The molecular architecture of engulfment during Bacillus subtilis sporulation

The study of cell biology is limited by the difficulty in visualizing cellular structures at high spatial resolution within their native milieu. Here, we have visualized sporulation in Bacillus subtilis using cryo-electron tomography coupled with cryo-focused ion beam milling, a technique that allows the 3D reconstruction of cellular structures in near-native state at molecular resolution. During sporulation, an asymmetrically-positioned septum divides the cell into a larger mother cell and a smaller forespore. Subsequently, the mother cell phagocytoses the forespore in a process called engulfment, which entails a dramatic rearrangement of the peptidoglycan (PG) cell wall around the forespore. By imaging wild-type sporangia, engulfment mutants, and sporangia treated with PG synthesis inhibitors, we show that the initiation of engulfment does not entail the complete dissolution of the septal PG by the mother cell SpoIIDMP complex, as was previously thought. Instead, DMP is required to maintain a flexible septum that is uniformly and only slightly thinned at the onset of engulfment. Then, the mother cell membrane migrates around the forespore by forming tiny finger-like projections, the formation of which requires both SpoIIDMP and new PG synthesized ahead of the leading edge of the engulfing membrane. We propose a molecular model for engulfment membrane migration in which a limited number of SpoIIDMP complexes tether the membrane to and degrade the new PG ahead of the leading edge, thereby generating an irregular engulfing membrane front. Our data also reveal other structures that will provide a valuable resource for future mechanistic studies of endospore formation.