I. The Secretory Pathway: How Cells Package and Traffic Proteins for Export
II. Genes and Proteins Required for Secretion
III. How Human Cells Secrete Small RNAs in Extracellular Vesicles
Part II: Genes and Proteins Required for Secretion
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|iBiology Archives: Randy Schekman iBioSeminar (2007)|
In his first lecture, Dr. Randy Schekman overviews the secretory pathway and reviews historical experiments that shaped our molecular understanding of this pathway. The journey begins at the endoplasmic reticulum (ER), where proteins that engage the secretory pathway get translated. The mRNA of these proteins codes for a signal sequence that serves as a “tag” to bring the mRNA-ribosome-newly-synthesized protein to the ER for continued translation and movement of the new secretory protein across the ER membrane into the interior or lumen of the organelle. Vesicles transport the recently translated proteins to the Golgi Apparatus, where they get “packaged” and sent to their final destination.
In his second lecture, Schekman explains how his laboratory used baker’s yeast to uncover major proteins involved in the secretory pathway, and describes proteins involved in budding, vesicle trafficking, and vesicle fusion. Schekman also presents data from his laboratory that helped to identify the ER channel through which proteins enter the secretory pathway. These series of experiments show how, step by step, scientific knowledge evolves, uncovering the fundamental mechanisms to better understand human disease.
In his third lecture, Schekman outlines exosome biogenesis. Exosomes are extracellular vesicles released by the cell, and in contrast to intracellular vesicles, exosomes contain small molecules of RNA. Schekman’s laboratory characterized the RNAs contained in exosomes and showed the importance of Ybx1 protein for the recruitment of certain miRNAs into exosomes.
Dr. Randy Schekman is a Professor in the Department of Molecular and Cell Biology, University of California, Berkeley, and an Investigator of the Howard Hughes Medical Institute. He studied the enzymology of DNA replication as a graduate student with Arthur Kornberg at Stanford University. His current interest in cellular membranes developed during a postdoctoral period with S. J. Singer at the University of California, San Diego. At Berkeley, he developed a genetic and biochemical approach to the study of eukaryotic membrane traffic.
Among his awards are the Eli Lilly Award in microbiology and immunology, the Albert Lasker Award for Basic Medical Research and the Louisa Gross Horwitz Prize. In 2013, Schekman was awarded the Nobel Prize in Medicine or Physiology jointly with Thomas Südhof and James Rothman for their contributions to understanding vesicle trafficking. Schekman is a member of the National Academy of Sciences and the American Academy of Arts and Sciences. From 2006-2011, he was Editor-in-Chief of the Proceeding of the NAS. Currently, he is Editor-in-Chief of the open access journal eLife. Learn more about Dr. Schekman’s research here.
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- Rachel Green iBioSeminar: Protein Synthesis
- Jennifer Lippincott-Schwartz iBioMagazine: How Do Lipids and Cholesterol Regulate Trafficking Across the Secretory Pathway
- Thomas Reese iBioMagazine: Visualizing Synaptic Signaling
- Kai Simons iBioSeminar: Lipids as Organizers in Cell Membranes
- Erik Jorgensen iBioSeminar: Recycling Synaptic Vesicles
- John Heuser and Thomas Reese Discovery talk: Imaging Synaptic Vesicle Transmission
Palade, G (1975) Intracellular aspects of protein secretion. Science 189(4200):347-5
This paper presents a version of Palade’s Nobel lecture reviewing his pioneering work to decipher the interrelationships among the organelles of the secretory pathway.
Singer, SJ & Nicolson, GL (1972) The fluid mosaic model of membrane structure. Science 175(4023):720-31
This review article synthesized the existing information about the organization of biological membranes to present an influential model that guided future work aimed at evaluating how membranes are built in cells.
Novick, P & Schekman, R (1979) Secretion and cell surface growth are blocked in a temperature sensitive mutant of Saccharomyces cerevisiae. Proc. Natl. Acad. Sci. USA 76, 1858‑1862
This paper introduced a genetic approach to the discovery of genes required for protein secretion in eukaryotic cells. The gene identified in this paper later was found to encode a key regulator of SNARE protein action in membrane fusion.
Deshaies, RJ & Schekman, R (1987) A yeast mutant defective at an early stage in import of secretory protein precursors into the endoplasmic reticulum. J. Cell Biol. 105, 633-645
The original collection of sec mutants did not define the initial stage of insertion of secretory and membrane proteins into the ER. This paper developed a novel genetic approach to discover SEC61, a gene that later was found to encode the channel-forming subunit of the polypeptide translocase in the ER.
Baker, D, et al. (1988) Reconstitution of Sec gene product-dependent intercompartmental protein transport. Cell 54, 335-344
This paper describes the development of a cell-free reaction that reproduces the requirement for Sec proteins in the traffic of secretory proteins from the ER to the Golgi.
Dissection of the reaction described in this paper led to the purification of functional forms of the Sec proteins.
Kaiser, CA & Schekman, R (1990) Distinct sets of SEC genes govern transport vesicle formation and fusion early in the secretory pathway. Cell 61, 723-733
This paper used a genetic and morphological approach to define the role of Sec genes required for vesicle budding from the ER and vesicle fusion at the Golgi membrane. The genes required for budding encode proteins that form a coat protein complex and the genes required for vesicle fusion encode two soluble proteins also required for fusion of vesicles in a mammalian transport reaction and a SNARE protein.
Barlowe, C, et al. (1994) COPII: A membrane coat formed by Sec proteins that drive vesicle budding from the endoplasmic reticulum. Cell 77, 895-907
This paper reports the discovery of a novel coat protein complex, COPII, responsible for vesicle budding from the ER. COPII is evolutionarily conserved and is required for traffic at this stage in all eukaryotic cells.
Fromme, J C, et al. (2007) The genetic basis of a craniofacial disease provides insight into COPII coat assembly. Developmental Cell 13, 623-634
This paper reports the molecular definition of the effect of a human mutation in a COPII subunit. A rare craniofacial disorder results from a lesion in one of two copies of Sec23 and produces a specific defect in the last step of coat assembly where the outer scaffold of the coat fails to polymerize on a surface presented by Sec23 bound to the ER membrane.
Colombo, M, Raposo, G & Théry, C (2014) Biogenesis, secretion and intercellular interactions of exosomes and other extracellular vesicles. Ann. Rev. of Cell and Dev. Biol 30:255-89
This review critically evaluates the current state of research on the isolation and characterization, biogenesis and function of extracellular vesicles and exosomes.
Shurtleff MJ, et al. (2016) Y-box protein 1 is required to sort microRNAs into exosomes in cells and in a cell-free reaction. Elife 5. pii: e19276
This paper describes our isolation of exosomes secreted by cultured human cells and our observation of a small number of highly select miRNAs that are enclosed within the exosomes. Further, we developed a cell-free reaction that reproduces the sorting of select miRNAs into exosome vesicles formed in an incubation of membranes and soluble cytosolic proteins obtained from mechanically disrupted cultured cells. Using this reaction, we identified an RNA-binding protein, Ybx1, required to sort miRNAs into vesicles in the cell-free reaction and in intact cells.