The Structure and Function of Ribosomes, Endoplasmic Reticulum Er and Colgi Body

The Structure and Function of the
Ribosomes, Endoplasmic reticulum (ER) and Golgi body
INTRODUCTION
The ribosome is composed of two subunits that work together to carry out mRNA-directed polypeptide synthesis. This process involves a highly dynamic interplay of two ribosomal subunits with each other and numerous cellular factors. Our understanding of protein biosynthesis is most advanced for bacteria which contain 70S ribosomes composed of a small (30S) and a large (50S) subunit. The activity of the ribosome involves initiation, elongation, termination and recycling step. The ribosome adopts many different functional states during each of the above steps. Understanding the complicated details of translation, therefore, requires, in addition to biochemical data, high resolution structures of each of the functional states of the ribosome. Our understanding of ribosomal structure has proceeded from the early reconstructions of the shapes of the two interacting subunits, to the current atomic- resolution structures of the prokaryotic 70S ribosome and of its large and small subunits captured in various functional states. Our intention is to present in this review how our knowledge about the ribosome’s structure evolved, starting from its discovery until nowadays.
Each subunit is made of one or more ribosomal RNAs (rRNAs) and many ribosomal proteins (r-proteins). The small subunit (30S in bacteria and archaea, 40S in eukaryotes) has the decoding function, whereas the large subunit (50S in bacteria and archaea, 60S in eukaryotes) catalysis the formation of peptide bonds, referred to as the peptidyl-
Lecture No.: 4 2016-2017 Third Stage
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transferase activity. The bacterial (and archaeal) small subunit contains the 16S rRNA and 21 r-proteins (Escherichia coli), whereas the eukaryotic small subunit contains the 18S rRNA and 32 r-proteins (Saccharomyces cerevisiae; although the numbers vary between species). The bacterial large subunit contains the 5S and 23S rRNAs and 34 r-proteins (E. coli), with the eukaryotic large subunit containing the 5S, 5.8S and 25S/28S rRNAs and 46 r-proteins (S. cerevisiae; again, the exact numbers vary between species).
The beginnings of the long and continuous discovery of the ribosomes lie in an excellent work with cell fractionation in the 1930s and 1940s performed by Albert Claude, the 1974 Nobel Prize laureate in Physiology or Medicine. The particular components of the cell were first seen in 1941 but were not recognized yet. By means of newly developed high-speed centrifugation, the cytoplasm no longer appeared as never ending space full of unknown substances, but as a powerful space in which the unknown substances showed up, waiting to be isolated, purified and characterized. The subcellular fragments could be obtained by many scientists by rubbing cells in a mortar, and further subjection to multiple cycles of sedimentations, washings and resuspensions. In addition to the nucleus, which was the most prominent feature of eukaryotic cell, mitochondria were also visualized in such way. In fact, mitochondria were detected under the light microscope as early as 1894, but despite extensive investigation by microscopy in the course of the following 50 years, no progress was achieved in this field. Finally, in 1940s, the staining properties of mitochondria led to the conclusion that they contained ribonucleic acids and thus put them as an object of new studies.
Ribosomes are