ATP-dependent zinc metalloprotease. Plays a role in the formation and regulation of the mitochondrial permeability transition pore (mPTP) and its proteolytic activity is dispensable for this function (By similarity). Binds 1 zinc ion per subunit. Forms heterooligomers with AFG3L1 and AFG3L2 (PubMed:22563492, PubMed:17101804, PubMed:19656850). Component of the mitochondrial permeability transition pore complex (mPTPC), at least composed of SPG7, VDAC1 and PPIF (By similarity). Interacts with AFG3L2; the interaction is required for the efficient assembly of mitochondrial complex I (PubMed:22563492, PubMed:19656850). Interacts with AFG3L1 (PubMed:19656850). Interacts with MAIP1. Interacts with VDAC1 and PPIF (By similarity). Expressed in the brain and retina (at protein level). Upon import into the mitochondrion, the N-terminal transit peptide is cleaved by the mitochondrial-processing peptidase (MPP) to generate an intermediate form which undergoes a second proteolytic cleavage mediated by proteases AFG3L1 and/or AFG3L2 removing an additional N-terminal fragment to generate the proteolytically active mature form. Mice are affected by a distal axonopathy of spinal and peripheral axons, characterized by axonal swelling and degeneration. Mitochondrial morphological abnormalities occur in synaptic terminals and in distal regions of axons long before the first signs of swelling and degeneration and correlate with onset of motor impairment during a rotarod test. In the N-terminal section; belongs to the AAA ATPase family. In the C-terminal section; belongs to the peptidase M41 family. According to PubMed:22563492, alternative splicing gives rise to an isoform (Paraplegin-2) which is identical to the sequence of the mature protein and localizes to the endoplasmic reticulum. Extended N-terminus. Extended N-terminus.