The structural characteristics of human preprotein translocase of the inner mitochondrial membrane Tim23: Implications for its physiological activities

Structural characterisation of a full-length mitochondrial outer membrane TOM40 preprotein translocase: implications for its interaction with presequence peptides

Tom40, the central component of the preprotein translocase of the mitochondrial outer membrane (TOM complex), forms the import pore that facilitates the translocation of preproteins across the outer membrane. Though the function of Tom40 has been intensively studied, the details of the interactions between presequence peptides and Tom40 remain unclear. In this study, we expressed rat Tom40 in Escherichia coli and purified it from inclusion bodies before investigating the refolded protein by fluorescence spectroscopy and circular dichroism (CD) spectroscopy. The far-UV CD spectra of the refolded Tom40 in various concentrations of urea revealed that the refolded protein has a well-defined structure consisting mainly of β-sheet. Moreover, the specific binding of presequence peptides to Tom40, which was demonstrated by fluorescence quenching, showed that the refolded purified protein is functional and that the interaction between Tom40 and presequence peptides is mainly electrostatic in nature.

Expression of the human TIMM23 and TIMM23B genes is regulated by the GABP transcription factor

The TIM23 protein is a key component of the mitochondrial import machinery in yeast and mammals. TIM23 is the channel-forming subunit of the translocase of the inner mitochondrial membrane (TIM23) complex, which mediates preprotein translocation across the mitochondrial inner membrane. In this paper, we aimed to characterize the promoter region of the highly similar human TIM23 orthologs: TIMM23 and TIMM23B. Bioinformatic analysis revealed putative sites for the GA-binding protein (GABP) and the recombination signal binding protein for immunoglobulin kappa J (RBPJ) transcription factors in both promoters. Luciferase reporter assays, electrophoretic mobility shift assays, and chromatin immunoprecipitation experiments showed three functional sites for GABP and one functional site for RBPJ in both promoters. Moreover, silencing of GABPA, the gene encoding the DNA-binding subunit of the GABP transcription factor, resulted in reduced expression of TIMM23 and TIMM23B. Our results show an essential role of GABP in activating TIMM23 expression. More broadly, they suggest that physiological signals involved in activating mitochondrial biogenesis and oxidative function also enhance the transcription but not the protein level of TIMM23, which is essential for maintaining mitochondrial function and homeostasis.

Interaction of divalent metal ions with human translocase of inner membrane of mitochondria Tim23

The preprotein translocase of the inner membrane of mitochondria (TIM23 complex) is the main entry gate for proteins of the matrix and the inner membrane. Tim23p, the core component of TIM23 complex, forms the import pore across the inner membrane and exerts a key function in the protein import. However, the interaction of divalent metal ions with Tim23p and the contribution in the interaction of presequence peptide with Tim23p are still unknown. Herein, we investigated the interaction of divalent metal ions with the intermembrane space domain of Tim23p (Tim23IMS) and the interaction of presequence peptides with Tim23IMS in presence of Ca(2+) ion by fluorescence spectroscopy in vitro. The static fluorescence quenching indicates the existence of strong binding between divalent metal ions and Tim23IMS. The order of the binding strength is Ca(2+), Mg(2+), Cu(2+), Mn(2+), and Co(2+) (from strong to weak). Moreover, the interaction of presequence peptides with Tim23IMS is weakened in presence of Ca(2+) ion, which implicates that Ca(2+) ion may play an important role in the protein import by TIM23 complex.

Interaction of presequence peptides with human translocase of inner membrane of mitochondria Tim23

The preprotein translocase of the inner membrane of mitochondria (TIM23 complex) is the main entry gate for proteins of the matrix and the inner membrane. Tim23p, the core component of TIM23 complex, forms the import pore across the inner membrane. However, the interaction between presequence peptides and Tim23p remains unclear. Herein, we investigated the interaction of presequence peptides with the intermembrane space domain of Tim23p (Tim23IMS) by fluorescence and micro-Raman spectroscopy. The fluorescence quenching revealed that the interaction between Tim23IMS and presequence peptides is mainly electrostatic interaction. Micro-Raman spectroscopy and ANS binding experiments showed that presequence peptides induce a more compact conformation of Tim23IMS. GST pull-down experiments and tryptophan fluorescence indicated that there is no interaction between Tim23IMS and Tim50IMS.