Mitochondrial intramembrane protease which plays a critical role in the regulation of mitochondrial function (PubMed:16713954, PubMed:21945938). Essential for mitochondrial development and fusion during spermatogenesis and the development of neurons and muscles (PubMed:16713954, PubMed:21945938). Essential for proteolytic processing of Pink1 and HtrA2 into their mature forms (PubMed:19048081). May also be involved, but not required, for the proteolytic processing of Opa1 (PubMed:19048081, PubMed:21945938). Cleaves type-1 transmembrane domains using a catalytic dyad composed of serine and histidine that are contributed by different transmembrane domains. Interacts with Pink1 and HtrA2. Lethal with most dying before pupation (PubMed:16713954). Mitochondria in the testis and skeletal muscles display morphological defects indicative of defective mitochondrial fusion (PubMed:16713954). The few escapers that survive to adulthood, display neurological defects, male sterility, and do not survive for longer than three days (PubMed:16713954). Also displays photoreceptor degeneration after prolonged light stimulation and abnormal photoreceptor synaptic function which specifically affects both the generation of light responses and the signal transfer across the first visual synapse (PubMed:16713954). Belongs to the peptidase S54 family. Extended N-terminus.