Figure 4

Model for the kinetics of T4 presynaptic filament formation in the presence and absence of UvsY (adapted from Liu, J., C. Berger, and S.W. Morrical: Kinetics of Presynaptic Filament Assembly in the Presence of SSB and Mediator Proteins, unpublished) . Left -- Under low-salt conditions in the absence of mediator protein UvsY, ATP-bound UvsX, a high affinity form, binds Gp32-ssDNA rapidly to form an unstable nucleation site or "pre-nucleation complex" (association constant K₁). A slow but almost irreversible conformational change (forward rate constant k₂) is required by UvsX to displace Gp32 and to secure this isolated nucleation site on the lattice. With successful nucleation, more ATP-bound UvsX is recruited to form an unstable cluster (association constant K₃). This rapidly formed UvsX cluster undergoes another slow but almost irreversible conformational change to displace Gp32 and to redistribute into a stable and productive presynaptic filament (forward rate constant k₄). Right -- Under high-salt conditions the mediator protein, UvsY, facilitates filament nucleation by stabilizing the salt-sensitive pre-nucleation complex (enhanced K₁), by forming a special quaternary complex with UvsX, Gp32, and ssDNA. Filament propagation (particularly k₄) is rate-limiting under all conditions.