Purification and characterization of human-derived Pneumocystis jirovecii dihydrofolate reductase expressed in Sf21 insect cells and in Escherichia coli

Pneumonia caused by Pneumocystis jirovecii is still a major opportunistic infection among patients with AIDS. This opportunitistic pathogen is susceptible to therapy with inhibitors of the enzyme dihydrofolate reductase (DHFR) that target cell growth. Recent studies have shown that recombinant human-derived Pneumocystis DHFR (pDHFR) differs from rat-derived pDHFR by 38% in amino acid sequence. However, characterization of drug susceptibility, kinetics, and the three-dimensional structure of human-derived pDHFR has been hampered by the limited availability of purified material. The present study was undertaken to develop procedures to prepare sufficient enzyme for structure/function studies. Protein yield was limited when human-derived pDHFR was expressed in Escherichia coli using a pET28a(+) vector with an N-terminal His-tag for the 25 kDa protein. Therefore, the protein was expressed in Sf21 insect cells by baculovirus infection. The soluble enzyme was purified from cell lysates via Ni-chelated chromatographic columns, yielding about 5.1 mg of human-derived pDHFR fusion protein per liter of Sf21 culture. The purified protein had the expected mass as determined from Western blots with antibody for the N-terminal His-tag. This His-tagged recombinant DHFR from human-derived Pneumocystis was catalytically active and demonstrated kinetics similar to the recombinant enzyme from rat-derived Pneumocystis. The present studies for production of soluble human-derived pDHFR indicated that the baculovirus expression system supported production of significantly purer catalytically active enzyme in higher yields than that expressed in bacterial cultures. These protocols now make it possible to facilitate screening of antifolates with selectivity for human-derived pDHFR and to determine its three-dimensional structure.