Most mitochondrial proteins are synthesized as precursors that carry N-terminal presequences. After import into mitochondria, these targeting signals are cleaved off by the mitochondrial processing peptidase MPP. Using the mitochondrial tandem protein Arg5,6 as model substrate, we demonstrate that MPP has an additional role in preprotein maturation, beyond the removal of presequences. Arg5,6 is synthesized as a polyprotein precursor that is imported into mitochondria and subsequently separated into two distinct enzymes. This internal processing is performed by MPP, which cleaves the Arg5,6 precursor at its N-terminus and at an internal site. The peculiar organization of Arg5,6 is conserved across fungi and reflects the polycistronic arginine operon in prokaryotes. MPP cleavage sites are also present in other mitochondrial fusion proteins from fungi, plants and animals. Hence, besides its role as “ticket canceller” for removal of presequences, MPP exhibits a second, conserved activity as internal processing peptidase for complex mitochondrial precursor proteins.

大多数线粒体蛋白都是作为携带 N 末端前序列的前体合成的。导入线粒体后，这些靶向信号被线粒体加工肽酶 MPP 裂解。使用线粒体串联蛋白 Arg5,6 作为模型底物，我们证明除了去除前序列之外，MPP 在前蛋白成熟中还具有额外的作用。 Arg5,6 被合成为多蛋白前体，被输入线粒体并随后分离成两种不同的酶。这种内部加工由 MPP 执行，它在 N 末端和内部位点切割 Arg5,6 前体。 Arg5,6 的特殊组织在真菌中是保守的，反映了原核生物中的多顺反子精氨酸操纵子。 MPP 切割位点也存在于来自真菌、植物和动物的其他线粒体融合蛋白中。因此，除了作为去除前序列的“取消票”的作用之外，MPP 还表现出第二种保守的活性，即复杂线粒体前体蛋白的内部加工肽酶。