AbstractThe signalling pathways underpinning cell growth and invasion use overlapping components, yet how mutually exclusive cellular responses occur is unclear. We developed 3-Dimensional culture analyses to separately quantify growth and invasion. We identify that alternate variants of IQSEC1, an ARF GTPase Exchange Factor, act as switches to promote invasion over growth by spatially enriching cortical phosphoinositide metabolism. All IQSEC1 variants activate ARF5- and ARF6-dependent PIP5-kinase to promote PI(3,4,5)P3-AKT signalling and growth. In contrast, select pro-invasive IQSEC1 variants restrict PI(3,4,5)P3 production to discrete cortical domains to form invasion-driving protrusions. Inhibition of IQSEC1 attenuates invasion in vitro and metastasis in vivo. Induction of pro-invasive IQSEC1 variants and elevated IQSEC1 expression occurs in a number of tumour types and is associated with higher-grade metastatic cancer, activation of PIP3-signalling, and predicts long-term poor outcome across multiple cancers. Spatial enrichment of phosphoinositide metabolism therefore is a switch to induce invasion over growth in response to the same external signal. Targeting IQSEC1 as the central regulator of this switch may represent a therapeutic vulnerability to stop metastasis.HighlightsSpatial enrichment of PI(3,4,5)P3 is a molecular switch to promote invasion.IQSEC1 is a GEF for ARF5/6, promoting PIP5K-dependent PI(3,4,5)P3 production downstream of the HGF receptor Met.Pro-invasive IQSEC1 variants restrict cortical PI(3,4,5)P3 production to subdomains that convert into invasive protrusions.IQSEC1 inhibition attenuates in vitro invasion and metastasis in vivo.IQSEC1 module is associated with poor outcome across tumour types.