␥ (PLC␥), Ras, and Rho GTPases (15). These events are followed by the activation of a number of different serine/threonine kinases, which include members of the protein kinase B (PKB), PKC, and PKD families as well as mitogen-activated protein 3 (MAP3) and MAP4 kinases, such as mitogen-activated protein kinase kinase kinase 1 (MEKK1) and hematopoietic progenitor kinase 1 (HPK1). Further downstream, mitogen-activated protein kinases (MAPKs) which contribute to the immune response are efficiently triggered; for instance, stress-activated protein kinases (SAPKs)/c-Jun N-terminal kinases (JNKs) have been shown to regulate critical steps in T-helper-cell differentiation (8). Transcription factors of the NF-B family constitute an additional major pathway involved in the regulation of lymphocytes and execution of immune functions. NF-B transcription factors determine life and death decisions in developing lymphocytes and also provide important costimulatory signals in T cells (16).Lymphocyte receptor signaling can be coupled to SAPK/ JNK and NF-B activation by HPK1 (18). HPK1 is a mammalian hematopoiesis-specific Ste20 homologue belonging to the germinal center (GC) kinase family, which contains at least 20 kinases (6). HPK1 is characterized by an N-terminal kinase domain, followed by an intermediate region and a C-terminal Citron homology domain. Both the N-terminal kinase domain and the C-terminal Citron homology domain are highly conserved among related kinases of the GC family. In the GC family, HPK1 is most closely related to germinal center kinase (GCK), germinal center-like kinase (GLK), and germinal center-related kinase (GCKR), which together form subfamily I of GCKs (19). The biological function of these Ste20 homologues is largely elusive. GCK and GCKR have been implicated in tumor necrosis factor receptor signaling (36, 42). More recently, HPK1 has been shown to be a positive regulator of cell death in T cells (35).HPK1 has been demonstrated to be a potent and highly specific activator of the SAPKs/JNKs in various cell types, a property shared with the related GCK family members (18,19). HPK1 likely acts at the level of a MAP4 kinase, resulting in the activation of MAP3 kinases, such...

The CUL7/F-box and WD Repeat Domain Containing 8 (CUL7/Fbxw8) Ubiquitin Ligase Promotes Degradation of Hematopoietic Progenitor Kinase 1

“…In addition, previous study also found that HPK1 autophosphorylation occurs predominantly on threonine residues (10). Using PhosphoSitePlus program, we identified two threonine residues, Thr-349 and Thr-355, in HPK1-PR region (Fig.…”

Identification of a novel phosphorylation site in c-jun directly targeted in vitro by protein kinase D

“…More recent reports advance the concept that PKD activity modulates JNK signaling to c-jun at multiple levels. Thus, PKD activates distinct MAP3Ks operating in JNK activation pathways, including hematopoietic progenitor kinase, HPK1 [10] and, when activated in response to oxidative stress [11], the apoptosis signaling kinase ASK1 [12]. In our own previous studies we found that purified PKD directly phosphorylated a GST-c-jun(1-89) (N-terminal) fusion protein in vitro, but the sites were not identified [13].…”

Sustained JNK signaling by proteolytically processed HPK1 mediates IL-3 independent survival during monocytic differentiation

“…9,10 HPK1 activation involves binding of cell-specific adaptor proteins, relocation to the plasma membrane, autophosphorylation and transphosphorylation by protein kinase D1. [11][12][13] While a role for HPK1 in the regulation of T-cell apoptosis was already suggested, 14 our recent studies have shown proteolytic processing of HPK1 into HPK1-C in non-apoptotic preactivated primary T cells. 15 In this study, the cleavage product HPK1-C sensitizes towards T-cell receptor-mediated cell death, while full-length HPK1 enables activation and survival of T cells.…”