Liquid soluble protein is needed for many experiments

I perform an experiment that needs to solubilize objective proteins to purify them. To purify a protein, I use an affinity tag purification method, which is based on the protein-protein interaction between the objective protein conjugated with a tag and a ligand which has an affinity against the tag. Fundamentally, the protein-protein interaction occurs in liquid, and then the method needs the liquid soluble proteins. It is sometimes difficult to establish a condition that an objective protein is efficiently soluble.