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The formation of axons and dendrites and maintenance of the neurons vastly expanded surface require the continuous addition of new membrane.During axon extension, the coordination of membrane traffic, adhesion to extracellular substrate, and reorganization of the cytoskeleton at the peripheral region of the growth cone mediate the protrusive activity of the axon growth cone.Previously we have found the involvement of Rab10 in directional membrane insertion of plasmalemmal precursor vesicles (PPV) during neuronal polarization.However the mechanism of Rab 10 action remains unknown.Here we report that the myristoylated alanine-rich C kinase substrate (MARCKS) interacts with Rab10 and mediates fusion of PPV to the plasma membrane.Down-regulation of MARCKs using siRNA against MARCKS abrogated polarized distribution of Rab10 and prevented neuronal polarization.Furthermore, blockade of the interaction between Rab10 and MARCKS by over-expressing the MARCKS domain that mediated the Rab10-MARCKS binding abolished membrane fusion of PPVs, which were labeled with BODIPY-ceramide.Membrane fusion of PPVs was also measured by analyzing the TIRF (Internal Reflection Fluorescence) signals of pHluorin-tagged receptors of axon growth promoting factors-TrkB for brain-derived neurotrophic factor (BDNF) and IGFR for insulin like factor 1 (IGF-1).We found that down-regulation of MARCKS or blocking its interaction with Rab10 inhibited the TIRF signals.Finally, blocking the interaction between Rab10 and MARCKS inhibited axon growth in vitro and in vivo.Thus, Rab10 interaction with MARCKS mediates fusion of PPV with the plasma membrane, a process required for axonal growth.