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Chapter 9

Protein Synthesis, Assembly,
and Degradation

9.1 From RNA to protein
9.2 Regulation of cytosolic protein biosynthesis in eukaryotes
9.3 Protein synthesis in chloroplasts
9.4 Post-translational modification of proteins
9.5 Protein degradation

Linda Spremulli




Protein synthesis is essential to cell function.

Proteins constitute a large percentage of the plant cell and carry out many different cell functions. It is therefore not surprising that protein synthesis is central to cell growth, differentiation, and reproduction. The processes of transcription and processing of messenger RNA (mRNA), discussed in Chapters 6 and 7, yield a template for the process of translation, which is described in detail in this chapter. Translation is the mechanism by which specialized riboprotein complexes “read” the information in the mRNA sequence and “write” a corresponding sequence of amino acids linked by peptide bonds to form a polypeptide chain. This chapter also presents how the polypeptide chain folds to form a precise three-dimensional structure that can carry out one or more specific biological functions (Fig. 9.1). Many proteins must assemble into large supramolecular complexes, called quaternary structures, to perform their specific functions. Photosynthetic complexes serve as examples of such multisubunit structures. To function properly, cells must regulate protein synthesis and degradation, responding to internal and external signals by adjusting the amounts of specific proteins present to suit cellular requirements. Although plants share many features of protein synthesis and metabolism in common with other eukaryotic organisms, certain aspects, such as protein synthesis in plastids and regulation of translation by light, are unique to plant cells.

Protein synthesis occurs in three distinct sites in plant cells.

In plants, protein synthesis occurs in three subcellular compartments. The cytoplasm, plastids, and mitochondria each contain different protein synthetic machinery (Fig. 9.2). About 75% of cell protein is made in the cytoplasm, where the mRNAs transcribed from the nuclear genome are translated. About 20% of the protein in a photosynthetically active cell (e.g., a young mesophyll cell) is synthesized in the chloroplast by means of mRNA templates transcribed from the chloroplast genome. A small amount of protein synthesis (approximately 2% to 5% of the total protein) occurs in mitochondria. This system translates mRNAs transcribed from mitochondrial DNA.
      The variety of proteins synthesized also differs among the three compartments. In the cytoplasm, more than 20,000 different proteins may be synthesized. Fifty to 100 proteins are synthesized in chloroplasts, and the number synthesized in mitochondria varies widely among species. About 30 to 40 proteins appear to be synthesized in the mitochondria of the liverwort Marchantia polymorpha, for example, whereas the mitochondrial genomes of seed plants typically encode even fewer proteins.
      The mechanisms responsible for protein synthesis in the cytoplasm, plastids, and mitochondria are distinct from each other and share few components, if any. Thus, plant cells contain three different types of ribosomes, three groups of transfer RNAs (tRNAs), and three sets of auxiliary factors for protein synthesis. Plastids and mitochondria presumably arose through the endo-symbiosis of ancient prokaryotic organisms (see Chapters 1 and 6). Consistent with this theory, the protein synthetic machinery in plastids and mitochondria is more closely related to bacterial systems than to the translation apparatus in the surrounding plant cell cytoplasm. For reasons unknown, chloroplasts and mitochondria have retained a small amount of DNA and have preserved their capacity to synthesize proteins. In contrast to protein synthesis in the cytoplasm and chloroplast, very little is known about mitochondrial protein synthesis.

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