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Molecular Mechanisms of Peroxisome Inheritance
Summary: With every round of cell division, organelles must be duplicated and distributed to daughter cells, while the mother cell retains some fraction of the organelles. Richard Rachubinski is investigating how peroxisomes, organelles involved in lipid metabolism, are inherited at cell division and the molecular factors required for this process.
Found in organisms from yeasts to mammals and in most cell types, peroxisomes are ubiquitous organelles. They are the site of diverse metabolic reactions that vary depending on the organism and cell type. Peroxisomes perform the β-oxidation of fatty acids, bile acid synthesis, plasmalogen synthesis, cholesterol metabolism, methanol oxidation, and activated oxygen decomposition. Mature peroxisomes are spherical in electron micrographs, have diameters between 0.5 and 1.0 μm, are delimited by a single unit membrane, and contain a fine granular matrix. All peroxisomal proteins are encoded in the nucleus and synthesized on cytoplasmic ribosomes.
Peroxisomes are essential for human survival. This fact is underscored by the existence of a number of inherited genetic disorders, collectively called the peroxisome biogenesis disorders, that are the result of dysfunctional peroxisome biogenesis. Such disorders include Zellweger syndrome, neonatal adrenoleukodystrophy, infantile Refsum disease, and rhizomelic chondrodysplasia punctata. These autosomal recessive disorders are caused by genetic mutations that impair the biogenesis of peroxisomes, affecting all the organelle's metabolic roles. Peroxisome biogenesis disorders are often fatal in early infancy. In keeping with the diverse roles of peroxisomes, the symptoms of these diseases are also diverse, including severe mental retardation, defects in neuronal migration, renal and hepatic dysfunction, craniofacial abnormalities, and hypotonia.
Our laboratory has been investigating and elucidating the molecules and mechanisms of peroxisome biogenesis to understand better the molecular bases of its disorders. We have defined how proteins are imported into peroxisomes, demonstrated a role for the secretory pathway and vesicular fusion in peroxisome assembly, and identified and characterized a number of genes and their products that are required for peroxisome biogenesis. We are now actively engaged in what can be considered the ultimate step of the peroxisome biogenesis program—the inheritance of peroxisomes from mother cell to daughter cell.
Compartmentalization of biochemical functions into membrane-bound organelles such as the peroxisome provides the eukaryotic cell with a level of control unavailable to the prokaryotic cell. To maintain the advantages afforded by compartmentalization over cell generations, eukaryotic cells have evolved molecular mechanisms to ensure the accurate duplication of organelles and their inheritance from mother cell to daughter cell at cell division. Controlled distribution of organelles is essential not only for autonomously replicating organelles that cannot form de novo, such as the mitochondrion, but also for other organelles that derive from existent endomembrane systems, such as the endoplasmic reticulum. The inheritance of peroxisomes is a largely uncharacterized event of the peroxisome biogenesis program. The goal of our HHMI-funded research is to characterize the molecular machinery and to elucidate the cellular mechanisms underlying peroxisome inheritance.
We use the budding yeast Saccharomyces cerevisiae to dissect how peroxisomes are inherited. S. cerevisiae has been used extensively as a model eukaryotic cell system to define the molecular pathways involved in the inheritance of various organelles including the mitochondrion, the endoplasmic reticulum, the Golgi complex, and the vacuole, the yeast equivalent of the mammalian lysosome. Cell division in S. cerevisiae is asymmetrical, with the formation of a bud that is initially much smaller than its mother. A highly polarized actin cytoskeleton is needed for bud formation and for the faithful delivery of each type of organelle to the emerging bud. Organelles are duplicated or fragmented within the S. cerevisiae mother cell before cell division. At division, half the organelle population is retained in the mother cell, and the other half is transported along actin tracks by myosin molecular motors to their proper location within the bud.
Myosin V motors are present in most eukaryotic cells and are ideally suited for organelle trafficking. They are molecular engines that move processively along actin filaments in 37-nm steps corresponding to the helical periodicity of the actin filament. Class V myosins function in the distribution of cellular components by interacting with cargo and then transporting it along actin tracks using the N-terminal motor domain. The divergent C-terminal tail domain of these myosins contains the information necessary for targeting myosins to specific intracellular compartments. These motors have been shown to have the ability to transport more than one type of cargo in one cell or in different cell types. For instance, mammalian myosin Va moves melanosomes in melanocytes, membranous vesicles in nerve cells, the smooth endoplasmic reticulum in brain Purkinje cells, and chromaffin vesicles in chromaffin cells.
The class V myosins Myo2p and Myo4p are the actin-based motors responsible for partitioning intracellular compartments in budding yeast. Myo2p plays a critical role in the bud-directed transport of various organelles, including the vacuole, secretory vesicles, late Golgi elements, and peroxisomes. Moreover, Myo2p also appears to play a role in mitochondrial inheritance and is required for proper orientation of the mitotic spindle. The other myosin V motor, Myo4p, has been shown to be involved in inheritance of the cortical endoplasmic reticulum that lines the periphery of the S. cerevisiae cell.
How can Myo2p have so many suitors and still manage to interact with all of them? A partial answer to this question came from the observation that the Myo2p-dependent delivery of the vacuole and secretory vesicles can be clearly dissected in the Myo2p tail. Therefore, the involvement of Myo2p in the transport of so many different organelles has been explained by the presence of distinct domains in the globular tail of Myo2p that bind to organelle-specific receptors in a temporal and spatial pattern characteristic for the transport of a particular organelle to the yeast bud. Given the large diversity of myosin ligands and the deleterious effect of competition among them, it is obvious that these organelle-specific factors are strictly regulated and act in coordination with cell cycle events.
We are combining cell, molecular, and global systems biology approaches to understand how peroxisomes are inherited. We are screening comprehensive libraries of S. cerevisiae strains expressing fluorescently tagged proteins to identify novel peroxisomal proteins and are using four-dimensional live cell video microscopy to ascertain their roles in peroxisome inheritance. We are providing a mechanistic understanding of how proteins function in peroxisome inheritance by isolating the complexes that contain them and by expressing mutant forms of the proteins to define specific regions that control aspects of the peroxisome inheritance program. Specifically, we aim to (1) identify novel peroxisomal proteins required for segregating peroxisomes from mother cell to bud, (2) characterize the interaction of proteins required for peroxisome inheritance, and (3) determine the mechanism of recruitment of the myosin molecular motor to peroxisomes.
Our HHMI-supported research will provide a more comprehensive and clearer molecular mechanistic understanding of peroxisome inheritance, an essential event of the eukaryotic cell cycle.
Last updated March 2007
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