Structure of type II-β PIP4K and models of type I PIP5K-α, -β, and -γ isoforms. (a–d) The reported structure of type II-β PIP4K (a) and the predicted structures of the PIP5K-α, -β, and -γ90 isoforms deduced by homology modeling are shown in an equivalent orientation with their putative membrane-interacting face pointing toward the observer. The surfaces are colored according to the range of electrostatic potential (red, <−10.0 kT/e; blue, >10.0 kT/e [where k = Boltzmann constant, T = absolute temperature, and e = electron]). The electrostatic surface potentials were computed using the continuum solvation model embodied in the Poisson–Boltzmann method, and the adaptive Poisson–Boltzmann solver was implemented in the APBS software. (e–h) The proteins are shown in an orientation corresponding to a 90° rotation from the orientations in a–d such that the presumed membrane-associated face of the proteins points downward. The ATP-binding site and a reported phosphorylation site are highlighted by yellow and green circles, respectively. The dipole moments (yellow arrows) were calculated directly from the atomic models and the atom partial charges using Protein Dipole Moments Server. The magnitude of the arrows is directly proportional to the strength on the dipole (Debye).