Recombinant proteins can be isolated from E. coli cells by repeated cycles of freezing and thawing.

Repeated cycles of freezing and thawing are sufficient to separate highly expressed recombinant proteins away from the cellular milieu of E. coli. Freezing and thawing liberates recombinant proteins from the bacterial cytoplasm, but does not release the bulk of endogenous E. coli proteins. Furthermore, protein secretion is not required. Fractionation of overexpressed proteins by freeze/thaw treatment does not depend on the identity of the recombinant protein and has been observed for thirty-five different recombinant proteins expressed in E. coli. These include proteins originally found in plant, animal or microbial sources, as well as several proteins designed de novo. Freezing and thawing typically yields approximately 50% of the recombinant protein in relatively pure form. Thus the freeze/thaw treatment can be utilized as a general method for the isolation of recombinant proteins from E. coli.