Polyethylene glycol (PEG) precipitation
PEG will precipitate proteins without denaturing or interfering with them, and so is a useful chemical to use.
The mechanism of PEG precipitation is not well understood, but it is thought that the protein molecules become surrounded by PEG and are denied access to water of solvation.
Selectivity of precipitation differs from that of (NH4)2SO4 precipitation, therefore the two methods might be effectively combined.