A detailed model for clamp loading mechanism. (1) Prior to ATP binding, the clamp loader AAA+ modules are not organized in a manner competent to bind the clamp. (2) Upon fully binding ATP, the clamp loader AAA+ modules assume a spiral shape that can hold the clamp in an open lockwasher conformation. (3) This binary complex is competent to bind to primer template DNA. We propose that the dimensions of the open clamp restrict DNA access to the central chamber such that only single-stranded regions or a major groove at a single-stranded/double-stranded junction can slip through the crack in the clamp. (4) The duplex region of primer-template DNA then slides up into the clamp loader central chamber, which activates the ATPase. (5) ATP hydrolysis or Pi release initiates at the end of the AAA+ spiral, which allows the clamp to close. Further hydrolysis events cause release of the clamp loader from the clamp and DNA, resulting in a loaded clamp. Figure adapted from .
Kelch et al. BMC Biology 2012 10:34 doi:10.1186/1741-7007-10-34