Actin monomers influence the interaction between Xenopus cyclase-associated protein 1 and actin filaments

Cyclase-associated protein (CAP) binds to both actin monomers and filaments and regulates multiple aspects of the actin dynamics including polymerization and depolymerization. CAP has been isolated from multiple species as a stable equimolar complex with actin monomer. However, functional significance of the CAP-actin complex is unknown. We previously demonstrated that native Xenopus cyclase-associated protein 1 (XCAP1) forms a 4:4 complex with actin. Here, we characterized how actin-free XCAP1 and the XCAP1-actin complex interact with actin filaments using high-speed atomic force microscopy and found that XCAP1-bound actin monomers influence dwell time and positional preference of XCAP1 on actin filaments. Actin-free XCAP1 bound to actin filaments transiently with a dwell time of ∼0.2 seconds. The XCAP1-actin complex also bound to actin filaments transiently but with a 3-to 5-fold longer dwell time than actin-free XCAP1. Actin-free XCAP1 bound to both side and ends of actin filaments with moderate preference to ends. However, the XCAP1-actin complex bound preferentially to the side of actin filaments. These results indicate that binding of actin monomers to XCAP1 affects its binding modes to actin filaments, suggesting that this might be a novel mechanism to regulate the effects of CAP on actin filaments.