Protein inclusion into ice can dissociate subunits

An antifreeze protein's inclusion into ice can be used to purify it from other proteins and solutes. Domains that are covalently attached to the antifreeze protein are also drawn into the ice such that the ice-binding portion of the fusion protein can be used as an affinity tag. Here we have explored the use of ice-affinity tags on multi-subunit proteins. When an ice-binding protein was attached as a tag to multisubunit complexes a substantial portion of each multimer dissociated during overgrowth by the ice. The protein subunit attached to the affinity tag was enriched in the ice and the other subunit was appreciably excluded. We suggest that step growth of the advancing ice front generates shearing forces on the bound complex that can disrupt non-covalent protein-protein interactions. This will effectively limit the use of ice-affinity tags to single subunit proteins.