ADP-induced rocking of the kinesin motor domain revealed by single-molecule fluorescence polarization microscopy

Kinesin is an ATP-driven molecular motor protein that moves processively al ong microtubules. Despite considerable research, the detailed mechanism of kinesin motion remains elusive. We applied an enhanced suite of single- and multiple-molecule fluorescence polarization microscopy assays to report th e orientation and mobility of kinesin molecules bound to microtubules as a function of nucleotide state. In the presence of analogs of ATP, ADP-Pi or in the absence of nucleotide, the kinesin head maintains a rigid orientatio n. In the presence of ADP, the motor domain of kinesin, still bound to the microtubule, adopts a previously undescribed, highly mobile state. This sta te may be general to the chemomechanical cycle of motor proteins; in the ca se of kinesin, the transition from a highly mobile to a rigid state after A DP release may contribute to the generation of the 8 nm step.