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Forschungsarbeit

The physiological role of the cancer associated protein MCJ in the human mitochondrial protein transport

by Christina Schusdziarra (19.12.2014)

Hypomethylation and hypermethylation are well-known gene modifications involved in cancer. These modifications can lead to silencing of tumor-suppressor genes and to the activation of growth-promoting genes. The methylation controlled J gene (MCJ) was first described to be differentially expressed in ovarian tumor cell lines and normal ovarian epithelial cells (Shridar V. et al., Cancer Res. 2001). The inhibition of its expression in primary ovarian tumors and ovarian carcinoma cell lines was linked to the methylation of a CpG island within the first exon and the first intron of the MCJ gene (Strathdee, G. et al., Carcinogenesis 2004). In addition, the epigenetic inactivation by hypermethylation of MCJ appears to play a role in Wilm’s tumors, in malignant pediatric brain tumors and in melanomas (Ehrlich, M. et al., Oncogene 2002, Lindsey, J. C. et al., Int. J. Cancer 2006, Muthusamy, V. et al., Cancer Res. 2006). Interestingly, the hypermethylation-controlled loss of MCJ expression correlates with resistance against various chemotherapeutic drugs, such as paclitaxel, topotecan and cisplatin, and also with poor survival and therapeutic response of ovarian tumor patients (Shridar V. et al., Cancer Res. 2001, Hatle, K. M. et al., Mol. Cell Biol. 2007, Strathdee, G. et al., Gynecol. Oncol. 2005). A parental MCJ-non-expressing ovarian carcinoma cell line was less chemoresistant, when it was stably transfected with MCJ, indicating that the chemoresistance phenotype can be rescued by expression of MCJ (Shridar V. et al., Cancer Res. 2001).  Moreover, MCJ is expressed in drug-sensitive, but not in multidrug-resistant breast cancer cells (Hatle, K. M. et al., Mol. Cell Biol. 2007).

Schusdziarra : Fig. 1[Bildunterschrift / Subline]: Model of MCJ in the human TIM23 translocase
© C. Schusdziarra, 2014
MCJ (Methylation Controlled J protein) resides in the inner mitochondrial membrane with its N-terminus facing the intermembrane space (IMS). MCJ interacts with components of the TIM23 translocase. The J domains of MCJ and DNAJC19 are able to stimulate the ATPase activity of mortalin, the human mtHsp70 chaperone. MAGMAS counteracts this stimulatory activity of MCJ and DNAJC19 upon formation of a stable sub-complex with either of them. MCJ plays a functional role in the import of preproteins (precursor) via the TIM23 translocase.

Since chemoresistance of tumor cells is a major problem in the therapeutic treatment of cancer patients, it is important to elucidate the underlying molecular mechanisms and to shed light on the proteins involved. In order to obtain insight into the pathological state of chemoresistant cells in absence of MCJ, it is necessary to investigate the physiological role of the MCJ protein. The physiological function of MCJ was completely unclear at the beginning of this study.

Subcellular fractionation revealed that MCJ is a mitochondrial protein, spanning the inner membrane once with its N-terminus facing the intermembrane space (IMS). This result corrected the reported localization of MCJ in the Golgi compartment (Hatle, K. M.  et al., Mol. Cell Biol. 2007). In line with the mitochondrial location, co-immunoprecipitation experiments demonstrated that MCJ interacts with components of the TIM23 (translocase of the inner membrane) translocase. Since the majority of the mitochondrial proteome is encoded in the nucleus of the cell and translated at cytosolic ribosomes as precursor proteins, several mitochondrial translocases exist which import hundreds of different proteins into the organelle. The TIM23 translocase is necessary for translocation of precursor proteins into the inner mitochondrial membrane and into the mitochondrial matrix. Although the yeast TIM23 translocase is well understood, little is known about the structure and the function of the human TIM23 translocase.

MCJ forms a highly stable sub-complex with MAGMAS, a component of the import motor of the TIM23 translocase, mediated by the soluble domains of both proteins. Moreover, MCJ is important for the mitochondrial import of precursor proteins translocated by the TIM23 translocase. In vitro, MCJ stimulates the ATPase activity of mortalin, the mitochondrial Hsp70 chaperone of the human TIM23 translocase, suggesting that MCJ acts as a J co-chaperone of the translocase and is important to drive the import process. Consistent with these results, MCJ was able to rescue the lethal deletion of its homologue in yeast (Schusdziarra, C. et al., Hum. Mol. Genet. 2013).

Taken together, the results of this study identified MCJ as a novel functional component of the human TIM23 translocase. MCJ contributes to the import of mitochondrial precursor proteins being a stimulating co-chaperone of the TIM23 translocase import motor. These results suggest that MCJ might provide a link between tumorigenesis, chemoresistance of cancer cells and mitochondrial protein transport.


Scientific career
  • 2010-2014
  • PhD at LMU Munich with Prof. Dr. Dr. W. Neupert (since May 2011 Prof. A. Ladurner)/Prof. Dr. A. Vollmar, lab of PD K. Hell "Functional characterization of MCJ in the protein import into human mitochondria”
  • 2009
  • Pharmaceutical license
  • 2004-2009
  • Pharmaceutical studies at Ludwig Maximilian University (LMU), Munich

Scholarships and Awards
  • * Oskar-Karl-Forster-Stipendium (2014)
  • * Participation at the Lindau Nobel Laureate Meeting (2013)
  • * Member of the graduate program “Protein Dynamics in Health and Disease” of the Elite Network of Bavaria (ENB) (2011-2013)
  • * Scholarship of the Universität Bayern (2011-2014)

Publications
  • * Christina Schusdziarra, Marta Blamowska, Abdussalam Azem, Kai Hell: Methylation-controlled J-protein MCJ acts in the import of proteins into human mitochondria Hum. Mol. Genet. 2013