Two of the three actin‐binding domains of gelsolin bind to the same subdomain of actin Implications for capping and severing mechanisms

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Abstract
Gelsolin binds two monomers in the nucleating complex with G‐actin in calcium and caps actin filaments. However, 3 actin‐binding domains have been identified within its 6 repeating sequence segments corresponding to S1,S2–3 and S4–6, S1 and S4–6 bind only G‐actin whereas S2–3 binds specifically to F‐actin. Two of the three domains (S2–3 and S4–6) are required for nucleation and a different pair (S1 and S2–3) for severing. Here we show for the first time that the domains unique to nucleation (S4–6) or severing (S1) compete for the same region on subdomain 1 of G‐actin. We further show that S2–3 binds actin monomers weakly in G‐buffer conditions and that this interaction persists when S1 or S4–6 are also bound. Thus gelsolin associates with two distinct regions on actin. Since S2–3 does not bind monomeric actin in F‐buffer, we suggest that its high affinity 1:1 stoichiometry for filament subunits reflects interaction with two adjacent subunits.