Interactome of Sec13 paralogs in Paradiplonema papillatum

The acquisition of new functions is frequently the result of gene duplication and is a major driver of evolutionary complexity. The β-propeller protein Sec13 plays roles in at least three distinct processes by virtue of being a component of the COPII endoplasmic reticulum export vesicle coat, the nuclear pore complex (NPC) and the Seh1-associated (SEA)/GATOR nutrient-sensing complex, suggesting that regulatory mechanisms coordinating these cellular activities may operate via Sec13. COPII, SEA and the NPC are ancient features of eukaryotic cells. We analyzed the Sec13 interactome in diplonemids, widespread marine protists, and found that Sec13 functions are divided between two paralogs, Sec13a and Sec13b. Sec13a interacts with COPII and the NPC, but by contrast Sec13b interacts with Sec16 and components of the SEA/GATOR complex. Hence, we infer that Sec13a is responsible for NPC functions and canonical anterograde transport activities while Sec13b acts within nutrient and autophagy-related pathways. Furthermore, duplication of Sec13 is near universal across the Euglenozoa, providing a potent exemplar of paralogy expansion and functional diversification.