Fig. 4.

Mechanical coupling between protein subunits in a bundle can influence hydrolysis chemistry. Here protein monomers can convert from GTP to GDP. If there is an associated structural change in the protein filaments upon hydrolysis, and the protofilaments can bind laterally, strain energy can develop as monomers convert GTP to GDP. The strain energy can prevent hydrolysis from proceeding further, as described in “Forces Influence Protein Enzymatic Activity.” This mechanochemical coupling in a protein bundle can lead to cooperative effects and explains microtubule instability. A model that describes this coupling was reported previously (155).