Role of the Deafness Dystonia Peptide 1 (DDP1) in Import of Human Tim23 into the Inner Membrane of Mitochondria

Abstract

Tim8 and Tim13 of yeast belong to a family of evolutionary conserved zinc finger proteins that are organized in hetero-oligomeric complexes in the mitochondrial intermembrane space. Mutations in DDP1 (deafness dystonia peptide 1), the human homolog of Tim8, are associated with the Mohr-Tranebjaerg syndrome, a progressive neurodegenerative disorder. We show that DDP1 acts with human Tim13 in a complex in the intermembrane space. The DDP1⅐hTim13 complex is in direct contact with translocation intermediates of human Tim23 in mammalian mitochondria. The human DDP1⅐hTim13 complex complements the function of the TIM8⅐13 complex in yeast and facilitates import of yeast and human Tim23. Thus, the pathomechanism underlying the Mohr-Tranebjaerg syndrome may involve an impaired biogenesis of the human TIM23 complex causing severe pleiotropic mitochondrial dysfunction.The vast majority of mitochondrial proteins are encoded as precursors in the nuclear genome. Mitochondrial biogenesis is, therefore, dependent on the import and sorting of the nuclear encoded precursor proteins into mitochondrial subcompartments. In eukaryotes three distinct preprotein import systems located in the mitochondrial outer and inner membrane have been described (1-5). The outer membrane contains a general preprotein translocase, the TOM 1 complex, which mediates the recognition and binding of preproteins and their transfer across the outer membrane. This complex is most likely used by all nuclear encoded precursors. Import into and across the inner membrane is mediated by two distinct inner membrane translocases, the TIM22 and the TIM23 complexes. Both TIM complexes cooperate with the TOM complex but differ in their substrate specificity (6 -11). The TIM23 complex mediates import of preproteins with a positively charged matrix targeting signal into the mitochondrial matrix space and into the inner membrane (6, 12, 13). The translocation of such precursors into the matrix requires the membrane potential ⌬ across the inner membrane and ATP in the matrix. The ⌬ drives the translocation of the presequences through the protein-conducting channel of the TIM23 complex which is formed by the membrane-integrated proteins Tim23 and Tim17 (6,12). A molecular motor that is attached to the inner side of this channel then promotes further translocation of the mature portion of the preproteins into the matrix. This motor consists of the peripheral membrane protein Tim44, the mitochondrial Hsp70, and the nucleotide exchange factor Mge1. Together, these components in repeated ATP-dependent reaction cycles facilitate the vectorial translocation into the matrix in a stepwise manner (14).The TIM22 complex mediates the insertion of a class of hydrophobic proteins with internal targeting signals into the inner membrane (7)(8)(9)(10)(11)(15)(16)(17). Typical substrates are members of the mitochondrial carrier family and other integral inner membrane proteins that are synthesized without a matrix-targeting signal. Insertion of these precursors into the inner membrane is s...

Protein Import Into Mitochondria

“…Therefore, mutations in DDP1 might considerably impair the biogenesis of Tim23 in humans, particularly under conditions when membrane potential is low. This would support the conclusion that MT syndrome is caused by an impaired biogenesis of the human TIM23 translocase, leading to severe pleiotropic mitochondrial dysfunction (147)(148)(149).…”

“…MT syndrome is the rst known human disease, which is caused by an impaired mitochondrial import machinery. Like their yeast homologous, human DDP1 and human Tim13 form a 70 kDa complex in the intermembrane space that facilitates import of the human Tim23 protein (147). The DDP1/hTim13 complex also rescues the cold-sensitive growth defect of a yeast Tim8/Tim13 deletion strain (81,147).…”

“…The DDP1/hTim13 complex also rescues the cold-sensitive growth defect of a yeast Tim8/Tim13 deletion strain (81,147). Further, the import of human Tim23 into isolated yeast mitochondria strongly requires the presence of the DDP1/hTim13 complex even if the membrane potential is high (147). The stronger dependence on the presence of the DDP1/hTim13 complex of the import of human Tim23 is probably due to a weaker insertion signal in its C-terminal half (35,81).…”

“…Like their yeast homologous, human DDP1 and human Tim13 form a 70 kDa complex in the intermembrane space that facilitates import of the human Tim23 protein (147). The DDP1/hTim13 complex also rescues the cold-sensitive growth defect of a yeast Tim8/Tim13 deletion strain (81,147). Further, the import of human Tim23 into isolated yeast mitochondria strongly requires the presence of the DDP1/hTim13 complex even if the membrane potential is high (147).…”

Mitochondrial ATP‐independent chaperones

“…Bioinformatics analysis of the small Tims from a wide range of eukaryotes indicates that the Tim12 proteins are not a distinct lineage, rather having arisen independently in different organisms by duplication of one of the other four small Tims (19). Although it is nonessential in yeast, mutations in the gene encoding the human homologue of Tim8, DDP1 (deafness/ dystonia peptide 1), can cause Mohr-Tranebjaerg syndrome, an X-linked neurodegenerative disorder, characterized by deafness, dystonia, mental retardation, and blindness (20)(21)(22). The small Tim family shares a characteristic conserved twin CX 3 C motif (where X is any amino acid), the two motifs themselves being separated by 11-16 amino acids (23).…”