SUMMARY The unicellular green alga Chlamydomonas reinhardtii is a biflagellated cell with two actin genes: one encoding a conventional actin (IDA5) and the other encoding a divergent novel actin-like protein (NAP1). Here, we probe how actin redundancy contributes to flagellar assembly. Disrupting a single actin allows complete flagellar assembly. However, when disrupting both actins using latrunculin B (LatB) treatment on the nap1 mutant background, we find that actins are necessary for flagellar growth from newly synthesized limiting flagellar proteins. Under total actin disruption, transmission electron microscopy identified an accumulation of Golgi-adjacent vesicles. We also find that there is a mislocalization of a key transition zone gating and ciliopathy protein, NPHP-4. Our experiments demonstrate that each stage of flagellar biogenesis requires redundant actin function to varying degrees, with an absolute requirement for these actins in transport of Golgi-adjacent vesicles and flagellar incorporation of newly synthesized proteins.

Flagella of the unicellular green alga Chlamydomonas reinhardtii are nearly identical to cilia of mammalian cells and provide an excellent model to study ciliogenesis. These biflagellated cells have two actin genes: one encoding a conventional actin (IDA5) and the other encoding a divergent novel actin-like protein (NAP1). Previously, we described a role for actin in regulation of flagella-building intraflagellar transport machinery and now probe how actin redundancy contributes to this process using a nap1 mutant Chlamydomonas strain. Treatment with Latrunculin B, a potent actin polymerization inhibitor on the nap1 mutant background acutely disrupts all filamentous actins in the cell. We find that actins are an absolute requirement for flagellar growth when the preexisting pool of flagellar precursors is depleted. Nap1 mutants treated with Latrunculin B also showed reduced protein synthesis during regeneration. Finally, loss of functional actins reduced the incorporation of existing flagellar proteins as well as caused mislocalization of a key transition zone gating protein, NPHP-4. These experiments demonstrate that each stage of flagellar biogenesis requires redundant actin function to varying degrees, with an absolute requirement for actin in incorporation of newly synthesized flagellar proteins.