Structure of a GEM‐complexed Gai Protein Provides Novel Insights into the Emerging Human GEMiome

Heterotrimeric G proteins are major signaling hubs and essential components of the signal gating machinery in eukaryotic cells. In their best‐studied capacity, G proteins transmit signals from integral membrane G protein coupled receptors (GPCRs) to intracellular effectors. Recently, a rapidly emerging paradigm revealed the existence of a novel class of G protein modulators: cytosolic GEMs (Guanine nucleotide Exchange Modulators). GEMs can fuel pathological networks by aberrantly coupling G protein signaling to a broad variety of cell surface receptors (growth factor RTKs, integrins, Wnt/Frizzled receptors, cytokine receptors, and others) and by amplifying this signaling in an unrestricted manner. They act pleiotropically via a conserved motif that activates cAMP‐inhibitory (Gαi) but inhibits cAMP‐stimulatory (Gαs) G proteins, thus helping to integrate downstream responses. The Ga‐interacting motif of the prototypical GEM GIV (Ga‐interacting Vesicle associated) was identified [1] based on distant homology with a bifunctional synthetic peptide KB‐752 [2]. Following the discovery of several additional GEMs with homologous G protein binding motifs [3, 4], it is now clear that the size of the human GEMiome and its footprint on diverse cellular processes are larger than initially expected. We recently solved an X‐ray structure of a Gai protein in complex with a GEM motif of GIV [5]. The structure demonstrated that GEMs act through a mechanism that is both unexpected and different from that employed by canonical G protein triggers, GPCRs. Using molecular modeling, we elucidated the structural basis of action of two additional GEMs, Daple and NUCB1. All studied GEMs appear to allosterically induce a low GDP affinity conformation of the nucleotide pocket by binding at the conformationally variable, nucleotide sensitive switch II region of the Gai molecule. Such binding is achieved through a precise set of hydrophobic and polar interactions involving residue positions conserved across all known GEMs. The obtained atomic resolution understanding of GEM action enabled prediction of novel members of the human GEMiome through structure‐ and model‐guided genome‐wide sequence searches. The GEMiome is an untapped source of revelations in the field of cancer biology as well as promising targets for anti‐cancer therapeutic discovery. The insights obtained through this study will help reveal novel cancer targets and enable rational design of truly effective therapeutics against them. Support or Funding Information Supported by NIH grants R01 AI118985 and R01 GM117424.