Model of Capping Protein and Arp2/3 Complex Turnover in the Lamellipodium based on Single Molecule Statistics

Capping protein (CP) and Arp2/3 protein complex regulate actin polymerization near the leading edge of motile cells. They assemble near the edge of the lamellipodium, undergo retrograde flow, and dissociate into the cytoplasm as single subunits or as part of actin oligomers. To better understand this cycle, we modeled the kinetics of CP and Arp2/3 complex in the lamellipodium using data from prior single molecule microscopy experiments [Miyoshi et al. JCB, 2006, 175:948]. In these experiments speckle appearance and disappearance events corresponded to assembly and dissociation from the F-actin network. We used the measured dissociation rates of Arp2/3 complex and CP (0.048 s−1 and 0.58 s−1, respectively) in a Monte Carlo simulation that includes particles in association with F-actin and diffuse in the cytoplasm. We explored the effect of slowly diffusing cytoplasmic pool to account for a big fraction of CP with diffusion coefficients as slow as 0.5 μm2/s measured by single molecule tracking [Smith et al. Biophys. J., 2011,101:1799]. These slowly diffusing species could represent severed actin filament fragments. We show that such slow diffusion coefficients are consistent with prior FRAP experiments by Kapustina et al. [Cytoskeleton, 2010, 67:525] who fitted their data using larger diffusion coefficients. We also show that the single molecule data are consistent with FRAP experiments by Lai et al. [EMBO J., 2008, 28:986] who found that the Arp2/3 complex recovers more quickly at the front of the lamellipodium as compared to the back. We discuss the implication of disassembly with actin oligomers and suggest experiments to distinguish among mechanisms that influence long range transport.