Expression of the S1S2 ligand binding domain [Kuusinen, A., Arvola, M. & Keinanen, K. (1995) EMBO J 14, 6327-6332] of the rat alpha-amino-3-hydroxy-5-methylisoxazole-3-propionic acid-selective glutamate receptor GluR2 in Escherichia coli under control of a T7 promoter leads to production of >100 mg/liter of histidine-tagged S1S2 protein (HS1S2) in the form of inclusion bodies. Using a novel fractional factorial folding screen and a rational, step-by-step approach, multiple conditions were determined for the folding of the HS1S2 alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid binding domain. Characterization of the HS1S2 ligand binding domain showed that it is water-soluble, monomeric, has significant secondary structure, and is sensitive to trypsinolysis at sites close to the beginning of the putative transmembrane regions. Application of a fractional factorial folding screen to other proteins may provide a useful means to evaluate E. coli as an economical and convenient expression host.
