Partially Redundant Actin Genes in Chlamydomonas Control Transition Zone Organization and Flagellum-Directed Traffic

Abstract

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. : Prevailing knowledge tells us that intracellular trafficking of flagellar proteins occurs on microtubule tracks. In this study, Jack et al. discover that filamentous actin is required for full flagellar assembly, normal flagellar protein synthesis, an intact flagellar gating region, and vesicle transport in Chlamydomonas reinhardtii. Keywords: Chlamydomonas reinhardtii, cilia, actin, trafficking, flagellar assembly, Goldi-adjacent vesicles, transition zone composition, flagellar protein synthesis