 Expansin

An expansin is a protein that promotes extension of a plant cell wall without requiring detectable hydrolysis of its structural polysaccharides. Wall extension and destruction of the wall are different processes: rearranging a load-bearing network can allow it to yield while leaving its components largely intact.^[ref]

The motivating 1992 experiment extracted proteins from growing seedling cell walls and added the extract to isolated walls. Fractionation separated two active proteins, about $29$ and $30\mathrm{kDa}$, each able to restore extension. The activity depended on conditions such as pH and was not equally effective on every tested wall type.^[ref]

A useful control compares equally loaded isolated walls with and without the active preparation. Extension after addition connects a measurable mechanical response to material in the preparation; measuring only the abundance of a protein in a growing stem would provide a weaker connection. The reconstitution result does not assign every change in whole-plant growth to expansin.