A crucial and challenging problem in proteomics is purification, identification, and characterization of proteins, some of which are expressed at very low levels. The preferred method for purification of low abundance proteins exploits multiple affinity purification tags on a single recombinant protein eg tandem affinity purification (TAP)[1]. However TAP is both experimentally lengthy, involving many sequential binding, washing and elution steps and costly, requiring two different and expensive resins to recover the purified recombinant protein. Hence processing large amounts of cell lysate makes it prohibitively expensive especially for scale-up.

Here we present a new and simple strategy to purify soluble recombinant proteins from E. coli at a protein concentration that approaches the limit of a single protein molecule per cell. This method utilizes the unique aggregation properties of elastin-like polypeptides (ELPs) to capture recombinant fusion proteins composed of a target protein and an ELP tag from cell lysate. ELPs are artificial, genetically encodable polypeptides composed the repeating pentapeptides sequence VPGXG, where the guest residue (X) can be any naturally occurring amino acid except Pro [2, 3]. ELPs exhibit a unique reversible inverse phase transition behavior; below a critical transition temperature (Tt) ELPs are highly soluble in aqueous solution, however at temperatures even a few degrees Celsius above Tt, ELP will undergo a solubilityinsolubility phase transition, leading to aggregation of the polypeptide [4]. The Tt of an ELP is a function of a number of variables including identity and stoichiometry of the guest residue, molecular weight, ELP and salt concentration in aqueous solution [5–9].