Crystal Structures of the Scaffolding Protein LGN Reveal the General Mechanism by Which GoLoco Binding Motifs Inhibit the Release of GDP from Gαi *

Background: GoLoco (GL) motif binds to Gα and inhibits its guanine nucleotide dissociation. Results: Crystal structures of LGN-GL3(4)·Gαi1(3) complexes reveal a double Arg finger-mediated GDP recognition mechanism, which is distinct from that shown in the RGS14·Gαi1 complex. Conclusion: LGN-GL/Gαi interaction might represent a general binding mode between GoLoco motifs and Gαi. Significance: Our findings shed new light on the GoLoco motif-mediated G protein signaling regulation. GoLoco (GL) motif-containing proteins regulate G protein signaling by binding to Gα subunit and acting as guanine nucleotide dissociation inhibitors. GLs of LGN are also known to bind the GDP form of Gαi/o during asymmetric cell division. Here, we show that the C-terminal GL domain of LGN binds four molecules of Gαi·GDP. The crystal structures of Gαi·GDP in complex with LGN GL3 and GL4, respectively, reveal distinct GL/Gαi interaction features when compared with the only high resolution structure known with GL/Gαi interaction between RGS14 and Gαi1. Only a few residues C-terminal to the conserved GL sequence are required for LGN GLs to bind to Gαi·GDP. A highly conserved “double Arg finger” sequence (RΨ(D/E)(D/E)QR) is responsible for LGN GL to bind to GDP bound to Gαi. Together with the sequence alignment, we suggest that the LGN GL/Gαi interaction represents a general binding mode between GL motifs and Gαi. We also show that LGN GLs are potent guanine nucleotide dissociation inhibitors.

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