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 (By similarity). Component of the IFT complex B, at least composed of IFT20, IFT22, HSPB11/IFT25, IFT27, IFT46, IFT52, TRAF3IP1/IFT54, IFT57, IFT74, IFT80, IFT81, and IFT88. 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 (By similarity). Interacts with RABL2/RABL2A; binding is equal in the presence of GTP or GDP. Interacts with IFT88 (By similarity). Interacts (via the IFT74/IFT81 heterodimer) with RABL2B (By similarity). The CH (calponin-homology)-like region shows high similarity to a CH (calponin-homology) domain and mediated binding to the globular domain of tubulin. Belongs to the IFT81 family.