Unexpected Ancient Paralogs and an Evolutionary Model for the COPII Coat Complex

The coat protein complex II (COPII) is responsible for the transport of protein cargoes from the Endoplasmic Reticulum (ER) to the Golgi apparatus. COPII has been functionally characterized extensively in vivo in humans and yeast. This complex shares components with the nuclear pore complex and the Seh1-Associated (SEA) complex, inextricably linking its evolution with that of the nuclear pore and other protocoatomer domain-containing complexes. Importantly, this is one of the last coat complexes to be examined from a comparative genomic and phylogenetic perspective. We use homology searching of eight components across 74 eukaryotic genomes, followed by phylogenetic analyses, to assess both the distribution of the COPII components across eukaryote diversity and to assess its evolutionary history. We report that Sec12, but not Sed4 was present in the Last Eukaryotic Common Ancestor along with Sec16, Sar1, Sec13, Sec31, Sec23, and Sec24. We identify a previously undetected paralog of Sec23 that, at least, predates the archaeplastid clade. We also describe three Sec24 paralogs likely present in the Last Eukaryotic Common Ancestor, including one newly detected that was anciently present but lost from both opisthokonts and excavates. Altogether, we report previously undescribed complexity of the COPII coat in the ancient eukaryotic ancestor and speculate on models for the evolution, not only of the complex, but its relationship to other protocoatomer-derived complexes.