Tubulin polyglutamylation is a modification that adds multiple glutamates to the gamma-carboxyl group of a glutamate residue in the C-terminal tails of alpha- and beta-tubulin [1, 2]. This modification has been implicated in the regulation of axonal transport and ciliary motility. However, its molecular function in cilia remains unknown. Here, using a novel Chlamydomonas reinhardtii mutant (tpg1) that lacks a homolog of human TTLL9, a glutamic acid ligase enzyme [3], we found that the lack of a long polyglutamate side chain in alpha-tubulin moderately weakens flagellar motility without noticeably impairing the axonemal structure. Furthermore, the double mutant of tpg1 with oda2, a mutation that leads to loss of outer-arm dynein, completely lacks motility. More surprisingly, when treated with protease and ATP, the axoneme of this paralyzed double mutant displayed faster microtubule sliding than the motile oda2 axoneme. These and other results suggest that polyglutamylation directly regulates microtubule-dynein interaction mainly by modulating the function of inner-arm dyneins.

Alternative polyadenylation leads to mRNAs with variable 3′ ends. Since a 3′-untranslated region (3′-UTR) often contains cis elements that impact stability or localization of mRNA or translation, selection of poly(A) sites in a 3′-UTR is regulated in mammalian cells. However, the molecular basis for alternative poly(A) site selection within a 3′-UTR has been unclear. Here we show involvement of cleavage factor Im (CFIm) in poly(A) site selection within a 3′-UTR. CFIm is a heterodimeric 3′ end-processing complex, which functions to assemble other processing factors on pre-mRNA in vitro. We knocked down 25 kDa subunit of CFIm (CFIm25) in HeLa cells and analyzed alternative poly(A) site selection of TIMP-2, syndecan2, ERCC6 and DHFR genes by northern blotting. We observed changes in the distribution of mRNAs in CFIm25 depleted cells, suggesting a role for CFIm in alternative poly(A) site selection. Furthermore, tissue specific analysis demonstrated that the CFIm25 gene gave rise to 1.1, 2.0 and 4.6 kb mRNAs. The 4.6 kb mRNA was ubiquitously expressed, while the 1.1 and 2.0 kb mRNAs were expressed in a tissue specific manner. We found three likely poly(A) sites in the CFIm25 3′-UTR, suggesting alternative polyadenylation. Our results indicate that alternative poly(A) site selection is a well-regulated process in vivo.

SUMMARY Intraflagellar transport (IFT) moves IFT trains carrying cargoes from the cell body into the flagellum and from the flagellum back to the cell body. IFT trains are composed of complexes IFT-A and IFT-B and cargo adapters such as the BBSome. The IFT-B core proteins IFT74 and IFT81 interact directly through central and C-terminal coiled-coil domains, and recently it was shown that the N-termini of these proteins form a tubulin-binding module important for ciliogenesis. To investigate the function of IFT74 and its domains in vivo, we have utilized Chlamydomonas reinhardtii ift74 mutants. In a null mutant, lack of IFT74 destabilized IFT-B, leading to flagella assembly failure. In this null background, expression of IFT74 lacking 130 aa of the charged N terminus stabilized IFT-B and promoted slow assembly of nearly full-length flagella. A further truncation (lacking aa 1-196 including part of coiled-coil 1) also stabilized IFT-B, but failure in IFT-A / IFT-B interaction within the pool at the base of the flagellum prevented entry of IFT-A into the flagellum and led to severely decreased IFT injection frequency and flagellar-assembly defects. Decreased IFT-A in these short flagella resulted in aggregates of stalled IFT-B in the flagella. We conclude that IFT74 is required to stabilize IFT-B; aa 197-641 are sufficient for this function in vivo. The N terminus of IFT74 may be involved in but is not required for tubulin entry into flagella. It is required for association of IFT-A and IFT-B at the base of the flagellum and flagellar import of IFT-A.

RABL2 interacts with the intraflagellar transport-B (IFT-B) complex and CEP19 in a mutually exclusive manner. A point mutation of RABL2 found in sperm motility–defective mice abolishes its binding to IFT-B but not to CEP19. A RABL2-defective Chlamydomonas strain exhibits a nonflagellated phenotype, suggesting a crucial role of RABL2 in ciliary assembly.

The assembly and maintenance of most cilia and flagella rely on intraflagellar transport (IFT). Recent in vitro studies have suggested that, together, the calponin-homology domain within the IFT81 N-terminus and the highly basic N-terminus of IFT74 form a module for IFT of tubulin. By using Chlamydomonas mutants for IFT81 and IFT74, we tested this hypothesis in vivo. Modification of the predicted tubulin-binding residues in IFT81 did not significantly affect basic anterograde IFT and length of steady-state flagella but slowed down flagellar regeneration, a phenotype similar to that seen in a strain that lacks the IFT74 N-terminus. In both mutants, the frequency of tubulin transport by IFT was greatly reduced. A double mutant that combined the modifications to IFT81 and IFT74 was able to form only very short flagella. These results indicate that, together, the IFT81 and IFT74 N-termini are crucial for flagellar assembly, and are likely to function as the main module for IFT of tubulin.