Lipid-Mediated Unfolding of 3β-Hydroxysteroid Dehydrogenase 2 Is Essential for Steroidogenic Activity

For inner mitochondrial membrane (IMM) proteins that do not undergo N-terminal cleavage, the activity may occur in the absence of a receptor present in the mitochondrial membrane. One such protein is human 3 beta-hydroxysteroid dehydrogenase 2 (3 beta HSD2), the IMM resident protein responsible for catalyzing two key steps in steroid metabolism: the conversion of pregnenolone to progesterone and dehydroepiandrosterone to androstenedione. Conversion requires that 3 beta HSD2 serve as both a dehydrogenase and an isomerase. The dual functionality of 3 beta HSD2 results from a conformational change, but the trigger for this change remains unknown. Using fluorescence resonance energy transfer, we found that 3 beta HSD2 interacted strongly with a mixture of dipalmitoylphosphatidylglycerol (DPPG) and dipalmitoylphosphatidylcholine (DPPC). 3 beta HSD2 became less stable when incubated with the individual lipids, as indicated by the decrease in thermal denaturation (T-m) from 42 to 37 degrees C. DPPG, alone or in combination with DPPC, led to a decrease in a-helical content without an effect on the beta-sheet conformation. With the exception of the 20 N-terminal amino acids, mixed vesicles protected 3 beta HSD2 from trypsin digestion. However, protein incubated with DPPC was only partially protected. The lipid-mediated unfolding completely supports the model in which a cavity forms between the alpha-helix and beta-sheet. As 3 beta HSD2 lacks a receptor, opening the conformation may activate the protein.