Cryo-EM resolves molecular recognition of an optojasp photoswitch bound to actin filaments in both switch states

Pospich S, Küllmer F, Nasufovic V, Funk J, Belyy A, Bieling P, Arndt HD, Raunser S. (2021) Angew Chem Int Ed Engl 

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Actin is essential for key processes in all eukaryotic cells. Cellpermeable  optojasps  provide  spatiotemporal control of the actin cytoskeleton, confining toxicity and potentially rendering F‐actin druggable by photopharmacology. Here, we report cryo electron microscopy (cryo‐EM) structures of both isomeric states of one  optojasp  bound to actin filaments. The high‐resolution structures reveal for the first time the pronounced effects of photoswitching a functionalized azobenzene. By characterizing the  optojasp  binding site and identifying conformational changes within F‐actin that depend on the  optojasp  isomeric state, we refine determinants for the design of functional F‐actin photoswitches.

High-resolution structures of OJ8-stabilized F-actinCryo-EM structures of OJ8-stabilized F-actin in the (left)) dark state – trans-OJ8 (magenta), and (right) bright state – cis-OJ8 (yellow). To highlight the double stranded helical arrangement of F-actin, the central and subsequent actin subunit are colored in shades of green (dark state) and blue (bright state), respectively. The central D-loop (*) and OJ8 binding site (#) are enlarged for direct comparison of the structures before and after activation of OJ8 with UV light (380 nm). (Top) While the D-loop solely adopts the open conformation with trans-OJ8 bound, it is mixed with a considerably higher population of the closed D-loop state after irradiation. The map is additionally shown at a lower threshold (mesh) to highlight the second, less populated D-loop conformation of the cis-OJ8 structure. (Bottom) The azobenzene photoswitch stacks onto the macrocycle and changes its configuration from trans to cis upon UV-irradiation.
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