In eukaryotic cells, mitochondrial movement or `transport'occurs by attaching the organelle to kinesin or myosin motor proteins that walk along microtubules or actin filaments, respectively. These mitochondrial transport events are required to distribute mitochondria within the cytoplasm and are also essential for inheritance of mitochondria by daughter cells during division. A molecular understanding of mitochondrial transport mechanisms is critical for human health, as genetic lesions that compromise mitochondrial distribution are linked to a growing list of human disorders, including some forms of muscular dystrophy, cardiomyopathy, paraplegia, and neurodegeneration. The basic mechanisms regulating mitochondrial transport are conserved from yeast to man. This proposal focuses on the molecules and machineries that mediate actin-based mitochondrial transport during division in the budding yeast, S. cerevisiae. Defects in yeast mitochondrial transport interfere with mitochondrial inheritance by daughter cells (buds). Previous studies identified two independent pathways that regulate yeast mitochondrial inheritance. The first pathway utilizes a peripheral mitochondrial adaptor protein called Mmr1. Mmr1 forms a complex with Myo2, a class V myosin motor protein. Experiments in this application will test the model that changes in Mmr1 abundance, activity and/or localization coordinate Mmr1-mediated mitochondrial inheritance with the cell cycle. The second mitochondrial inheritance pathway requires an endoplasmic reticulum (ER)- localized Rab GTPase called Ypt11. Ypt11 also forms a complex with Myo2. How an ER-anchored Rab controls inheritance of mitochondrial membranes is not understood. In this application, two alternative models to explain how ER-localized Ypt11 influences mitochondrial inheritance are proposed and tested. The principle investigator's laboratory recently described a third pathway required for mitochondrial inheritance. This pathway utilizes Gem1/Miro, a tail-anchored outer mitochondrial membrane protein containing two GTPase domains and two Ca2+binding EF-hand motifs. Gem1/Miro is conserved from yeast to man. Expression of mutant Miro in cultured cells leads to apoptosis, and flies with defective Gem1/Miro display neuronal defects. Experiments are proposed to determine whether Gem1/Miro acts via Myo2 and to identify specific effectors of its GTPase domains. Finally, the principle investigator's laboratory will study novel components of mitochondrial inheritance pathways identified in genetic screens. These studies will likely identify mammalian homologs that will allow future investigation of mitochondrial distribution mechanisms in human cells and tissues. Public Health Relevance: The studies outlined will advance the understanding of actin-based mitochondrial transport mechanisms. The results will be relevant to human development and health, as several of the molecules of interest have human homologs, including Myo2 (human myosin Va) and Gem1 (human Miro1 and Miro2). Defects in human myosin Va cause embryonic lethality, and patients with reduced myosin Va function suffer from Griscelli's syndrome, an often fatal disorder characterized by immunodeficiency, and neurological and pigmentation defects. Expression of mutant mammalian Miro alters mitochondrial morphology and induce cell death in cultured cells, and defects in fly Miro disrupt axonal mitochondrial transport, a function that is likely conserved in humans. Some of the proposed experiments may lead to the identification of homologs that control actin-based mitochondrial distribution processes in human cells and tissues. What is learned in the process may allow scientists and clinicians to manipulate the activities of these molecules for the benefit of human health.
