Multivesicular bodies in the enigmatic amoeboflagellate Breviata anathema and the evolution of ESCRT

Abstract

Endosomal sorting complexes required for transport (ESCRTs) are heteromeric protein complexes required for multivesicular body (MVB) morphogenesis. ESCRTs I, II, III and III-associated are ubiquitous in eukaryotes and presumably ancient in origin. ESCRT 0 recruits cargo to the MVB and appears to be opisthokont-specific, bringing into question aspects of the current model of ESCRT mechanism. One caveat to the restricted distribution of ESCRT 0 was the previous limited availability of amoebozoan genomes, the supergroup closest to opisthokonts. Here, we significantly expand the sampling of ESCRTs in Amoebozoa. Our electron micrographic and bioinformatics evidence confirm the presence of MVBs in the amoeboflagellate Breviata anathema. Searches of genomic databases of amoebozoans confirm the ubiquitous nature of ESCRTs I-III-associated and the restriction of ESCRT 0 to opisthokonts. Recently, an alternate ESCRT 0 complex, centering on Tom1 proteins, has been proposed. We determine the distribution of Tom1 family proteins across eukaryotes and show that the Tom1, Tom1L1 and Tom1L2 proteins are a vertebrate-specific expansion of the single Tom1 family ancestor, which has indeed been identified in at least one member of each of the major eukaryotic supergroups. This implies a more widely conserved and ancient role for the Tom1 family in endocytosis than previously suspected.