Arp2/3 complex-mediated actin assembly drives microtubule-independent motility and phagocytosis in the evolutionarily divergent amoeba Naegleria

Much of our current understanding of actin-driven phenotypes in eukaryotes has come from the "yeast to human" opisthokont lineage, as well as the related amoebozoa. Outside of these groups lies the genus Naegleria, which shared a common ancestor with humans over a billion years ago, and includes the deadly "brain-eating amoeba." Unlike nearly every other known eukaryotic cell type, Naegleria amoebae are thought to lack cytoplasmic microtubules. The absence of microtubules suggests that these amoebae rapidly crawl and phagocytose bacteria using actin alone. Although this makes Naegleria a powerful system to probe actin-driven functions in the absence of microtubules, surprisingly little is known about Naeglerias actin cytoskeleton. Here, we use microscopy and genomic analysis to show that Naegleria amoebae have an extensive actin cytoskeletal repertoire, complete with nucleators and nucleation promoting factors. Naegleria use this cytoskeletal machinery to generate Arp2/3-dependent lamellar protrusions, which correlate with the capacity to migrate and phagocytose bacteria. Because human cells also use Arp2/3-dependent lamellar protrusions for motility and phagocytosis, this work supports an evolutionarily ancient origin for these actin-driven processes and establishes Naegleria as a natural model system for studying microtubule-independent cytoskeletal phenotypes.