Cryo-electron tomography reveals the nanoscale architecture of the actin cortex in cellular blebs

In animal cells, cellular deformations driving cytokinesis, migration, and epithelial constriction are driven by contractile tension in the actomyosin cortex, a thin network of actin and myosin underlying the plasma membrane. Cortical tension results from myosin-generated forces and as such, cortical myosin organization and dynamics have received significant attention. However, recent studies highlight that alongside myosin motor activity, the organization of the cortical actin network is a key regulator of tension. Yet, very little is known about the structural arrangement of cortical actin filaments. This is mostly due to the small thickness and high density of the cortex, which makes the visualization of cortical actin filaments challenging. Here, we use cryo-electron tomography (cryo-ET) to unveil the structural organization of cortical actin. As a model, we use isolated cellular blebs, which assemble an actin cortex comparable to the cortex of entire cells, but are small enough to be amenable to cryo-ET. We find that the bleb actin cortex is mostly composed of short and straight actin filaments. We then characterize cortex structural parameters, including the density of potential cross-linking and membrane attachment points. Our study unveils the nanoscale three-dimensional organization of the cortical actin network in cellular blebs. As such, it provides a quantitative framework for models of cortical tension generation, and will help understanding the nanoscale basis of cell surface contractions.