Oral Presentation The 44th Lorne Conference on Protein Structure and Function 2019

The translocase of the inner membrane 22: a modern view of a mitochondrial insertase (#47)

Yilin Kang 1 , David Stroud 1 , Michael James Baker 1 , David Peter De Zouza 2 , Ann Frazier 3 , Dedreia Tull 2 , Malcolm McConville 2 , David R Thorburn 3 , Michael T Ryan 4 , Diana Stojanovski 1
  1. Biochemistry and Molecular Biology, Bio21 institute, the University of Melbourne, Melbourne, VIC, Australia
  2. Metabolomics Australia, Bio21 Institute, The University of Melbourne, Melbourne, VIC, Australia
  3. Department of Paediatrics, Murdoch Children’s Research Institute, Royal Children’s Hospital , Melbourne, VIC, Australia
  4. Department of Biochemistry and Molecular Biology, Monash Biomedicine Discovery Institute, Monash University, Melbourne, VIC, Australia

The Translocase of the Inner Membrane 22 (TIM22) is a mitochondrial inner membrane insertase that functions in the import and assembly of polytopic carrier proteins, which function in cellular metabolism and bioenergetics. We set out to determine the subunit composition of the human TIM22 complex, as the complex showed significant divergence from the well-characterised yeast complex. 

Utilising immunoprecipitation and mass spectrometric analyses, we identified two novel, metazoan-specific TIM22 complex subunits: Tim29 (Kang et al., eLife, 2016) and Acylglycerol Kinase, AGK (Kang et al., Molecular Cell, 2017). These discoveries are the first reports revealing additional subunits of the human TIM22 machine since its initial identification in 1999. Our investigations revealed exciting and unexpected functions of these human-specific subunits. We showed that Tim29 mediates the assembly and stability of the TIM22 complex and also creates contacts with the general entry gate in the outer mitochondrial membrane, the TOM complex. The TOM-TIM22 interaction is unique to human mitochondria and provides a novel mechanism of translocation for hydrophobic proteins across the soluble intermembrane space within mitochondria.

Characterisation of AGK (a previously described mitochondrial lipid kinase) revealed a lipid-kinase independent role of this protein in maintaining the integrity of TIM22 complex and the import of carrier proteins. Absence of this protein also leads to perturbation in TCA cycle, disrupting the central carbon metabolism. Interestingly, mutations in AGK cause Sengers syndrome, a metabolic disorder characterised by congenital cataracts, hypertrophic cardiomyopathy, skeletal myopathy and lactic acidosis. Our analysis with Sengers patient cells/tissues revealed a destabilisation of TIM22 complex accompanied by impaired carrier proteins biogenesis. Sengers syndrome was previously described as a disease of defective mitochondrial lipid metabolism. Our identification of AGK as a bona fide subunit of TIM22 resulted in a paradigm-shift, revealing that dysfunctional TIM22-mediated protein import is a new pathomechanism of Sengers syndrome.

  1. Kang Y, Baker MJ, Liem M, Louber J, McKenzie M, Atukorala I, Ang CS, Keerthikumar S, Mathivanan S & Stojanovski D (2016). Tim29 is a novel subunit of the human TIM22 translocase and is involved in complex assembly and stability. eLife
  2. Kang Y, Stroud DA, Baker MJ, De Souza D, Frazier AE, Liem M, Tull D, Mathivanan M, McConville MJ, Thorburn DR, Ryan MT, & Stojanovski, D (2017). AGK, a mitochondrial kinase associated with Sengers syndrome, is a subunit of the human TIM22 complex. Molecular Cell