Plus-end directed microtubule-dependent motor protein which displays processive activity. Is involved in regulation of microtubule dynamics, synapse function and neuronal morphology, including dendritic tree branching and spine formation. Plays a role in lerning and memory. Involved in delivery of gamma-aminobutyric acid (GABA(A)) receptor to cell surface. Interacts with TRIM3; the interaction positively affects motility of KIF21B. Interacts with GABARAP and GABA(A) receptor subunits: GABRG2, GABRA1 and GABRA2. May interact with GABA(A) receptor subunits: GABRB2 and GABRB3. Belongs to the TRAFAC class myosin-kinesin ATPase superfamily. Kinesin family.