Rufy3 is an adapter protein for small GTPases that activates a Rac guanine nucleotide exchange factor to control neuronal polarity.

RUN and FYVE domain-containing 3 (Rufy3) is an adapter protein for small GTPase proteins and is bound to activated Rap2, a Ras family protein in the developing neuron. Previously, we reported the presence of a rapid cell polarity determination mechanism involving Rufy3, which is likely required for in vivo neuronal development. However, the molecular details of this mechanism are unclear. To this end, here we produced Rufy3 knock-out (Rufy3-KO) mice to study the role of Rufy3 in more detail. Examining Rufy3-KO neurons, we found that Rufy3 is recruited via glycoprotein M6A to detergent-resistant membrane domains, which are biochemically similar to lipid rafts. We also clarified that Rufy3, as a component of a ternary complex, induces the assembly of Rap2 in the axonal growth cone, whereas in the absence of Rufy3, the accumulation of a Rac guanine nucleotide exchange factor, T-cell lymphoma invasion and metastasis 2 (Tiam2/STEF), is inhibited downstream of Rap2. We also found that Rufy3 regulates the cellular localization of Rap2 and Tiam2/STEF. Taken together, we conclude that Rufy3 is a physiological adapter for Rap2 and activates Tiam2/STEF in glycoprotein M6A-regulated neuronal polarity and axon growth.