Restrained activation of CYFIP2-containing WAVE complexes controls membrane protrusions and cell migration

Branched actin networks polymerized by the Arp2/3 complex are critical for cell migration. The WAVE complex is the machinery that activates Arp2/3 in a RAC1-dependent manner at the leading edge of migrating cells. Multiple WAVE complexes are assembled in a cell through various combinations of paralogous subunits. Here we report the surprising phenotype associated with loss-of-function of CYFIP2, a subunit of the WAVE complex. In three different human mammary cell lines and in prechordal plate cells of gastrulating zebrafish embryos, CYFIP2 depletion promoted, rather than impaired, membrane protrusions and migration persistence. CYFIP2, however, assembled WAVE complexes that polymerize branched actin at the cell cortex and rescued membrane protrusions ofCYFIP1/2double knock-out cells, although less efficiently than CYFIP1. Point mutations of CYFIP2 associated with intellectual disability in children were gain-of-function, as they made CYFIP2 as active as CYFIP1 in this rescue experiment. Biochemical reconstitutions of CYFIP2-containing WAVE complexes showed that they bound equally well to active RAC1 as CYFIP1-containing WAVE complexes, yet they were poorly activated in response to RAC1 binding. Together these results suggest that CYFIP2-containing WAVE complexes titrate active RAC1 and thereby prevent efficient CYFIP1-containing complexes from being activated. In this context, where cell migration is governed by the balance of CYFIP1/2 expression, releasing the restrained activity of CYFIP2-containing WAVE complexes leads to pathology.