Hematopoietic Progenitor Kinase-1 Structure in a Domain-Swapped Dimer

Abstract

Highlights d The active and inactive HPK1 kinase in apo and ligand bound forms are characterized d Structures show rare domain-swapped dimer involving helical activation segment d Dimerization observed in solution suggesting potential biologically relevance d Ligand complex structure provides insights for structurebased drug design In BriefEnhancement of antigen-specific T cell immunity has shown significant therapeutic benefit in infectious diseases and cancer. Hematopoietic progenitor kinase-1 (HPK1) is one of the intracellular regulators that dampens T cell receptor signaling. Wu et al. studied the molecular structure of HPK1 kinase with bound ligand, which provides insights for structure-based drug design. SUMMARYEnhancement of antigen-specific T cell immunity has shown significant therapeutic benefit in infectious diseases and cancer. Hematopoietic progenitor kinase-1 (HPK1) is a negative-feedback regulator of T cell receptor signaling, which dampens T cell proliferation and effector function. A recent report showed that a catalytic dead mutant of HPK1 phenocopies augmented T cell responses observed in HPK1knockout mice, indicating that kinase activity is critical for function. We evaluated active and inactive mutants and determined crystal structures of HPK1 kinase domain (HPK1-KD) in apo and ligand bound forms. In all structures HPK1-KD displays a rare domain-swapped dimer, in which the activation segment comprises a well-conserved dimer interface. Biophysical measurements show formation of dimer in solution. The activation segment adopts an a-helical structure which exhibits distinct orientations in active and inactive states. This face-to-face configuration suggests that the domain-swapped dimer may possess alternative selectivity for certain substrates of HPK1 under relevant cellular context.

A perspective on HPK1 as a novel immuno-oncology drug target

“…The caspase-cleaved N-terminal domain of HPK1 becomes catalytically active upon separation from its C-terminal fragment, which suggest that the C-terminal region of HPK1 is involved in the auto-or trans-regulation of HPK1 kinase activity. This speculation is strengthened by the recent X-ray crystallographic studies revealing that the three-dimensional structure of HPK1 kinase domain exists as a head-to-head domain-swapped dimer in both the crystal form, as well as in the liquid form Wu et al, 2019;Johnson et al, 2019. In the non-phosphorylated, inactive state, the activation loop of each HPK1 monomer partially occupies the ATP-and substrate-binding sites of its partner monomer, thus serving as a possible mechanism to quiesce the basal kinase activity of HPK1. This trans-inhibitory posture of the activation loop is disrupted when the threonine residue 165 and serine residue 171 are phosphorylated (Johnson et al, 2019).…”

“…The uncommon domain-swapped dimeric conformation and the structural changes accompanying HPK1 trans-regulation revealed by these studies may facilitate future insights regarding the strategy to enhance selectivity of the small molecule inhibitor of HPK1. (Wu et al, 2019;Johnson et al, 2019;Sawasdikosol and Burakoff, 2019). As an alternative approach, large pharmaceutical companies with sufficient resources might engage in an unbiased high-throughput screen of their small molecule libraries for allosteric inhibitors of HPK1.…”

The Structure of HPK1 Kinase Domain: To Boldly Go Where No Immuno-Oncology Drugs Have Gone Before

“…In this issue of Structure, Wu et al (2018) report several apo and small-molecule inhibitor-bound structures of the kinase domain of hematopoietic progenitor kinase 1, a ser/thr kinase that functions as an inhibitor of T cell activation. The studies reveal that the HPK1 kinase domain exists as a domain-swapped dimer.…”

“…Structures of HPK1 revealed by Wang and colleagues represent one of the critical initial steps necessary to facilitate the development of exquisitely specific small-molecule-based immune response modulators of HPK1 that are capable of augmenting anti-tumor immunity (Wu et al, 2018). There are many compelling reasons why Wu et al chose to determine the structures of this important hematopoietic cell-restricted kinase.…”

“…The HPK1 structures solved by the Wang group (Wu et al, 2018) provide a number of interesting insights into the structural characteristics of the HPK1 kinase domain in several conformational states. While possessing the overall conserved bilobed structure of a typical kinase domain, two distinctive features were observed in all HPK1 variants reported in this study: the presence of a three-turn a-helical structure at the N terminus of the activation segment (AS) and the existence of HPK1 as a ''face-toface'' domain-swapped dimer in the crystallographic state as well as in solution.…”