Animal cell shape is controlled by gradients in contractile tension of the actin cortex. The cortex is a thin actomyosin network supporting the plasma membrane. At the molecular level, contractile tension is generated by myosin motors pulling on actin filaments. Along- side myosin, actin connectivity has been shown to be key to cortical tension regulation. Understanding molecular organisation of the actin cortex is thus key to understanding cortical tension.

To understand how cortical composition changes when tension changes, and to identify potential molecular regulators of cortical tension, I firstly compared protein composition of interphase and mitotic cortices. Indeed, interphase and mitotic cells were previously shown to di↵er in cortical tension. I isolated cortical fractions from cells in these stages of cell cycle, by isolating cortex-enriched blebs. Using mass spectrometry, we detected over 922 proteins in blebs isolated from synchronised cells. Among 238 actin-related proteins, we showed a role for septins in the regulation of the mitotic cell shape. Overall, we created a comprehensive dataset of potential regulators of cortex mechanics.

In the second part of my PhD, I focused on the role of actin crosslinkers in cortex tension regulation. In particular, I focused on the role of actin crosslinker size for their localisation and in tension regulation. To this aim, we created artificial crosslinkers, for which I was able to modulate size independently of other features. We created artificial crosslinkers between 5 and 35 nm long, which successfully localised to actin structures. I investigated the role of artificial crosslinkers in the control of cortical thickness, tension and cell division.

Together, in this thesis, I investigate new levels of regulation of cortical organisation and tension at the molecular level.