The Emerging Role of Proteolysis in Mitochondrial Quality Control and the Etiology of Parkinson's Disease

Mitochondria are highly dynamic organelles that are important for many diverse cellular processes, such as energy metabolism, calcium buffering, and apoptosis. Mitochondrial biology and dysfunction have recently been linked to different types of cancers and neurodegenerative diseases, most notably Parkinson's disease. Thus, a better understanding of the quality control systems that maintain a healthy mitochondrial network can facilitate the development of effective treatments for these diseases. In this perspective, we will discuss recent advances on two mitochondrial quality control pathways: the UPS and mitophagy, highlight how new players may be contributing to regulate these pathways. We believe the proteases involved will be key and novel regulators of mitochondrial quality control, and this knowledge will provide insights into future studies aimed to combat neurodegenerative diseases

OPA1 disease alleles causing dominant optic atrophy have defects in cardiolipin-stimulated GTP hydrolysis and membrane tubulation

The dynamin-related GTPase OPA1 is mutated in autosomal dominant optic atrophy (DOA) (Kjer type), an inherited neuropathy of the retinal ganglion cells. OPA1 is essential for the fusion of the inner mitochondrial membranes, but its mechanism of action remains poorly understood. Here we show that OPA1 has a low basal rate of GTP hydrolysis that is dramatically enhanced by association with liposomes containing negative phospholipids such as cardiolipin. Lipid association triggers assembly of OPA1 into higher order oligomers. In addition, we find that OPA1 can promote the protrusion of lipid tubules from the surface of cardiolipin-containing liposomes. In such lipid protrusions, OPA1 assemblies are observed on the outside of the lipid tubule surface, a protein-membrane topology similar to that of classical dynamins. The membrane tubulation activity of OPA1 is suppressed by GTPγS. OPA1 disease alleles associated with DOA display selective defects in several activities, including cardiolipin association, GTP hydrolysis and membrane tubulation. These findings indicate that interaction of OPA1 with membranes can stimulate higher order assembly, enhance GTP hydrolysis and lead to membrane deformation into tubules

Understanding the Molecular Mechanism of Mgm1 Function in Mitochondrial Dynamics

Given the debilitating effect that mitochondrial dysfunction has on human health, it is important to understand mitochondrial dynamics that are vital for the maintenance of mitochondrial function, genome, morphology, and quality control. Mitochondrial dynamics result from a balance in mitochondrial fusion and fission. Although the mechanism and regulation of mitochondrial fission are largely elucidated, less is known about mitochondrial fusion. Mgm1 is a protein that mediates mitochondrial fusion in yeast. However, the molecular mechanism of Mgm1 function in mediating mitochondrial fusion is unclear. In this thesis, first, I show that Mgm1 contains a lipid-binding domain by demonstrating that purified Mgm1 has lipid-binding activity and by identifying mutations in conserved residues that abrogate these interactions. Second, I show that Mgm1 assembles into hexameric rings and undergoes nucleotide-dependent structural transitions that, I believe, initiate membrane fusion. Lastly, I demonstrate that Mgm1 exhibits membrane-remodeling activities that are crucial for the tethering and lipid-mixing steps in the membrane fusion event. Together, I propose a mechanistic model of Mgm1 function in mediating mitochondrial fusion that advances the fields of mitochondrial biology, cellular protein-membrane dynamics, and the etiology of neurodegenerative diseases.Ph

Phospholipid Association Is Essential for Dynamin-related Protein Mgm1 to Function in Mitochondrial Membrane Fusion*

Mgm1, the yeast ortholog of mammalian OPA1, is a key component in mitochondrial membrane fusion and is required for maintaining mitochondrial dynamics and morphology. We showed recently that the purified short isoform of Mgm1 (s-Mgm1) possesses GTPase activity, self-assembles into low order oligomers, and interacts specifically with negatively charged phospholipids (Meglei, G., and McQuibban, G. A. (2009) Biochemistry 48, 1774–1784). Here, we demonstrate that s-Mgm1 binds to a mixture of phospholipids characteristic of the mitochondrial inner membrane. Binding to physiologically representative lipids results in ∼50-fold stimulation of s-Mgm1 GTPase activity. s-Mgm1 point mutants that are defective in oligomerization and lipid binding do not exhibit such stimulation and do not function in vivo. Electron microscopy and lipid turbidity assays demonstrate that s-Mgm1 promotes liposome interaction. Furthermore, s-Mgm1 assembles onto liposomes as oligomeric rings with 3-fold symmetry. The projection map of negatively stained s-Mgm1 shows six monomers, consistent with two stacked trimers. Taken together, our data identify a lipid-binding domain in Mgm1, and the structural analysis suggests a model of how Mgm1 promotes the fusion of opposing mitochondrial inner membranes