  Rigidity analysis using the "pebble game" can usefully be applied to protein
crystal structures to obtain information on protein folding, assembly and the
structure-function relationship. However, previous work using this technique
has not made clear how sensitive rigidity analysis is to small structural
variations. We present a comparative study in which rigidity analysis is
applied to multiple structures, derived from different organisms and different
conditions of crystallisation, for each of several different proteins. We find
that rigidity analysis is best used as a comparative tool to highlight the
effects of structural variation. Our use of multiple protein structures brings
out a previously unnoticed peculiarity in the rigidity of trypsin.
