One fascinating aspect of the cellular signaling network is the crosstalk between G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). GPCRs, known for their involvement in a myriad of physiological processes, mediate mostly signaling through heterotrimeric G proteins. On the other hand, RTKs play pivotal roles in growth factor signaling, regulating cell growth, differentiation, and survival. The interaction between these two receptor types raises intriguing questions about how their signaling pathways intersect and influence each other to orchestrate cellular responses.
The cross-talk between growth factor receptors and GPCRs is an intriguing area of study. Recently, Suchismita Roy et al. have explored how growth factors can modulate canonical G protein signaling, shedding light on molecular mechanisms with significant implications for both physiology and pathology.
The primary research question addressed in this study was: How do growth factors, specifically through RTKs, modulate canonical heterotrimeric G protein signaling?
The researchers focused on the phosphorylation of the G protein subunit Gαi at specific residues within three strategic hotspots: the P loop, the interdomain cleft, and the C terminus. They employed a series of phospho-mimicking mutants and assays such as linear ion trap mass spectrometry, BRET, pull down, and transwell cell migration assays to investigate the effects of these modifications on key signaling events such as receptor recruitment, trimer dissociation, cAMP inhibition, and chemotaxis.
The study revealed that the epidermal growth factor-induced phosphorylation of Gαi at specific residues predominantly inhibits ligand-induced signaling while promoting constitutive Gβγ signaling.
Key findings include:
Phosphorylation Hotspots:
P Loop (Ser44, Ser47, Thr48): Impairs ligand-stimulated Gβγ release and cAMP suppression, potentially acting as dominant-negative proteins.
Interdomain Cleft (Ser151, Tyr154, Tyr155): Phosphorylation at Tyr154 and Tyr155 impaired ligand-stimulated Gβγ release, cAMP suppression, and chemotaxis while enhancing constitutive Gβγ signaling. Computational analyses indicated that these modifications favor the nucleotide-free state of Gαi.
C Terminus (Tyr320): Phosphorylation at Tyr320 disrupted Gβγ binding, receptor coupling, and ligand-stimulated signaling, resulting in impaired chemotaxis. Notably, the Y320F mutation restored some signaling capabilities, emphasizing Tyr320's role in membrane localization and signaling efficiency.
Mechanisms of Inhibition: The study identified three distinct mechanisms by which phosphorylation inhibits canonical signaling, based on the location of the phosphosites within the G protein.
Signaling Pathway Segregation: Phosphorylation events in the interdomain cleft and P loop uncouple G proteins from GPCRs, leading to segregation of RTK-to-Gαi pathways from canonical GPCR-to-Gαi pathways.
These findings have broad implications for understanding cellular signaling in both normal physiology and pathological conditions. The ability of growth factors to modulate G protein signaling through specific phosphorylation events highlights a nuanced regulatory mechanism that could be crucial in contexts such as cancer progression and wound healing. For instance, the sequestration of G proteins from canonical GPCR-dependent pathways by growth factor signaling might contribute to altered cellular behaviors in cancer, promoting uncontrolled cell migration and invasion.
Moreover, the study's insights into the structural and functional consequences of specific phosphorylation events provide a deeper understanding of the molecular basis for cross-talk between RTKs and GPCRs. This knowledge could inform the development of therapeutic strategies aimed at targeting specific phosphorylation events to modulate G protein signaling in disease.
In conclusion, this research elucidates the molecular mechanisms by which growth factors influence G protein signaling, revealing the strategic importance of phosphorylation at specific residues. These findings enhance our understanding of cellular signaling dynamics and open new avenues for therapeutic intervention in diseases characterized by dysregulated signaling pathways.
Reference
Roy, S., Sinha, S., Silas, A. J., Ghassemian, M., Kufareva, I., & Ghosh, P. (2024). Growth factor-dependent phosphorylation of Gαi shapes canonical signaling by G protein-coupled receptors. Science signaling, 17(839), eade8041. https://doi.org/10.1126/scisignal.ade8041
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