|
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.
|
