Cell-autonomous immunity and the endosymbiotic theory: Septins at the origin?

Masayuki Onishi , Wallace Marshall, Gautam Dey , Serge Mostowy

The origin of complex eukaryotic cells remains a long-standing mystery. Endosymbiosis gave rise to mitochondria and chloroplasts, the bacteria-derived organelles that underpin eukaryotic productivity. Early eukaryotes had to establish these partnerships while avoiding pathogens, using limited ancestral tools. Septins are cytoskeletal GTP-binding proteins best known for roles in cytokinesis, yet their core function remains unclear. Their presence in algae and ciliates places septins in the last eukaryotic common ancestor (LECA). We propose that the ancestral septin recognised foreign cells, and that this single function later diversified into chloroplast and mitochondrial fission and into innate immunity against pathogens. Here we examine the mechanisms and consequences of bacterial recognition by septins, and outline the organisms and approaches needed to test this model.