The structures of the same protein, determined under different conditions, provide clues toward understanding the role of structural changes in the protein's function. Structural changes are usually identified as rigid-body motions, which are defined using a particular threshold of rigidity, such as domain motions. However, each protein actually undergoes motions with various size and magnitude ranges. In this study, to describe protein structural changes more comprehensively, we propose a method based on hierarchical clustering. This method enables the illustration of a wide range of protein motions in a single tree diagram, named the "Motion Tree". We applied the method to 432 proteins exhibiting large structural changes and classified their Motion Trees in terms of the characteristic indices of the trees. This classification of the Motion Trees revealed clear relationships to their protein functions. Especially, complex structural changes are significantly correlated with multi-step protein functions. © 2013.
