The wall-less bacterium Spiroplasma poulsonii builds a polymeric cytoskeleton composed of interacting MreB isoforms [article]

Florent Masson, Xavier Pierrat, Bruno Lemaitre, Alexandre Persat
2021 bioRxiv   pre-print
A rigid cell wall defines the morphology of most bacteria. MreB, a bacterial homologue of actin, plays a major role in coordinating cell wall biogenesis and defining cell shape. In contrast with most bacteria, the Mollicutes family is devoid of cell wall. As a consequence, many Mollicutes have undefined morphologies. Spiroplasma species are an exception as they robustly grow with a characteristic helical shape, but how they maintain their morphology remains unclear. Paradoxal to their lack of
more » ... ll wall, the genome of Spiroplasma contains five homologues of MreB (SpMreBs). Since MreB is a homolog of actin and that short MreB filaments participate in its function, we hypothesize that SpMreBs form a polymeric cytoskeleton. Here, we investigate the function of SpMreB in forming a polymeric cytoskeleton by focusing on the Drosophila endosymbiont Spiroplasma poulsonii. We found that in vivo, Spiroplasma maintain a high concentration of all five MreB isoforms. By leveraging a heterologous expression system that bypasses the poor genetic tractability of Spiroplasma, we found that strong intracellular levels of SpMreb systematically produced polymeric filaments of various morphologies. Using co-immunoprecipitation and co-expression of fluorescent fusions, we characterized an interaction network between isoforms that regulate the filaments formation. Our results point to a sub-functionalization of each isoform which, when all combined in vivo, form a complex inner polymeric network that shapes the cell in a wall-independent manner. Our work therefore supports the hypothesis where MreB mechanically supports the cell membrane, thus forming a cytoskeleton.
doi:10.1101/2021.06.08.447548 fatcat:u2kalxfezvckhavqkyfxgjczey