Functional role of histidine in the conserved His-x-Asp motif in the catalytic core of protein kinases

The His-x-Asp (HxD) motif is one of the most conserved structural components of the catalytic core of protein kinases; however, the functional role of the conserved histidine is unclear. Here we report that replacement of the HxD-histidine with Arginine or Phenylalanine in Aurora A abolishes both the catalytic activity and auto-phosphorylation, whereas the Histidine-to-tyrosine impairs the catalytic activity without affecting its auto-phosphorylation. Comparisons of the crystal structures of wild-type (WT) and mutant Aurora A demonstrate that the impairment of the kinase activity is accounted for by (1) disruption of the regulatory spine in the His-to-Arg mutant, and (2) change in the geometry of backbones of the Asp-Phe-Gly (DFG) motif and the DFG-1 residue in the His-to-Tyr mutant. In addition, bioinformatics analyses show that the HxD-histidine is a mutational hotspot in tumor tissues. Moreover, the H174R mutation of the HxD-histidine, in the tumor suppressor LKB1 abrogates the inhibition of anchorage-independent growth of A549 cells by WT LKB1. Based on these data, we propose that the HxD-histidine is involved in a conserved inflexible organization of the catalytic core that is required for the kinase activity. Mutation of the HxD-histidine may also be involved in the pathogenesis of some diseases including cancer.