Combined use of extraction and genetic engineering for protein purification: recovery of beta-galactosidase fused proteins.

Partitioning of beta-galactosidase in aqueous two-phase systems of poly(ethylene glycol) and potassium phosphate is reviewed. The affinity of Escherichia coli beta-galactosidase for the PEG-rich phase dominates also in beta-galactosidase fusion proteins and the concept of using beta-galactosidase as an affinity handle for extraction of other proteins, after fusion, is discussed. A hypothesis is presented, assuming that tryptophan residues at the surface of beta-galactosidase is responsible for its partitioning to the PEG rich phase, and the concept of poly-tryptophan handles fused to the target protein for extraction is introduced.