High impact information on TOM40

• The central receptor Tom22 binds preproteins through both its cytosolic domain and its intermembrane space domain and is stably associated with the channel protein Tom40 (refs 11-13) [1].
• ISP42 is the first mitochondrial membrane protein shown to be indispensable for protein import and cell viability [2].
• This protein (termed import-site protein 42 or ISP 42) is exposed on the mitochondrial surface; antibodies against it block protein import into mitochondria [3].
• Two novel mitochondrial outer membrane proteins in yeast, Tom13 and Tom38/Sam35, mediate assembly of mitochondrial beta-barrel proteins, Tom40, and/or porin (VDAC) in the outer membrane [4].
• One of the tom40 mutants, tom40-97, carries a single point mutation (W(243)R) resulting in an ineffective transfer of precursors to the TIM23 complex [5].

Biological context of TOM40

• The additional function of the TOM complex channel is probably facilitated by the upregulation of nuclear-encoded components of the TOM complex as has been shown for Tom40 (a major component of the channel) and Tom70 (one of the surface receptors) [6].
• Mitochondria lacking Mdm10 are selectively impaired in the final steps of the assembly pathway of Tom40, including the association of Tom40 with the receptor Tom22 and small Tom proteins, while the biogenesis of porin is not affected [7].
• To search genetically for additional components of the protein translocation apparatus of mitochondria, we have used low fidelity PCR mutagenesis to generate temperature-sensitive mutants in the outer membrane translocation pore component ISP42 [8].
• To investigate the sorting of precursor proteins to these various sub-mitochondrial compartments, we created a library of tom40 mutants and screened for alleles selectively corrupt in protein sorting [5].
• In vivo this fusion protein lost its ability to suppress the temperature-sensitive phenotype of an isp42 mutant, while the other fusion protein DHFR-6, which was found targeted correctly to mitochondria, suppressed the mutant as well as the wild-type ISP6 [9].

Anatomical context of TOM40

• Most mitochondrial proteins are synthesized in the cytosol, imported into mitochondria via the TOM40 (translocase of the mitochondrial outer membrane 40) complex, and follow several distinct sorting pathways to reach their destination submitochondrial compartments [10].
• Protein import channel of the outer mitochondrial membrane: a highly stable Tom40-Tom22 core structure differentially interacts with preproteins, small tom proteins, and import receptors [11].
• The GIP complex isolated from outer membranes exhibits characteristic TOM channel activity with two coupled conductance states, each corresponding to the activity of purified Tom40, suggesting that the complex contains two simultaneously active and coupled channel pores [11].
• These results indicate that, in contrast to the nonstick tunnel of the ribosome for polypeptide exit, the Tom40 channel offers an optimized environment to translocating non-native precursor proteins by preventing their aggregation [12].

Associations of TOM40 with chemical compounds

• Neither the NH(2) nor the COOH termini are necessary to target Tom40 to the outer membrane [13].
• The complex containing stuck precursor, mitochondrial hsp70, and ISP42 could be solubilized from mitochondria by the non-ionic detergent Triton X-100 even without crosslinking, suggesting tight association of these three components [14].
• Although the binding of the targeting sequence to Tom40p is inhibited by urea concentrations in excess of 1 M, it is moderately resistant to 1 M salt [15].
• An equivalent proline residue is important in targeting each of the other three tail-anchored proteins that associate with Tom40 to form the core of the TOM translocase [16].

Physical interactions of TOM40

• Tom40 exists in a homo-oligomeric assembly and dynamically interacts with Tom6 [17].
• The mitochondrial outer membrane contains two integral proteins essential for cell viability, Tom40 of the translocase of the outer membrane (TOM complex) and Sam50 of the sorting and assembly machinery (SAM complex) [18].

Regulatory relationships of TOM40

• The Tom40 assembly is influenced by a block of negatively charged amino acid residues in the cytosolic domain of Tom22, indicating a cross-talk between preprotein receptors and the translocation pore [17].

Other interactions of TOM40

• Yeast mutants of TOM40, MAS37, and SAM50 also show aberrant mitochondrial morphology [7].
• (iii) Tom6 functions as an assembly factor for Tom22, promoting its stable association with Tom40 [19].
• Rpm2p also increases the level of Tom40p, as well as Hsp60p, but not Atp2p, suggesting that some, but not all, nucleus-encoded mitochondrial components are affected [20].
• A mechanism for this partitioning is suggested by our observations that interaction with a 40 kDa protein of the translocase of outer mitochondrial membrane (Tom40p), occurs preferentially with unfolded, unphosphorylated forms of Ynk1p [21].
• Cross-linking studies showed that the Tom40 precursor contacts the Tim8-Tim13 complex [22].

Analytical, diagnostic and therapeutic context of TOM40

• Immunoprecipitation of the gene product, ISP42p, from mitochondria solubilized under mild conditions reveals a multi-protein complex containing ISP6p and ISP42p [8].
• We have directly compared expressed and renatured Tom40 from both species and find a high content of beta-structure in circular dichroism measurements in agreement with refined secondary structure predictions [23].

References