Endoplasmic Reticulum in yeast cells

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The study "A 3D Analysis of Yeast ER Structure Reveals How ER Domains Are Organized by Membrane Curvature" investigates the structural organization of the endoplasmic reticulum in yeast cells. Using advanced three-dimensional electron tomography, the researchers identified three major domains of the endoplasmic reticulum: the plasma membrane-associated endoplasmic reticulum, the central cisternal endoplasmic reticulum, and the tubular endoplasmic reticulum. The plasma membrane-associated endoplasmic reticulum is tightly associated with the plasma membrane and consists of both tubular structures and fenestrated cisternal regions. The central cisternal endoplasmic reticulum, which had not been characterized before, spans the cytoplasm and appears to play a key role in the inheritance of the endoplasmic reticulum during cell budding. The tubular endoplasmic reticulum forms highly curved tubular networks that connect various regions of the endoplasmic reticulum.
The study highlights the crucial role of membrane-shaping proteins, specifically reticulons and Yop1, in maintaining the curvature and structural integrity of the endoplasmic reticulum. These proteins are localized to regions of high membrane curvature and are essential for preserving the shape of tubules and the edges of cisternae. In cells lacking reticulons and Yop1, the structure of the endoplasmic reticulum is significantly altered. Tubules are replaced by flat, unfenestrated cisternae, and the overall ribosome density on the endoplasmic reticulum changes. However, despite these structural disruptions, the inheritance of bloated endoplasmic reticulum tubules into the budding cell still occurs. This suggests that reticulons and Yop1 primarily stabilize membrane curvature rather than generating it.
The researchers also examined how membrane curvature influences ribosome density. They found that ribosome binding is not solely determined by the degree of curvature. For example, the central cisternal endoplasmic reticulum and the cytoplasmic face of the plasma membrane-associated endoplasmic reticulum exhibit high ribosome densities, while the tubular endoplasmic reticulum has fewer ribosomes bound to it. This finding challenges the traditional assumption that curvature directly limits ribosome binding and indicates that other factors contribute to ribosome distribution.
A significant aspect of the study was understanding how the endoplasmic reticulum is inherited during cell division. The researchers tracked the spatial organization of endoplasmic reticulum domains during six stages of yeast budding. They observed that the central cisternal endoplasmic reticulum and the tubular endoplasmic reticulum are the primary domains inherited into the budding cell, while the plasma membrane-associated endoplasmic reticulum remains attached to the plasma membrane of the mother cell. This suggests that the central cisternal endoplasmic reticulum plays a unique role in transferring endoplasmic reticulum material to the growing bud, emphasizing the functional specialization of different endoplasmic reticulum domains.
To achieve these insights, the researchers employed high-pressure freezing and advanced imaging techniques to reconstruct endoplasmic reticulum structures at nanometer resolution. This detailed analysis provided a comprehensive view of how the endoplasmic reticulum domains are organized and how they interact with other cellular components, such as ribosomes and the plasma membrane. The findings offer valuable insights into the mechanisms that maintain the structure and function of organelles in eukaryotic cells.
Overall, this study provides a detailed and comprehensive view of how membrane curvature and associated proteins influence the structural organization of the endoplasmic reticulum in yeast. It highlights the interplay between structure and function, shedding light on critical cellular processes such as protein synthesis, lipid trafficking, and organelle inheritance. These insights could have broader implications for understanding similar mechanisms in more complex eukaryotic systems, including human cells.