(A) Left panel, cartoon representation of structure of AtITPK4 coloured by domain: HAD domain (pink), kinase N-terminal domain (light blue), kinase central domain (lime green) and kinase C-terminal domain (sand). The residues of the tab insertion in AtITPK4 are coloured magenta while those of the tether are shown in blue. Broken lines in the backbone trace indicate residues unresolved in the model due to disorder. The sidechains of residues forming the unique ion pair in AtITPK4 are shown in stick format with interactions indicated by dotted black lines. Bound ATP is shown in stick format with atom colouring as follows: carbon-green, oxygen-red, nitrogen-blue and phosphorus-orange. Inset: the 2F_o−F_c electron density map (grey mesh) in the region of ATP is contoured at 1.5 σ. Right panel, molecular surface representation of the structure of AtITPK4 coloured by domain and insertion. (B) Left panel, cartoon representation of the structure of human ITPK1 (HsITPK1). Bound ADP is shown in stick format. The orientation of view, and atom and domain colouring follows that shown in panel (a). Right panel, molecular surface representation of the HsITPK1 structure coloured by domain and insertion. (C) Left panel, a view of the molecular surface of AtITPK4 coloured by electrostatic potential (red-acidic, blue-basic). The orientation of the molecule is the same as that in panel (A). Right panel, view of the electrostatic potential of the HAD-like domain. The view represents a 90° rotation to that shown the left panel such that the HAD-like domain is viewed head on.