SpoIVA contributes to efficient engulfment through a cytoskeletal-like mechanism during Bacillus subtilis sporulation
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During endospore (spore) development in bacteria, polar cell division generates two transcriptionally distinct cellular compartments, the mother cell and future spore (forespore). Signalling between these cells leads to sequential and compartmentalized transcription, along with key morphogenetics events, including the phagocytic-like process of engulfment and the recruitment of coat proteins to the engulfing membrane. The SpoIVA ATPase is an essential sporulation protein that assembles into static filaments at the forespore surface during engulfment, where it functions as the basement layer for coat assembly. Here, using Bacillus subtilis, we reveal an additional role for SpoIVA during engulfment. Cytological analysis of a spoIVA null mutant ( Δ spoIVA ) revealed engulfment defects such as septal membrane bulges and asymmetric membrane migration, similar to those typically associated with impaired peptidoglycan remodelling during engulfment. Engulfment defects were exacerbated when Δ spoIVA was combined mutants known to impact engulfment progression and efficiency. Importantly, a spoIVA mutant (K30A) impaired for ATP hydrolysis and filament formation in vitro but partially functional for coat assembly in vivo , closely phenocopies the spoIVA null mutant engulfment defects. Based on these data, we propose a model whereby SpoIVA polymerisation at the spore surface, independently of coat assembly, plays a mechanical and structural role during engulfment, akin to the cytoskeletal proteins that drive phagocytosis in eukaryotic cells.
IMPORTANCE
Endospore formation relies on membrane remodeling to generate highly resistant dormant spores in human, animal and insect pathogens. During engulfment, the mother-cell membrane migrates around the forespore in a phagocytic-like process while the spore coat is simultaneously assembled. SpoIVA is widely recognized as the ATPase that polymerizes at the forespore surface to nucleate assembly of the multilayered spore coat. Here, we demonstrate that SpoIVA also performs a distinct function during engulfment that is independent of its role in coat formation. Cells lacking SpoIVA, or carrying a polymerization-defective SpoIVA variant, exhibit membrane deformations characteristic of impaired engulfment. We propose that SpoIVA polymers provide structural and mechanical support for membrane migration, analogous to the role of cytoskeletal systems during eukaryotic phagocytosis. These findings reveal an unexpected function for SpoIVA and demonstrate that bacterial protein polymers can provide mechanical support for membrane remodeling during complex developmental processes, analogous to cytoskeletal systems in eukaryotic cells
