Coordinated Membrane Deformation Driven by a Minimal Set of Spiroplasma MreB Isoforms

Abstract

Spiroplasma are wall-less helical bacteria that swim by switching the handedness of their helices. In most bacteria, MreB, a bacterial actin, forms relatively static filaments beneath the membrane to organize cell wall synthesis. In contrast, Spiroplasma eriocheiris encodes five MreB isoforms (SpeMreBs), and swimming requires a pair of isoforms, SpeMreB5 together with SpeMreB4 (or the related isoform SpeMreB1) (1) Yet how these MreBs generate force and membrane deformation remains unclear. To examine the membrane-deforming activities of SpeMreBs, we demonstrated a simple reconstitution system using the non-motile synthetic bacterium JCVI-syn3B and purified SpeMreBs expressed in Escherichia coli. Lysates of syn3B expressing SpeMreB5 deformed liposomes in a concentration-dependent manner. In contrast, lysates co-expressing both SpeMreB5 and SpeMreB4 showed a plateau in the frequency of deformation, suggesting that SpeMreB4 suppresses membrane deformation driven by SpeMreB5. Deformed liposomes exhibited either fluctuating or stable behaviors. ATP depletion changed both the frequency and behavior of deformation, indicating that membrane remodeling depends on the nucleotide state of SpeMreBs. Reconstitution with purified SpeMreBs from E. coli confirmed that SpeMreB5 alone deforms membranes, whereas SpeMreB1, a member of the same class as SpeMreB4, suppresses deformation. These results suggest that membrane shape in Spiroplasma is dynamically regulated by antagonistic interactions among isoforms of SpeMreBs isoforms and their nucleotide-dependent assembly states.