Actin's functional "switch": Constraining C-terminal conformational flexibility disrupts functionally important communication networks.

Actin's C terminus contributes to interactions with actin-binding proteins (ABPs) in cellular processes. Historical resolution constraints have prevented a complete understanding of how actin's C terminus mediates actin's interactions, but evidence suggests that it undergoes conformational shifts upon ABP binding. Actin's C terminus also shifts in response to actin's nucleotide state, guiding nucleotide-dependent ABP binding. Previously, we proposed that C-terminal shifts regulate actin's functions through allosteric communication networks. To further examine the role of C-terminal shifts in actin communication networks and understand how C-terminal modifications influence actin's functional state, we utilized in silico modeling of actin filaments crosslinked by N,N'-para-phenylenebismaleimide (PBM). Our modeling examined how constraining C-terminal flexibility with PBM crosslinks affects the conformation of distant structural elements and shifts internal communication networks critical for actin's functions. We present evidence that disruption of the stabilizing F375-R116 interaction by PBM-crosslinks reshapes allosteric communication networks through the protomer, altering nucleotide cleft architecture and nucleotide dynamics. We therefore propose that actin's C terminus acts as a nexus for actin structural changes, shifting in response to stimuli, and propagating changes that allow actin to adopt different functional conformations that influence ABP binding.