Role of adenosine 5'-triphosphate hydrolysis in the assembly of the bacteriophage T4 DNA replication holoenzyme complex.

Steady-state and pre-steady-state rates of ATP hydrolysis by the 44/62 accessory protein were determined to elucidate the role of ATP hydrolysis in bacteriophage T4 holoenzyme complex formation. Steady-state ATPase measurements of the 44/62 protein under various combinations of 45 protein, DNA substrate, and T4 exo- polymerase indicate that although the 44/62 protein synergistically hydrolyzes ATP in the presence of 45 protein and DNA substrate, the ATPase activity of 44/62 is diminished substantially upon the formation of the holoenzyme complex. The decrease in activity is primarily in kcat while the K(m) for ATP is changed unsubstantially by the various combinations. Data suggest that the decrease in the rate of ATP hydrolysis upon the addition of T4 exo- polymerase in the presence of 45 protein and DNA substrate is due to formation of a stable holoenzyme complex consisting of only the 45 protein and T4 exo- polymerase in a 1:1 ratio. The 44/62 protein acts catalytically to load 45 protein onto the DNA substrate and does not remain a component of the holoenzyme complex. Pre-steady-state kinetic analysis of the ATP hydrolysis reaction catalyzed by the 44/62 protein loading the 45 protein onto the DNA substrate in the absence or presence of polymerase is biphasic, in which a burst in ATP hydrolysis precedes the steady-state rate of ATP hydrolysis. An identical burst in ATP consumption is obtained under either condition, indicating that ATP hydrolysis is not required to load polymerase into the holoenzyme complex. The data suggest one turnover of ATP at each of the four ATPase active sites of the 44/62 protein per 45 protein loaded. ATP hydrolysis by the 44/62 protein under conditions of holoenzyme complex formation is the rate-limiting step in holoenzyme complex formation. The process of holoenzyme formation appears to be identical for leading and lagging strand synthesis.