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. Component of the IFT complex B, the core composed of IFT25, IFT27, IFT46, IFT52, IFT74, IFT81 and IFT88 as well as associated subunits IFT20, IFT57, IFT80 and IFT172. Interacts with IFT81; the interaction is direct: within the IFT complex B, IFT74 and IFT81 mediate the transport of tubulin within the cilium. Interacts with tubulin; interaction is direct. 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.