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Structural and Biochemical Characterization of Eukaryotic Protein Complexes Involved in Interorganelle Tethering and Protein Quality Control


This dissertation describes a comprehensive set of experiments aimed at understanding two distinct biological phenomena that are essential to the cell—1) Interorganelle communication, and 2) regulation of protein unfolding and degradation.

The compartmentalization of cells permits the segregation and regulation of diverse and specialized biochemical reactions. The tremendous benefits of intracellular compartmentalization also come at a price; to function harmoniously, cells must coordinate the exchange of biological information between compartments. Numerous mechanisms have evolved to facilitate these exchanges. One that has not been well appreciated until the last few years is the transmission of signals between organelles at regions where the organelles are closely apposed, often called membrane contact sites (MCSs). Emerging evidence highlight essential roles for interorganelle tethering complexes in the formation and maintenance of MCS. These molecular tethers are widespread and function primarily by bridging two organelles, including the endoplasmic reticulum (ER) and mitochondria.

Chapter 2 discusses the structural and biochemical characterization of the multi-subunit interorganelle tethering complex ERMES (endoplasmic reticulum-mitochondrial encounter structure), which bridges the ER and mitochondria in the yeast, Saccharomyces cerevisiae. To our knowledge, this is the first structure of a multi-subunit tether known to date. Our findings highlight novel roles for the conserved SMP domain (synaptotagmin-like mitochondrial and lipid-binding protein)—present in three ERMES subunits—in ERMES assembly and phospholipid binding, and suggest a structure-based mechanism for the facilitated transport of phospholipids between the ER and mitochondria.

Intracellular proteolysis is regulated by a wide array of molecular systems consisting of molecular chaperones and proteases. These surveillance systems ensure that misfolded proteins and short-lived regulatory proteins are targeted for degradation. The Clp family of chaperones and proteases play an important role in protein homeostasis in the cell. They function mainly in the disaggregation, unfolding and degradation of native as well as misfolded proteins. Several members of the Clp family were recently identified in Plasmodium falciparum, the etiologic agent of malaria. Chapters 3 and 4 detail the structures of three members of the Plasmodium Clp family—the AAA+ chaperones ClpB1 and ClpB2, and the adaptor protein ClpS. These structures provide novel insights into the role of the elusive Plasmodium Clp protease machinery in protein unfolding and degradation, and open new venues for the design of novel anti-malarial drugs aimed at disrupting parasite-specific, protein quality control pathways.

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