Abstract
Background: The eukaryotic endomembrane system likely arose via paralogous expansion of genes encoding proteins specifying organelle identity, coat complexes and government of fusion specificity. While the majority of these gene families were established by the time of the last eukaryotic common ancestor (LECA), subsequent evolutionary events molded these systems, likely reflecting adaptations retained for increased fitness. As well as sequence evolution, these adaptations include loss of otherwise canonical subunits, emergence of lineage-specific proteins and paralog expansion. The exocyst complex is involved in late exocytosis, and possibly additional pathways, and is a member of the complexes associated with tethering containing helical rods (CATCHR) tethering complex family, which includes conserved oligomeric Golgi (COG), homotypic fusion and vacuole protein sorting (HOPS), class C core vacuole/endosome tethering (CORVET) and others. The exocyst is integrated into a complex GTPase signaling network in animals, fungi and other lineages. Prompted by discovery of Exo99, a non-canonical subunit in the excavate protist Trypanosoma brucei, and significantly increased genome sequence data, we examined evolution of the exocyst. Methods: We examined evolution of the exocyst by comparative genomics, phylogenetics and structure prediction. Results: The exocyst is highly conserved, but with substantial losses of subunits in the Apicomplexa and expansions in Streptophyta plants and Metazoa. Significantly, few taxa retain a partial complex, suggesting that, in the main, all subunits are required for functionality. Further, the ninth exocyst subunit Exo99 is specific to the Euglenozoa with a distinct architecture compared to the other subunits and which possibly represents a coat system. Conclusions: These data reveal a remarkable degree of evolutionary flexibility within the exocyst complex, suggesting significant diversity in exocytosis mechanisms.
Highlights
A sophisticated level of cellular compartmentalisation is the major feature differentiating prokaryotic and eukaryotic cells and underpins the origins of the nucleus
It is becoming clear that these systems predate the origins of what would be classically recognised as eukaryotes, as some ancestral genes for constructing an endomembrane system were present in prokaryotes, and Archaea (Eme et al, 2018; Spang et al, 2018) (Figure 1A)
Identifying exocyst subunits across the eukaryotes The earlier failure to identify Sec5 and Exo84 in excavates by comparative genomics (Koumandou et al, 2007), and subsequent identification in trypanosomes by immunoisolation and mass spectrometry, indicated that the earlier study lacked sensitivity, and suggested other false negatives within the dataset (Boehm et al, 2017)
Summary
A sophisticated level of cellular compartmentalisation is the major feature differentiating prokaryotic and eukaryotic cells and underpins the origins of the nucleus. MTCs have splendid names that include transport protein particle (TRAPP) I, II and III, conserved oligomeric Golgi (COG), homotypic fusion and vacuole protein sorting (HOPS), class C core vacuole/endosome tethering (CORVET) (plus class C homologs in endosome-vesicle interaction, CHEVI and factors for endosome recycling and retromer interactions, FERARI), dorsalin-1 (Dsl1), Golgi-associated retrograde protein/endosome-associated recycling protein (GARP/EARP) and the exocyst. These complexes vary considerably in the number of subunits they possess. Using updated methodology and genome resources, we find evidence for considerable evolutionary flexibility in exocyst retention, with essentially complete loss from some lineages and a tentative suggestion of a connection to novel coat proteins
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