title: Mitochondrial import receptors Tom20 and Tom22 have chaperone-like activity.
abstract: Mitochondrial preproteins are synthesized in the cytosol with N-terminal signal sequences (presequences) or internal targeting signals. Generally, preproteins with presequences are initially recognized by Tom20 (translocase of the outer membrane) and, subsequently, by Tom22, whereas hydrophobic preproteins with internal targeting signals are first recognized by Tom70. Recent studies suggest that Tom70 associates with molecular chaperones, thereby maintaining their substrate preproteins in an import-competent state. However, such a function has not been reported for other Tom component(s). Here, we investigated a role for Tom20 in preventing substrate preproteins from aggregating. In vitro binding assays showed that Tom20 binds to guanidinium chloride unfolded substrate proteins regardless of the presence or absence of presequences. This suggests that Tom20 functions as a receptor not only for presequences but also for mature portions exposed in unfolded preproteins. Aggregation suppression assays on citrate synthase showed that the cytosolic domain of Tom20 has a chaperone-like activity to prevent this protein from aggregating. This activity was inhibited by a presequence peptide, suggesting that the binding site of Tom20 for presequence is identical or close to the active site for the chaperone-like activity. The cytosolic domain of Tom22 also showed a similar activity for citrate synthase, whereas Tom70 did not. These results suggest that the cytosolic domains of Tom20 and Tom22 function to maintain their substrate preproteins unfolded and prevent them from aggregating on the mitochondrial surface.
authors.affiliation: Department of Molecular Genetics, Graduate School of Medical Sciences, Kumamoto University, Honjo 1-1-1, Kumamoto 860-8556, Japan. myano@gpo.kumamoto-u.ac.jp
authors.forename: Masato
authors.initials: M
authors.lastname: Yano
authors.forename: Kazutoyo
authors.initials: K
authors.lastname: Terada
authors.forename: Masataka
authors.initials: M
authors.lastname: Mori
mesh_terms: D000818:Animals; D001665:Binding Sites; D003600:Cytosol / Q000737:chemistry / Q000378:metabolism; D017353:Gene Deletion; D005982:Glutathione Transferase / Q000378:metabolism; D049452:Green Fluorescent Proteins; D019791:Guanidine / Q000494:pharmacology; D006801:Humans; D008164:Luminescent Proteins / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000502:physiology*; D008928:Mitochondria / Q000378:metabolism; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D024101:Mitochondrial Proteins; D018832:Molecular Chaperones / Q000502:physiology*; D010957:Plasmids / Q000378:metabolism; D011485:Protein Binding; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D011956:Receptors, Cell Surface / Q000502:physiology*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D015398:Signal Transduction; D013552:Swine; D013997:Time Factors
chemical_list: D008164:Luminescent Proteins; D026901:Membrane Transport Proteins; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D024101:Mitochondrial Proteins; D018832:Molecular Chaperones; D011956:Receptors, Cell Surface; D011993:Recombinant Fusion Proteins; C084008:TOMM20 protein, human; C489971:TOMM22 protein, human; D049452:Green Fluorescent Proteins; D005982:Glutathione Transferase; D019791:Guanidine