Catalytic component of the mitochondrial inner membrane peptidase (IMP) complex. IMP catalyzes the removal of signal peptides required for the targeting of proteins from the mitochondrial matrix, across the inner membrane, into the inter-membrane space. The two catalytic IMP subunits seem to have non-overlapping substrate specificities. IMP1 substrates include nuclear encoded CYB2, mitochondrially encoded COX2, NADH-cytochrome b5 reductase and GUT2. Component of the mitochondrial inner membrane peptidase (IMP) complex which at least consists of IMP1, IMP2 and SOM1. Belongs to the peptidase S26 family. IMP1 subfamily.