The evolutionary origins of the lysosome-related organelle sorting machinery reveal fundamental homology in post-endosome trafficking pathways

The major organelles and pathways of the endomembrane system were in place by the time of the last eukaryotic common ancestor (LECA) (~1.5 billion years ago) and their acquisition were defining milestones during the process of eukaryogenesis itself. Comparative cell biology and evolutionary analyses show multiple instances of homology in the protein machinery controlling distinct inter-organelle trafficking routes. Resolving these homologous relationships allows us to explore processes underlying the emergence of new cellular compartments, infer ancestral states pre-dating LECA, and can even provide insight into the process of eukaryogenesis itself. Here we undertake a molecular evolutionary analysis, including providing a transcriptome of the jakobid flagellate Reclinomonas americana, exploring the origins of the machinery responsible for the biogenesis of lysosome-related organelles, the so-called Biogenesis of Lysosome-related Organelle Complexes (BLOCs 1,2, and 3). This pathway has been studied only in animals and is not considered a feature of the basic eukaryotic cell plan. We show that this machinery, and by inference the corresponding sorting pathway, was likely in place prior to the divergence of eukaryotes and is found in a much more diverse array of eukaryotes than is currently assumed. As such, this sorting pathway is likely an underappreciated facet of broader eukaryotic cellular function. Moreover, we resolve multiple points of ancient homology between all three BLOCs and other post-endosomal retrograde trafficking machinery (BORC, CCZ1/MON1, and a newly identified relationship with HOPS/CORVET) offering a mechanistic and evolutionary unification of these trafficking pathways. Overall, this study provides a comprehensive account of the rise of the LRO biogenesis machinery from prokaryotic origins to current eukaryotic diversity, Asgard archaea to animals, integrating it into the larger mechanistic framework describing endomembrane evolution. ### Competing Interest Statement The authors have declared no competing interest.