The molecular architecture of engulfment during Bacillus subtilis sporulation

• Main Authors: Kanika Khanna, Javier Lopez-Garrido, Ziyi Zhao, Reika Watanabe, Yuan Yuan, Joseph Sugie, Kit Pogliano, Elizabeth Villa
• Format: Article
• Language: English
• Published: eLife Sciences Publications Ltd 2019-07-01
• Series: eLife
• Subjects: engulfment; spoIIDMP; membrane migration; peptidoglycan; cryo-focused ion beam milling; cryo-electron tomography
• Online Access: https://elifesciences.org/articles/45257

The study of bacterial cell biology is limited by difficulties in visualizing cellular structures at high spatial resolution within their native milieu. Here, we visualize Bacillus subtilis sporulation using cryo-electron tomography coupled with cryo-focused ion beam milling, allowing the reconstruction of native-state cellular sections at molecular resolution. During sporulation, an asymmetrically-positioned septum generates a larger mother cell and a smaller forespore. Subsequently, the mother cell engulfs the forespore. We show that the septal peptidoglycan is not completely degraded at the onset of engulfment. Instead, the septum is uniformly and only slightly thinned as it curves towards the mother cell. Then, the mother cell membrane migrates around the forespore in tiny finger-like projections, whose formation requires the mother cell SpoIIDMP protein complex. We propose that a limited number of SpoIIDMP complexes tether to and degrade the peptidoglycan ahead of the engulfing membrane, generating an irregular membrane front.