  Conventional analysis of in vitro assays of motor proteins rests on the
assumption that all proteins with the same chemical composition function
identically; however molecule-to-molecule variation is often seen even in
well-controlled experiments. In an effort to obtain a statistically meaningful
set of time traces that simultaneously avoid any experimental artifacts, we
performed quantum-dot labeled kinesin experiments on both surface and levitated
microtubules. Similar to glassy systems, we found that mean velocities of
individual kinesin motors vary widely from one motor to another, the variation
of which is greater than that expected from the stochastic variation of
stepping times. In the presence of heterogeneity, an ensemble-averaged quantity
such as diffusion constant or randomness parameter is ill-defined. We propose
to analyze heterogeneous data from single molecule measurements by decomposing
them into homogeneous subensembles.
