Mechanically-driven closure of extreme membrane wounds in a single cell*

Wound healing is a fundamental aspect of living systems and is increasingly recognized in single cells. However, our understanding of single-cell wound healing is limited as many cell types used in prior studies cannot survive wounds larger than a few percent of the cell membrane area. In contrast, the giant single-celled ciliate Stentor coeruleus is a unique model that can robustly heal and regenerate from drastic wounds. Here, we study the interplay of wound healing capacity, wound size, and cell size in Stentor. Remarkably, at all cell sizes tested, Stentor easily survives wounds up to 60% of the cell membrane area, larger than any wounds reported in other single-cell models. This critical wound size corresponds to the geometric limit where the intact membrane area equals the minimum area needed to cover the cell volume (i.e., that of a sphere). In contrast to prior studies that only reported local wound healing events, we observe large-scale wrapping of intact cell membrane at a rate of ~100 – 500 μm²/s to aid in wound closure. We then show membrane wrapping is driven by KM fiber extension and/or myoneme contraction. Our work highlights how single cells can act as mechanical systems to enable large-scale cell functions.