The present invention provides polypeptide purification reagents that are, themselves, proteins and methods for using these to purify almost any protein of interest. A polypeptide purification reagent comprises a polypeptide domain that can bind to a protein of interest and a polypeptide domain that confers a propensity to precipitate under conditions where the vast majority of proteins will not precipitate. In addition or as an alternative, a polypeptide purification reagent can comprise amino acid sequences that confer an ability to partition into a phase in a liquid--liquid phase system that the majority of proteins do not partition into. Methods for purifying a protein of interest using such polypeptide purification reagents are also provided. The methods include forming a precipitate comprising the polypeptide purification reagent and the protein of interest under conditions in which the vast majority of proteins do not precipitate and in which the biological activities of both proteins are substantially preserved. The methods further include an optional repurification by one or more reprecipitation steps or by affinity chromatography. In addition or as an alternative, the methods include recovering the protein of interest bound to the polypeptide purification reagent in a phase of a liquid--liquid phase system. The methods further include separating the protein of interest from the polypeptide purification reagent.