A NUMB–EFA6B–ARF6 recycling route controls apically restricted cell protrusions and mesenchymal motility

How the endocytic protein NUMB modulates mesenchymal migration is unclear. Zobel et al. show that NUMB limits the formation of apically restricted, actin-based protrusions called circular dorsal ruffles (CDRs). NUMB activates the ARF6 guanine nucleotide exchange factor EFA6B and controls the trafficking rate of ARF6-dependent cargos relevant for CDR formation, such as RAC1, and negatively regulates HGF-induced cell migration and invasion.
The endocytic protein NUMB has been implicated in the control of various polarized cellular processes, including the acquisition of mesenchymal migratory traits through molecular mechanisms that have only been partially defined. Here, we report that NUMB is a negative regulator of a specialized set of understudied, apically restricted, actin-based protrusions, the circular dorsal ruffles (CDRs), induced by either PDGF or HGF stimulation. Through its PTB domain, NUMB binds directly to an N-terminal NPLF motif of the ARF6 guanine nucleotide exchange factor, EFA6B, and promotes its exchange activity in vitro. In cells, a NUMB–EFA6B–ARF6 axis regulates the recycling of the actin regulatory cargo RAC1 and is critical for the formation of CDRs that mark the acquisition of a mesenchymal mode of motility. Consistently, loss of NUMB promotes HGF-induced cell migration and invasion. Thus, NUMB negatively controls membrane protrusions and the acquisition of mesenchymal migratory traits by modulating EFA6B–ARF6 activity.