Component of the intraflagellar transport (IFT) complex B: together with IFT74, forms a tubulin-binding module that specifically mediates transport of tubulin within the cilium. Binds tubulin via its CH (calponin-homology)-like region. Required for ciliogenesis. Required for proper regulation of SHH signaling (By similarity). Plays an important role during spermatogenesis by modulating the assembly and elongation of the sperm flagella (PubMed:32233951). May be involved in cardiac hypertrophy caused by carnitine deficiency. May play a role in development of the testis and spermatogenesis. Component of the IFT complex B, at least composed of IFT20, IFT22, HSPB11/IFT25, IFT27, IFT46, IFT52, TRAF3IP1/IFT54, IFT57, IFT74, IFT81, and IFT88 (PubMed:19253336, PubMed:23810713). Interacts with IFT74; the interaction is direct: within the IFT complex B, IFT74 and IFT81 mediate the transport of tubulin within the cilium. Interacts with tubulin; the interaction is direct (By similarity). Interacts with IFT57 and TTC30B (PubMed:23810713). Interacts with RABL2/RABL2A; binding is equal in the presence of GTP or GDP (PubMed:23055941). Interacts with IFT88 (PubMed:19253336). Interacts (via the IFT74/IFT81 heterodimer) with RABL2B (By similarity). Highly expressed in the testis (at protein level) (PubMed:32233951). Co-localizes with RABL2/RABL2A in the midpiece of elongated spermatids within the testis (at protein level) (PubMed:23055941). Expressed predominantly in the testis. Expressed specifically in heart and suppressed specifically in ventricles, but not in auricles, of carnitine-deficient juvenile visceral steatosis (JVS) mice. First detected at postnatal day (PND) 8, its level increased dramatically from PND 16 to 42 (at protein level). Poorly expressed in the testis 15 days after birth. Levels increase sharply between days 15 and 30 reaching a maximum by day 45 and the onset of spermatogenesis. The CH (calponin-homology)-like region shows high similarity to a CH (calponin-homology) domain and mediated binding to the globular domain of tubulin. Mice show impaired spermiogenesis, abnormal sperm morphology and significantly reduced sperm number and motility (PubMed:32233951). In addition to oligozoospermia, spermatozoa show dysmorphic and non-functional flagella (PubMed:32233951). Belongs to the IFT81 family.