Plays an essential role in male fertility, sperm intra-flagellar transport, and tail assembly. Binds, in a GTP-regulated manner, to a specific set of effector proteins including key proteins involved in cilia development and function and delivers them into the growing sperm tail. Interacts with IFT27, IFT81, IFT172, ATP6V1E1, HK1, LDHC, MAPRE1 and HSPA2. Expressed in the testis. Belongs to the small GTPase superfamily. Rab family.