Twinfilin-Mediated Uncapping and Post-Uncapping Barbed-End Fate

Twinfilin regulates actin monomers, filament ends, and capping protein (CP), yet its perturbation produces different F-actin responses across cellular systems. We propose that CP dissociation and subsequent barbed-end fate are sequential events that can be regulated separately. Evidence from budding-yeast actin patches and mammalian lamellipodia supports uncapping followed by shortening or turnover. Exposed ends may also be recapped or elongate, depending on CP rebinding, nucleotide state, polymerizable actin, and end-associated factors. A reconstituted mDia1 pathway demonstrates one elongation route, whose cellular relevance remains unresolved. State-matched assays show that filament age affects cooperative CP dissociation without establishing subsequent fate. The hypothesis predicts that changing uncapping kinetics need not proportionally change F-actin, and that fate distributions can shift when uncapping kinetics remain comparable. Continuous tracking of CP departure and subsequent behavior at the same end would test these predictions.