Kinesin superfamily proteins (KIFs) are molecular motors that transport cellular cargo along the microtubule cytoskeleton. KIF21B is a neuronal kinesin that is highly enriched in dendrites. The regulation and specificity of microtubule transport involves the binding of motors to individual cargo adapters and accessory proteins. Moreover, posttranslational modifications of either the motor protein, their cargos or tubulin regulate motility, cargo recognition and the binding or unloading of cargos. Here we show that the ubiquitin E3 ligase TRIM3, also known as BERP, interacts with KIF21B via its RBCC domain. TRIM3 is found at intracellular and Golgi-derived vesicles and co-localizes with the KIF21B motor in neurons. Trim3 gene deletion in mice and TRIM3 overexpression in cultured neurons both suggested that the E3-ligase function of TRIM3 is not involved in KIF21B degradation, however TRIM3 depletion reduces the motility of the motor. Together, our data suggest that TRIM3 is a regulator in the modulation of KIF21B motor function. © 2013 Labonté et al.