Recombinant human KRAS (GTPase), isoform a, encompassing amino acids 2-186 (end). In this construct the N-terminal end of the protein contains a His-tag (6xHis) followed by an optimal TEV protease target sequence that allows cleavage of the His-tag from the protein (TEV: tobacco etch virus cysteine protease, BPS Bioscience #50308). The protein also contains cancer-associated mutation G12C. KRAS (G12C) His-Tag (BPS Bioscience #100413) was affinity purified and was loaded with BODIPY-GDP. Unbound BODIPY-GDP was removed by spin column and the protein is ready for a GDP/GTP exchange assay.