Carolyn K. Suzuki, Ph.D. Assistant Professor

Lab Members:

Modulation of apoptosis by Tid1- a human homolog of the Drosophila tumor suppressor Tid156

Mitochondria are essential organelles that have emerged as a central control point for the regulation of apoptotic cell death. Recent work has shown that the activation and inhibition of apoptosis are mediated by a number of mitochondrial proteins such as cytochrome c, Apoptosis Inducing Factor and Smac-1/Diablo. It is likely that other mitochondrial proteins will also function as key regulators of apoptosis.

Human Tid1-Long and -Short (hTid1-L and hTid1-S respectively), are mitochondrial matrix proteins that show a high degree of sequence similarity to the tumor suppressor Tid56 of Drosophila melanogaster and belong to the DnaJ family of molecular chaperones. The absence of Tid56 expression in Drosophila results in defective differentiation and morphogenesis giving rise to tumorous imaginal discs that proliferate to form lethal tumors, which maintain their neoplastic characteristics when transplanted into wild-type flies.

hTid1-L and -S are produced by alternative splicing and are 98% identical, differing only at their carboxyl-terminal tails. Work by Karl Münger's lab strikingly showed that hTid1-L and a dominant negative mutant of the Short isoform augment cell death induced by an apoptotic signal, whereas hTid1-S and a dominant negative mutant of the Long isoform inhibit cell death. Although these proteins are principally localized to the mitochondrial matrix, recent data suggest that hTid1-L and -S also function within the cytosol or nucleus by interacting with the human papillomavirus oncoprotein- E7, the Ras GTPase activating protein- Ras-GAP and the Janus kinase- Jak2.

Our lab is interested in understanding the mechanisms by which hTid1-L and -S carry out their distinctly different functions, not only as modulators of apoptotic cell death but also as regulators of normal cellular processes such as the folding and degradation of proteins.

Regulation of Mitochondrial Genome Stability and Expression by the ATP-dependent Lon Protease

ATP-dependent proteases are highly conserved enzymes present in humans as well as in archaea and bacteria. In eukaryotes, regulated degradation by the ubiquitin- proteasome system is understood in great detail. Far less is known however, about the regulatory function of ATP-dependent proteases localized to mitochondria. Recent work has shown that the human disease Hereditary Spastic Paraplegia is caused by mutations in a gene that has high sequence similarity to ATP-dependent proteases localized to the mitochondrial inner membrane of yeast. Studies performed in the budding yeast Saccharomyces cerevisiae have demonstrated the crucial role of mitochondrial proteases and chaperones in organelle biogenesis, respiration and mitochondrial DNA stability and gene expression.

Our lab aims at a mechanistic understanding of the multi-function mitochondrial ATP-dependent Lon protease. Mitochondrial Lon possesses a protease and chaperone-like activity in addition to a DNA-binding activity that we believe monitors the folding and assembly of newly synthesized proteins as well as the integrity and expression of the mitochondrial genome.

We use a combination of mammalian and yeast genetics, biochemistry, cell biology and structural biology to understand the mechanisms by which the mitochondrial ATP-dependent Lon protease and Tid1 chaperones regulate both protein and DNA function and how these proteins function in modulating apoptotic cell death.

Recent Publications:

1) Suzuki, C.K., K. Suda, N. Wang and G. Schatz. Requirement of the yeast gene LON in intramitochondrial proteolysis and maintenance of respiration. Science, 264 : 273-276 (1994).
2) Rep, M., J.M. van Dijl, K. Suda, G. Schatz, L.A. Grivell and C.K. Suzuki, Promotion of mitochondrial membrane complex assembly by a proteolytically inactive yeast Lon. Science, 274 : 103-106 (1996).
3) Suzuki, C.K., M. Rep, J. M. van Dijl, K. Suda, L.A. Grivell and G. Schatz. ATP-dependent proteases that also chaperone protein biogenesis. Trends Biochem. Sci. 22:118-123 (1997).
4) van Dijl, J.M., E. Kutejova, K. Suda, D. Perecko, G. Schatz and C.K. Suzuki. The ATPase and protease domains of yeast mitochondrial Lon: Roles in proteolysis and respiration-dependent growth. Proc. Natl. Acad. Sci. USA,18 : 10584 - 10589 (1998).
5) Beilharz, T., C. K. Suzuki and T. Lithgow, A toxic fusion protein accumulating between the mitochondrial membranes inhibits protein assembly in vivo., J. Biol. Chem., 273:35268-35272 (1998).
6) Stahlberg, H., E. Kutejova, K. Suda, B. Wolpensinger, A. Lustig, G. Schatz, A. Engel, and C.K. Suzuki. Mitochondrial Lon of Saccharomyces
cerevisiae is a ring-shaped protease with seven flexible subunits. Proc. Natl. Acad. Sci., USA 96: 6787-6790 (1999).
7) Lu, B., T. Liu, J.A. Crosby, J. Thomas -Wohlever, J., I. Lee and C.K. Suzuki, The ATP-dependent Lon Protease of Mus musculus is a DNA-binding Protein that is Functionally Conserved between Yeast and Mammals, (Submitted, Sept. 2002).
8) Lu, B., J.A. Crosby. S. Kotenko, K. Münger, and C.K. Suzuki, Modulation of Jak-Stat signaling by human Tid1- a DnaJ-like protein homologous to the Drosophila tumor suppressor Tid56 (Manuscript in preparation).