PPIase that catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides and may therefore assist protein folding (PubMed:20676357). Involved in regulation of the mitochondrial permeability transition pore (mPTP) (PubMed:26387735). It is proposed that its association with the mPTP is masking a binding site for inhibiting inorganic phosphate (Pi) and promotes the open probability of the mPTP leading to apoptosis or necrosis; the requirement of the PPIase activity for this function is debated (PubMed:26387735). In cooperation with mitochondrial p53/TP53 is involved in activating oxidative stress-induced necrosis (PubMed:22726440). Involved in modulation of mitochondrial membrane F(1)F(0) ATP synthase activity and regulation of mitochondrial matrix adenine nucleotide levels (By similarity). Has anti-apoptotic activity independently of mPTP and in cooperation with BCL2 inhibits cytochrome c-dependent apoptosis (PubMed:19228691). [protein]-peptidylproline (omega=180) = [protein]-peptidylproline (omega=0) Inhibited by cyclosporin A (CsA) (PubMed:20676357). Is displaced by CsA from the mPTP leading to a lower open probability of the mPTP. Associates with the mitochondrial membrane ATP synthase F(1)F(0) ATP synthase; the association is increased by inorganic phosphate (Pi) and decreased by cyclosporin A (CsA) (By similarity). Interacts with ATP5F1B; ATP5PD and ATP5PO (By similarity). Interacts with SLC25A3; the interaction is impaired by CsA (By similarity). Interacts with BCL2; the interaction is impaired by CsA (PubMed:19228691). Interacts with TP53; the association implicates preferentially tetrameric TP53, is induced by oxidative stress and is impaired by CsA (PubMed:22726440). Interacts with C1QBP (PubMed:20950273). Interacts with MCUR1 (PubMed:26976564). Component of the mitochondrial permeability transition pore complex (mPTPC), at least composed of SPG7, VDAC1 and PPIF (PubMed:26387735). Interacts with SPG7 (PubMed:26387735). Acetylated at Lys-167; deacetylated at Lys-167 by SIRT3. Belongs to the cyclophilin-type PPIase family.