Vesicular stomatitis virus (VSV) visualized under electron microscopy. Source: http://en.wikipedia.org/wiki/File:Vesicular_
stomatitis_virus_(VSV)_EM_18_lores.jpg
The cell — the most basic and functional unit of life in an organism — is constantly busy. It must rebuild, reproduce, consume nutrients, excrete waste, respond to stimuli, and follow all of the requirements for the processes of life (2). Because of this, cells require a robust transportation system that ensures that the millions of different proteins that fulfill these metabolic demands end up where they need to go. Scientists James Rothman, Randy Schekman, and Thomas Sudhof have conducted groundbreaking research in the complicated field of cell trafficking mechanisms throughout the last 30 years and were awarded the 2013 Nobel Prize for Physiology or Medicine for solving “the mystery of how the cell organizes its transport system” (3).
At the heart of cell transportation is the vesicle, a tiny, membrane-enclosed sac that can pinch off from existing structures to store and transport substances (5). Vesicles carry everything from proteins that need to be delivered to the cell membrane, to massive bacterial cells that are moved deep inside of white blood cells for digestion. They are also required for cell-to-cell communication by way of neurotransmitters, which are chemical signals exerted by the nervous system in order to regulate bodily functions such as movement, breathing, heart rate, and abstract mental activity. Vesicle ubiquity in biology is difficult to overstate, as it is vital for many intracellular and extracellular activities.
During the 80’s, Rothman and his team at the University of Stanford were primarily concerned with how vesicles fuse with their target membranes (3). By isolating cell components from their natural environment, Rothman was able to purify the crucial proteins involved in the vesicle fusion process. Specifically, Rothman took advantage of Vesicular Stomatitis Virus or VSV (3), which causes infected cells to produce large amounts of a protein. The protein VSV-G is marked with a sugar modification during its travel through the cell (6). That is, the protein undergoes a change in its molecular structure that allows it to be identified and tracked. Rothman used this modification to study vesicle budding and fusion in infected cells in-depth, and his team soon isolated the first vesicle specific protein: the N-ethylmaelimide-sensitive factor or NSF.
The discovery of NSF served as a point of convergence between Rothman’s work and that of fellow Nobel Laureate Randy Schekman. Schekman’s on S. cerevisiae (baker’s yeast) at UC Berkeley had shown that the mutation of a certain gene, sec18, prevented vesicle fusion (4). Following this vein, Rothman used NSF as bait to purify three more proteins, which he later dubbed SNAREs (soluble NSF-attachment receptors) (1). The definite proportions with which the SNAREs were purified in the assays suggested that they worked together in specific, stoichiometric combinations to allow the vesicle to fuse with the target membrane. Rothman hypothesized that NSF, along with the appropriate amount of SNARE proteins, served as the minimal machinery with which vesicles can bind to their target (8).
Since the efforts of Rothman, Shekman, and Sudhof, the model of vesicle transport within the cell has become more refined (3). Scientists have confirmed that not only was Rothman’s minimal machinery for vesicles correct, but that there exists a large variety of SNARE proteins that interact to ensure that only certain vesicles can bond to select surfaces. This works in tandem with current knowledge of neurotransmitters and protein specificity and explains why these tiny, membrane-enclosed sacs serve as the backbone for organization within the cell and, by extension, the organism itself.
Reference
1. Block, G. W. (1988). Purification of an N-ethylmaleimide-sensitive Protein Catalyzing Vesicular Transport. Stanford, CA: Department of Biochemistry, Stanford University.
2. Can We Define Life? (2009, March). Retrieved from University of Colorado Boulder: http://artsandsciences.colorado.edu/magazine/2009/03/can-we-define-life/
3. Institutet, T. N. (2013, October). Machinery Regulating Vesicle Traffic, A Major Transport System In Our Cells. Retrieved from NobelPrize.Org: http://www.nobelprize.org/nobel_prizes/medicine/laureates/2013/advanced-medicineprize2013.pdf
4. Kaiser, S. (1990, May). Distinct Sets of SEC Genes Govern Transport Vesicle Formation and Fusion. Cell, pp. 723-733.
5. Learn Genetics – Genetics Science Learning Center. (n.d.). Retrieved from University of Utah Genetics: http://learn.genetics.utah.edu/content/begin/cells/vesicles/
6. Moyer, S. (1974, May). Vesicular Stomatitis Virus Envelope Glycoprotein Alterations Induced by Host Cell Transformations. Cell, pp. 63-70.