IQGAP1 binds the Axl receptor tyrosine kinase and inhibits its signaling

Authors

• L Gorisse
• Z Li
• AC Hedman
• DB Sacks

Abstract

IQGAP1 is a scaffold protein that integrates cell signaling pathways by binding growth factor receptors and intracellular signaling molecules. The Axl tyrosine kinase receptor is important in hematopoiesis, platelet aggregation, engulfment of apoptotic cells and cell survival. Binding of growth arrest-specific protein 6 (Gas6) to Axl activates intracellular signaling. However, the mechanism of inactivation of the Axl receptor is poorly understood. Here, we investigate the interaction between IQGAP1 and Axl. Using GST pulldown assays and immunoprecipitation from mammalian cell lysates, we found that IQGAP1 associates with Axl. Analysis with multiple discrete fragments revealed that the binding is direct and mediated by the IQ domain of IQGAP1. We observed by proximity ligation assays that Axl:IQGAP1 complexes are located in the cytoplasm, and this interaction is reduced by 64% when cells are incubated with Gas6. Consistent with these findings, reducing IQGAP1 levels in cells by CRISPR/Cas9 enhanced the ability of Gas6 to stimulate both Axl phosphorylation and Akt activation. Moreover, reduction of IQGAP1 promoted Gas6-stimulated transcription of matrix metalloproteinase (MMP)-2, MMP-3, MMP-9 and urokinase-type plasminogen activator genes. Finally, we observed by confocal microscopy that IQGAP1 downregulates the heterodimerization of Axl with the epidermal growth factor receptor. In summary, we identify IQGAP1 as a previously undescribed suppressor of Axl. These findings differ from the previously reported functions for IQGAP1 as a scaffold that promotes signaling by growth factor receptors. Our data expand the repertoire of binding partners for both Axl and IQGAP1 and provide additional insight into the regulation of Axl function.

Scientific Focus Area: Molecular Biology and Biochemistry

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