The endoplasmic reticulum (ER) is a universally conserved component of eukaryotic cells. The ER is organized into three morphologically and functionally distinct domains: ER tubules, ER sheets, and the nuclear envelope (NE). Professional secretory cells, such as pancreatic ? cells and plasma cells, are packed with ribosome-coated ER sheets, demonstrating the importance of ER sheets to efficient protein secretion. Although ER sheets play a critical role as the major entry point of the secretory pathway, it is not known how they are generated. The proposed work will identify the mechanisms underlying the specialized structure and function of ER sheets.
Aim 1 will characterize ER sheets generated by the known ER-shaping protein Climp63 and the novel ER sheet regulator Lyric. These proteins generate ER sheets when individually overexpressed, but they do not colocalize when coexpressed. This observation is consistent with the idea that cells contain different types of ER sheets. The structures of these distinct ER sheets will be compared by 3D electron microscopy (EM) tomography to define their shapes at nanometer resolution. ER morphology will also be examined in cells depleted for Lyric and Climp63. Finally, cells codepleted for Climp63 and Lyric will be examined for loss of ER sheets to test the notion that these two regulators act through parallel ER sheet biogenesis pathways.
Aim 2 will address how Lyric and Climp63 form segregated ER sheet domains. Truncation mutants of Lyric and Climp63 will be generated to define the protein domains that control their ability to generate ER sheets and exclude one another from sheet domains. These experiments will test the hypothesis that their luminal domains act as spacers?a property already known for Climp63?and that this spacing activity controls their segregated localization. In addition, the influence of ER-bound ribosomes on Lyric and Climp63 protein distribution will be tested by monitoring the responses of Climp63 and Lyric ER sheets to translational inhibitors.
Aim 3 will define the functional relevance of Climp63 versus Lyric ER sheets. The distribution and density of ribosomes on ER sheets generated by Climp63 and Lyric will be assessed by 3D EM tomography to test if the proteins generate rough ER. In addition, interactors of Climp63 and Lyric will be identified by TurboID to identify potential downstream regulators of ER sheet morphology and function. The proposed training includes comprehensive, one-on-one instruction in 3D EM tomography methods. In addition, the program will provide training in new imaging and biochemical techniques, learning how to write peer-reviewed papers, manage a laboratory, and establish an independent research program. The laboratory and institutional environments will provide state-of-the-art technical resources to support the execution of this work in addition to providing an enriching intellectual environment for scientific and professional development.
The endoplasmic reticulum is an essential component of cells and exhibits diverse shapes and functions. The endoplasmic reticulum surrounds the DNA and provides a conduit for protein traffic through the cell, made possible by its specialized membrane domains. By understanding the processes that shape the endoplasmic reticulum into these distinct domains, we can understand how they go awry in disease and design strategies to counteract cellular dysfunction.
