Fig. 6.
A working model for how CAP catalyzes nucleotide exchange on actin monomers in cells. (1) CAP can interact with both free and cofilin-bound ADP-G-actin using CARP domain. This interaction puts the WH2 domain in position to competitively replace cofilin, leading to cofilin dissociation from the ADP-actin monomer, as previously observed biochemically20. (2) The tight association of both CARP and WH2 domains with ADP-actin monomers, together with the penetration of the C-terminal tail of CAP into the nucleotide-binding pocket, catalyzes a change in conformation and dynamics of the ADP-actin monomer to enhance the rate of nucleotide exchange. (3) Nucleotide in actin is rapidly exchanged from ADP to ATP. (4) The CARP domain has little if any affinity for ATP-actin, leaving dimeric CAP molecules associated with ATP-G-actin solely through their WH2 domains. (5) Profilin has high affinity for ATP-actin monomers, and binds directly to the PP1 domain of CAP31, adjacent to the WH2 domain. Thus, as ATP-actin monomers dissociate from CAP, they are rapidly bound by profilin, replenishing the pool of ATP-actin monomers available for assembly. This leaves CAP primed for the next round of nucleotide exchange