4.4 Ribosomes: the sites of protein synthesis

The mRNA molecules exported from the nucleus are translated into protein in the cytoplasm by the ribosomes (which are RNA-protein complexes, not organelles). Evidence suggests that some mRNA molecules leaving the nucleus are directed to specific sites in the cell before they are translated. Ribosomal structure is similar in prokaryotic and eukaryotic cells. Ribosomes are organised into a large subunit and a small subunit, as shown in Figure 15 (prokaryotic ribosomes contain over 50 different types of protein and those of eukaryotic cells contain more than 80 different types of protein). The large subunit contains two different ribosomal RNAs in prokaryotes and three in eukaryotes, while the small subunits contain one ribosomal RNA'.

Figure 15 The ribosome is organised into a small subunit and a large subunit. It is useful to be aware that the sizes of the subunits and their component ribosomal RNAs (rRNAs) are described by a so-called sedimentation coefficient 'S', which depends on their relative molecular mass and shape (S stands for Svedberg unit). The small and large subunits are referred to as 30S and 50S, respectively, for prokaryotic ribosomes; and 40S and 60S, respectively, for eukaryotic ribosomes. The complete ribosome is referred to as 70S for prokaryotic ribosomes and 80S for eukaryotic ribosomes.

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This diagram summarises the structural organisation of prokaryotic and eukaryotic ribosomes. Prokaryotic ribosomes are 70S particles comprising a small, 30S subunit and a large, 50S subunit. The 30S subunit is made up of 16S rRNA and 21 proteins; and the 50S subunit is made up of 23S rRNA, 5S rRNA and 34 proteins. Eukaryotic ribosomes are 80S particles comprising a small, 40S subunit and a large, 60S subunit. The 40S subunit is made up of 18S rRNA and 33 proteins; and the 50S subunit is made up of 28S rRNA, 5.8S rRNA, 5S rRNA and 50 proteins.

Figure 15 The ribosome is organised into a small subunit and a large subunit. It is useful to be aware that the sizes of the subunits and...

Ribosomes occur both 'freely' in the cytosol, i.e. not attached to any organelle (Figure 16), or attached to the membrane of the rough endoplasmic reticulum in eukaryotic cells (Figure 1a and b). Cytosolic proteins and proteins that are destined for the nucleus, mitochondria, chloroplasts and peroxisomes (you will learn about these other organelles later in this course) are synthesised by the free ribosomes in the cytosol. Cytosolic proteins, such as cytoskeleton components, are simply released into the cytosol once their synthesis is complete, but many other proteins are delivered to specific locations by a complex system of protein targeting or protein sorting. A signal sequence, a characteristic short sequence of particular amino acids in the protein itself, acts rather like a postal address that targets the protein for import into the correct organelle. Nuclear proteins, for example, contain a nuclear localisation sequence and are recognised by proteins that transport them to the nuclear membrane and through the nuclear pore. Proteins destined for mitochondria, chloroplasts or peroxisomes also have a characteristic signal sequence directing them to the appropriate organelle. This 'tagging' of polypeptides by signal sequences comprising specific amino acids is an important general mechanism used by all cells to recognise and deliver proteins to specific sites in the cell. Prokaryotic cells lack organelles, but they use a similar mechanism to target proteins to the cell membranes, cell wall, or to various types of inclusions in the cytosol.

Figure 16 Electron micrograph of 'free' ribosomes attached to newly transcribed mRNA. Several ribosomes joined by an mRNA molecule in this way constitute what is called a polyribosome, or polysome.

As you will learn in the next section, lysosomal proteins (Section 4.9), and proteins that are destined for export from the cell, or to reside in the cell membrane, are translated by ribosomes that are attached to the endoplasmic reticulum.