Cyanobacteria are in large characterized by their capacity for aerobic photolithoautotrophic growth. The latter is driven by oxygenic photosynthesis whose molecular machinery resides within a specialized intracellular membrane system termed "thylakoid" in all but the most early-branching cyanobacterial species. Thylakoid membranes have undergone severe morphological changes during cyanobacterial evolution and the transition into endosymbiotic plastids. Here, the CurT/CURT1-like transmembrane proteins constitute a key determinant of thylakoid architecture in both cyanobacteria and the green lineage of algae and land plants. The evolutionary origin of this protein family, however, remains highly elusive, with the appearance of "fully-formed" CurT-like proteins roughly coinciding with the evolution of thylakoids themselves.
Microscopic analyses reveal a number of hitherto undescribed defects is cell division and thylakoid partitioning in curT-depleted mutants of Synechocystis sp. PCC 6803 which may provide the first relevant cue towards the origin of these fascinating proteins. CurT-depleted Synechocystis sp. PCC 6803 cells show impaired Z-ring formation, while a filamentous phenotype can be observed in Synechococcus sp. PCC 7942 curT mutants, all of which suggests an (original) involvement of CurT in cell division. Through phylogenetic analyses we identified a candidate for a distant sister clade to cyanobacterial/plastid CurT/CURT1 within the inherently thylakoid-less Firmicutes phylum. With Firmicutes and cyanobacteria being closely related phyla with overlapping cell division features, CurT/CURT1-like proteins having been originally obtained through horizontal gene transfer and stemming from a non-photosynthetic context becomes conceivable. Initially aiming at unravelling a tentative secondary function of CurT/CURT1 in cyanobacteria, we are now confident to have uncovered the evolutionary origins of these prominent photosynthesis-associated proteins in Gram-positive-like cell division.