The Rab and Arf small GTPases coordinate numerous membrane trafficking routes. However, little is known about the functional crosstalk between Rab- and Arf-mediated signals. By examining the role of Rab35 and Arf6 in neurite outgrowth, Hotaka Kobayashi and Mitsunori Fukuda (p. ) now uncover a signalling cascade that involves the coordinated activity of both small GTPases. They show that Rab35 as well as the Rab35-binding protein centaurin-β2 (also known as ACAP2) are required for neurite outgrowth. Both of these proteins colocalise with Arf6 on pericentrosomal endosomes in PC12 cells, and Rab35 is required for the recruitment of centaurin-β2 to this location. Furthermore, knockdown and rescue experiments reveal that this sequential recruitment is required for neurite outgrowth in response to nerve growth factor (NGF) stimulation. Centaurin-β2 is a known Arf6 GTPase-activating protein (GAP), which led the authors to probe the role of Arf6 in NGF-induced neurite outgrowth. They report that knockdown of Arf6 reduces neurite length, whereas the expression of a constitutively active Arf6 mutant inhibits NGF-induced neurite outgrowth. Thus, Rab35-mediated recruitment of centaurin-β2 to Arf6-positive endosomes in response to NGF stimulation is essential for inactivating Arf6 to allow proper neurite outgrowth to take place.
