Fig. 5. Stabilization of the DHFR domain of MTS-GFP-DHFR and FGF-2-GFP-DHFR in the presence of aminopterin. Doxicycline-induced CHO_MTS-GFP-DHFR cells and CHO_{FGF-2-GFP-DHFR} cells were lysed in a detergent-containing buffer. The lysates were cleared by centrifugation and subjected to protease treatment (200 mg trypsin/ml) in the presence or absence of aminopterin (50 mM). Lanes 1-3: MTS-GFP-DHFR; lanes 4-6: FGF-2-GFP-DHFR. The various samples were subjected to SDS-PAGE and western blotting followed by antigen detection using monoclonal anti-DHFR antibodies. As can be seen in lanes 1 (MTS-GFP-DHFR) and 4 (FGF-2-GFP-DHFR), post-lysis degradation results in the appearance of the DHFR fragment without adding exogenous protease.