Mechanisms of axoneme and centriole elimination inNaegleria gruberi

Abstract

The excavate Naegleria gruberi, a basal eukaryote related to the "brain eating" Naegleria fowleri, can transform transiently from an amoeboid life form lacking flagella and centrioles to a flagellate life form where these elements are present, followed by reversion to an amoeboid state. The mechanisms imparting elimination of axonemes and centrioles during this reversion process are not known. Here, we uncover that flagella primarily fold onto the cell surface and fuse within milliseconds with the plasma membrane. Once internalized, axonemes are severed by Spastin into equally-sized fragments, which are then enclosed by membranes, after which their contents are eliminated through the lysosomal pathway. Moreover, we discovered that centrioles undergo progressive K63 autophagy-linked poly-ubiquitination and K48 proteasome-promoting poly-ubiquitination, and that ubiquitination occurs next to centriolar microtubules. Most centrioles are eliminated in lysosomes or the cytoplasm in a lysosomal- and proteasome-dependent manner. Strikingly, we uncover in addition that centrioles can be shed in the extracellular milieu and taken up by other cells. Collectively, these findings reveal fundamental mechanisms governing the elimination of essential cellular constituents in Naegleria that may operate broadly in eukaryotic systems.