Alternating site mechanism of the kinesin ATPase

The processivity of the microtubule-kinesin ATPase has been investigated using stopped-flow kinetic methods to measure the binding of each motor domain of the dimeric kinesin (K401) to the microtubule and the release of the fluorescent ADP analog, 2'(3')-O-(N-methylanthraniloyl)adenosine 5'-diphosphate (mantADP) from the active site of the motor domain. The results show that the release of two molecules of ADP from dimeric kinesin (K401) after the binding of kinesin ADP to the microtubule is a sequential process leading to biphasic kinetics. The maximum rate of release of mantADP from the first motor domain of K401 or monomeric K341 is fast (300 s-1) and independent of added nucleotide. The rate of mantADP release from the second motor domain of K401 is slow in the absence of added nucleotide (0.4 s-1) and reaches a maximum rate of 300 s-1 at saturating concentrations of ATP. High concentrations of ADP stimulate mantADP release from the second head to a maximum rate of 3.8 s-1. The nonhydrolyzable analog AMP-PNP and ATP-gamma S also stimulate ADP release from the second head (maximum rate of 30 s-1), suggesting that ATP hydrolysis is not necessary to stimulate the ADP release. These experiments establish an alternating site mechanism for dimeric kinesin whereby ATP binding to one kinesin active site stimulates the release of ADP from the second site such that the reactions occurring at the active sites of the two monomer units are kept out of phase from each other by interactions between the heads. These results define the steps of the ATPase pathway that lead to the efficient coupling of ATP hydrolysis to force production in a processive reaction whereby force production in forming a tight microtubule complex by one head is coupled to the rate-limiting release of the other head from the microtubule.