Ribosomes and protein synthesis.

Abstract

Let us start at the very beginning. Between 1897 and 1899, G. Gamier, in France, published elegant microscope studies describing a basophilic component ofthe cytoplasm of glandular cells (1) . Because of what he thought its role might be in the elaboration and transformation of secretory products, he gave a Greek name to these concepts-ergastoplasm (work plasm) . Gamier's research was extended by others-particularly A. Prenant, R. R. Bensley, and A. Matthews-to include other cell types, so that by the early part of this century ergastoplasm came to be a generally accepted term for a specific basophilic area of the cytoplasm . These early studies are extensively reviewed by F. Haguenau (1) . The next major advance was to show that basophilia was due to RNA: in 1933, J. Brachet used RNase (2) ; in 1939, T. Caspersson used ultraviolet spectrophotometry (3); in 1943, J. N. Davidson and C. Waymouth used chemical methods (4) . The high correlation which was shown between the amount ofRNA in various cells and the postulated protein-synthesizing capacity of those cells led Caspersson in 1941 (5) and Brachet in 1942 (6) to proclaim the importance of RNA in the process of protein synthesis . As can be imagined, this conjecture spurred many scientists in the next decade to try to answerthree questions. In what form was this cytoplasmic RNA? Did it really have a role in protein synthesis? Ifso, what was the role? Various methods were used: extraction and chemical procedures; extraction and physicochemical procedures, such as ultracentrifugation; and, because electron microscopy was becoming more and more refined, visualization . We now know that the RNA is in the form of ribosomes, and that the proteins of the ribosomes are involved in the many individual steps of protein synthesis; however, the function of ribosomal RNA is still elusive .The intensity of the research in the 1940s is caught very well in Haguenau's chapter on the visualization aspect (1) and in Magasanik's 1955 monograph ( 7) on the extraction and chemical properties of what were then called "pentose nucleoprotein." Confusion abounded, in good part due to the terminologies developed for the different techniques, such as Gamier's ergastoplasm, K. Porter's "endoplasmic reticulum" (8), A. Claude's "microsome" fraction (9), G. Palade's "small particulate component" (10), and the "nucleoprotein" preparations or particles discovered by various workers . The last are re-

Localization of mRNAs to the endoplasmic reticulum

“…These particles were sensitive to RNase treatment and were thus eventually named ribosomes (i.e., the ribonucleo‐protein particles of the microsome fraction) . By using these cell fractions to reconstitute protein synthesis in vitro , it was then shown by several groups that ribosomes play a central role in this biological process (for an excellent overview, see Ref ).…”

Association of methionyl-tRNA synthetase with detergent-insoluble components of the rough endoplasmic reticulum.

“…In eucaryotic cells some of the components required for protein biosynthesis are bound to membranes of the endoplasmic reticulum and some are free in the cytoplasm (34). Recently, it has been suggested that the "free" ribosomes and mRNA are actually bound to a detergent-resistant "cytoskeleton" (4-6, 12, 23, 24, 38, 40, 43) or "microtrabeculae" (45).…”

From Water and Ions to Crowded Biomacromolecules:In VivoStructuring of a Prokaryotic Cell

“…The simplest model of a prokaryotic cell has been described as a semipermeable "bag" that encloses catalytic reaction systems of diffusing metabolites and biomacromolecules (13,139,140,146,189,234,243). Though the bag itself has been known to be a highly structured entity-the "fluid mosaic" of integral membrane proteins energized by the membrane potential (146,199) and supported by the cell wall and extracytoplasmic layers (196)-the contents of the bag showed only the chemically distinct nucleoid (177,228) and the ribosomes (198) in an unstructured cytoplasm when electron microscopy became established (35,188). Because cells in vivo are crowded with biomacromolecules, typically about a 20 to 30% volume fraction (77,78,265), the bag model has been challenged by concepts of higher-order structural elements derived from biochemical and physiological functions.…”