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Could a mundane concept like an “address” be used to explain a complicated physiological process that occurs countless times within cells? Dr. Günter Blobel, the 1999 Nobel Prize Laureate, applied this rationale to explain the mechanism by which newly synthesized proteins are transported to their destinations of function within and out of cells. Blobel theorized that proteins must have chemical “address tags,” or signals that direct them to their correct locations (Press Release: The 1999 Nobel Prize in Physiology or Medicine), and proved the validity of this in his research. His research has contributed to the plethora of biological knowledge accrued in the twentieth century. Blobel’s discovery has manifested a novel utilization of cells in which they are genetically engineered to secrete therapeutic proteins, and will continue to be at the center of various therapeutic strategies. The mid-twentieth century bore great biological findings, adding immensely to our expanding knowledge of the life sciences, and contributing to new medical advances. With the development of the electron microscope in the first half of the century (Bellis), scientists were able to examine the intricate design of life’s simplest constituent- the cell- and its processes. Scientists were able to discover the major components of the cell and the basic structure of the cell in the late 1940s and early 1950s (Gallwitz). Cells are bound by lipid bilayer membranes that have many proteins between them. Membranes composed of proteins and lipids enclose many organelles that perform important life processes in the cell. The endoplasmic reticulum (ER) and the ribosomes synthesize proteins. Proteins play various important roles within cells, such as forming the composition of the cell, and catalyzing chemical reactions as enzymes. Such findings helped to enhance the knowledge of cellular processes (Gallwitz). With this knowledge at hand, Blobel began to explore the cellular functions of these major components. The discovery of the structure of DNA in 1953 provided great insight into the way by which cells pass down their hereditary information (genes) to daughter cells and organisms to their offspring. This finding helped scientists to discover the mechanism for protein synthesis, as the helical shape of the DNA molecule could easily “unzip” into two strands which could be replicated or transcribed into mRNA. While the mechanism of protein synthesis, through messenger RNA translation on the ribosome, was manifesting itself to scientists in the 1960s (Gallwitz), many questions were still left to be answered. Biologists began to delve into the issue of cellular function from a molecular aspect; that is, they attempted to explain functions of various cellular structures and processes using molecular analysis and experimentation. Under the mentorship of George E. Palade (Nobel Prize Laureate in 1974), who had already found the intracellular path of secretory proteins, Blobel began to perform intense research at Rockefeller University in New York into the issue of cellular protein targeting. Palade had already found that secretory proteins traverse the rough endoplasmic reticulum (ER) membrane, where they are modified after which they enter the Golgi Apparatus for packaging, before being exported from the cell. The biochemical/molecular mechanisms were unknown at that time, and Blobel attempted to explain this process by these means. For his great discoveries, Blobel was awarded the Nobel Prize in 1999 (Gallwitz; Press Release: The 1999 Nobel Prize in Physiology or Medicine; Blobel 86-102). First, Blobel and David Sabatini tried to discover whether free and ER-bound ribosomes were unlike each other in protein structure, in an attempt to explain how these proteins arrive at their cellular destinations. Using SDS polyacrylamide gel electrophoresis (PAGE), they found no difference in their relative structures, suggesting that ribosome differences were not behind the selection and direction of newly synthesized proteins to their targets (Blobel 86-102). In 1971, Blobel and collaborators developed the first “signal hypothesis”, in which they theorized that proteins targeted to leave the cells have chemical signals that direct them to and across membranes (Press Release: The 1999 Nobel Prize in Physiology or Medicine). The experiments reproducing protein translation/translocation without a cell helped to support their theory. Blobel’s experiments produced a peptide larger than the functional form, suggesting that the extra components played a role in protein targeting (Blobel 86-102). Blobel was able to explain how newly synthesized proteins that are to be transported out of the cell, are directed to the ER, after some biochemical experiments in 1975; he found that the signal is a peptide that is part of the synthesized protein, and that proteins cross the ER through channels (protein-conducting channels). The peptide, called the “signal peptidase,” was shown to mediate the attachment of the ribosome to the ER membrane, as it could be solubilized when microsomes were treated with detergent (Gallwitz; Blobel 86-102). Over the next twenty years, Blobel and fellow scientists discovered the molecular niceties of the process of protein allocation and transport within the cell, ultimately proving his signal hypothesis true for cells of yeast, plant, mammalian, and animal cells. The theory was controversial until conclusive evidence for it was presented in 1991 (Blobel 86-102). Blobel was also able to show that peptide signals direct the transport of proteins to other cell compartments. In explaining the mechanism of this in 1980, he said that peptides govern whether or not proteins cross a membrane into an organelle, are inserted into the membrane, or are transferred out of the cell. These peptide signals have been characterized, and are analogous to what we know as address tags. From 1980-1981, Blobel discovered the “signal recognition particle (SRP)” that regulates protein movement into the ER (Gallwitz). Experiments using SDS-PAGE, and tests using Nikkol helped to identify this particle and its role in the receptor mediated mechanism of protein internalization in the ER (Blobel 86-102). The mechanism for this process was consequently characterized. The SRP is a particle in the cytosol of cells that binds to the proteins with the correct signal sequence and guides them to the SRP receptor, found on the ER. The complex connects the ribosome, from which the protein is being translated, to the protein translocator that imports the protein into the ER. The proteins are then modified through detoxification (in the smooth ER) and glycosylation, and aided in proper folding and assembly. Blobel’s work helped to explain this process in its molecular niceties, contributing greatly to the present understanding of such cellular mechanisms (Gallwitz). Günter Blobel’s work has had a profound influence on biological research from his discovery of the signal peptides to the present day. His work has greatly enhanced the scientific community’s knowledge of the molecular scheme behind many of the cell’s most fundamental processes. The original “signal hypothesis” is now greatly antiquated, as it has been characterized in minute detail. The signal peptidase, protein-conducting channel, SRP and SRP receptor have all been purified and assayed for their activity (Blobel 86-102). Blobel’s contributions have improved the understanding of protein subcellular transport and export out of the cell, and have augmented the knowledge of medical processes, such as the synthesis of antibodies. Blobel’s work has been used to elucidate the means by which genetic diseases take their effect; a mutated signal peptide can target proteins to the incorrect place in the cell, thus triggering pernicious results in the body. One example is primary hyperoxaluria, a hereditary disease which causes kidney stones in youth. Such fatal diseases like cystic fibrosis can be explained by Blobel’s findings, as this lethal illness is caused by defective channel protein transport to the plasma membrane. Also, in some forms of hypercholesterolemia, extremely high cholesterol levels are caused by faulty target signals (Gallwitz). Blobel’s findings may prove to be invaluable in medical applications and the search for treatment of innumerable illnesses. Valuable, practical, and therapeutic biotechnological applications have been performed using the knowledge manifested by Blobel’s research. The understanding of protein targeting sequences has been used to modify bacteria and eukaryotic cells to produce therapeutic proteins. These proteins are altered so that they are secreted by and exported from the cells after which the gene is inserted into the genome of the secreting organism (Gallwitz). After finishing the work for which he was awarded the Nobel Prize, Blobel continued to show remarkable dedication to science, as he become a leading researcher in the area of protein transport into/out of the nucleus and ribonucleoprotein complexes. Proteins targeted for the nucleus have “nuclear localization sequences,” but move into and out of the nucleus via nuclear pores (Strambio-de-Castillia et al. 839-55). Today, he studies macromolecular transport across the nuclear pore complex, chromatin structure within the nucleus, and continues his previous work on protein transport across the ER (Blobel). Blobel’s work today is as notable as the research for which he won the Nobel Prize in 1999. As the human genome has been mapped to completion, the structure and signals (“topogenic”) of the proteins for which human genes code can be discovered. This information will aid in the explanation of disease mechanism and may be used to create novel therapeutic methods. At the present, many protein drugs are produced by bacteria genetically engineered with the desired gene inserted within plasmid DNA which is taken up by the bacteria. However, more complex cells must sometimes be used when producing human cells. Our increasing knowledge of protein targeting may be used to make drugs targeted for a specific organelle, which could mend cell defects. In the future, it is quite possible that the ability to reprogram cells will play an important role in cell biology and gene therapy. In the future, Blobel’s findings will continue to play a vital role in advancing biology and medicine. Biologists will continue (as they are doing now) to explore the genetic scheme of protein translocation within the cell, discovering the genes that control SRP expression as well as expression of other vital peptides in the process. Sec61 has already been identified as a protein-conducting channel in yeast (Blobel 86-102). Also, a genetic understanding of the process will aid in explaining the mechanisms by which hereditary diseases cause debilitating and fatal effects. These inherited diseases result because of genetic defects that cause defective proteins to be synthesized, many of which play vital roles in regulating the processes explained by Blobel. Further research may result in the discovery of novel topogenic sequences, as Blobel’s findings have manifested the enormous challenge of explaining the process on a genetic level and attempting to correct defects in the process should they be inherited or acquired. Future improvements in biotechnology and molecular analysis techniques may result in cheap and quick diagnosis of patients with genetic diseases, manipulation of genes to prevent offspring from inheriting the same defects, and possible intake of lacking proteins, produced by engineered bacteria or other organisms on a wide-scale. Thus, Blobel’s work has provided the means for newer, lower costing, and easier therapeutic strategies. Blobel’s zeal and determination made him ideal for tackling the once esoteric issue of cellular protein transport. Rather than using endless calculations, Blobel used great intuition and imagination in formulating possible explanations for cell processes. His experimentation and research proved just how far this method of thinking could go, as his hypotheses were proven valid. Blobel’s work ethic and scientific attitude have exemplified the ideal characteristics of successful scientists. Works Cited |
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