All Atom Simulations Reveal Adaptive and Multimodal Membrane Binding of Contractile Ring Anchoring Protein Mid1

The positioning of the cytokinetic ring at the cell equator in animals and fungi depends crucially on the anillin scaffold proteins. In fission yeast, anillin-like Mid1 binds to the plasma membrane and helps establish a broad band of cytokinetic nodes near the cell center. Mid1 consists of a C terminal globular domain with two membrane binding candidate regions, the Plekstrin Homology (PH) and C2 domains, both of which prefer PIP2 lipids, and an N terminal intrinsically disordered region. The PH and C2 domains are joined by a connector domain (CNCT) and the C2 domain contains a predicted flexible region that is important for membrane binding. Previous studies have shown that both PH and C2 domains can associate with the membrane. However, it's unclear if they can simultaneously bind to the membrane in a way allowing Mid1 dimerization or oligomerization, and if one domain plays a dominant role. In order to elucidate Mid1's membrane binding mechanism, we used the available structural information of the PH, C2, and CNCT in all atom molecular dynamics simulations of Mid1 near membrane compositionally based on experimental measurements. The results indicate that Mid1 initially binds through the C2's L3 loop, and can further bind through the PH domain and C2's L1 loop. Mid1's multiple modes of binding may reflect multiple interactions with membranes and other node proteins, and ability to sustain mechanical forces.