A CapG gain-of-function mutant reveals critical structural and functional determinants for actin filament severing

CapG is the only member of the gelsolin family unable to sever actin filaments. Changing amino acids 84–91 (severing domain) and 124–137 (WH2‐containing segment) simultaneously to the sequences of gelsolin results in a mutant, CapG‐sev, capable of severing actin filaments. The gain of severing function does not alter actin filament capping, but is accompanied by a higher affinity for monomeric actin, and the capacity to bind and sequester two actin monomers. Analysis of CapG‐sev crystal structure suggests a more loosely folded inactive conformation than gelsolin, with a shorter S1–S2 latch. Calcium binding to S1 opens this latch and S1 becomes separated from a closely interfaced S2–S3 complex by an extended arm consisting of amino acids 118–137. Modeling with F‐actin predicts that the length of this WH2‐containing arm is critical for severing function, and the addition of a single amino acid (alanine or histidine) eliminates CapG‐sev severing activity, confirming this prediction. We conclude that efficient severing utilizes two actin monomer‐binding sites, and that the length of the WH2‐containing segment is a critical functional determinant for severing.