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 (PubMed:23990561). Required for ciliogenesis (PubMed:23990561, PubMed:27666822). Required for proper regulation of SHH signaling (PubMed:27666822). Plays an important role during spermatogenesis by modulating the assembly and elongation of the sperm flagella (By similarity). {ECO:0000250|UniProtKB:O35594, ECO:0000269|PubMed:23990561, ECO:0000269|PubMed:27666822}. This is the function of IFT81 (intraflagellar transport 81, Ensembl gene identifier ENSG00000122970).