Structural homology discloses a bifunctional structural motif at the N-termini of G_α proteins

In a family of proteins, often the three-dimensional structure has been experimentally determined only for one member or a few members of the family. Homology modeling can be used to model the structures of all other members of the family and thus allow comparison of these structures. This approach was applied to heterotrimeric G proteins that require anchorage to the plasma membrane to properly interact with membrane-bound receptors and downstream effectors. Lipid modification by palmitoylation is a fundamental contributor to this localization, but the signals leading to this modification are still unknown. In this work, homology models of all the different human G_α paralogs were generated using automated homology modeling, and the electrostatic potential of these proteins was calculated and visualized. This approach identifies a basic, positively charged, structural motif in the N-termini of heterotrimeric G proteins, which is not readily discernible from sequence alone. The basic motif is much reduced in those G_α subunits that also undergo myristoylation, suggesting that the basic patches and myristoylation play overlapping roles. These motifs can affect both membrane affinity and orientation and determine the palmitoylation of G_α subunits in cooperation with the G_βγ subunits, as has been corroborated by previous experimental studies. Furthermore, other palmitoylated proteins such as GAP-43 and RGS proteins share this α-helical basic motif in their N-terminus. It therefore appears that this structural motif is more widely applicable as a membrane-targeting and palmitoylation-determining signal. The work presented here highlights the possibilities available for experimentalists to discover structural motifs that are not readily observed by analysis of the linear sequence.