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. Isoform 2 is expressed in the testis and localizes to the mid-piece of the sperm tail (at protein level). Isoform 2 is expressed at higher levels in testis than isoform 1. Isoform 1 and isoform 2 are widely expressed and notably within other tissues containing motile cilia including the lung, trachea, brain, ovary and kidney. Exhibits phenotypes characteristic of ciliopathies. Pre-axial polydactyly. Retinal degeneration, characterized by loss of the outer nuclear layer that contains the cell body of photoreceptor cells. Belongs to the small GTPase superfamily. Rab family.