Maltose binding protein (MBP) is the 370 amino acid product of the E.coli mal E gene. MBP is a useful affinity tag that can increase the expression level and solubility of the resulting tagged protein. The MBP tag also promotes proper folding of the attached protein. Plasmid vectors have been constructed utilizing the MBP domain that allow the synthesis of high levels of MBP-fusion proteins that can be purified in a one step procedure by affinity chromatography cross linked amylose resin. Once bound to amylose, the MBP protein can then be separated from the target protein by cleavage by coagulation Factor Xa at a specific four residue site. Alternatively, the intact fusion protein can be specifically eluted from the resin by the addition of excess free maltose.