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A typical plant cell contains 5000 to 10,000
different polypeptide sequences and billions of
individual protein molecules. If such a cell is
to function properly, it must direct these proteins
to specific metabolic compartments, cytoplasmic
structures, and membrane systems. Accurate protein
sorting is required at all times, both when cellular
structures are formed in dividing and differentiating
cells, and when proteins in mature structures
are degraded and replaced. Examples of the proteins
that must be sorted include soluble enzymes, intrinsic
membrane proteins, and structural proteins in
the cell wall matrix.
Soluble enzymes
are present in all subcellular compartments, including
the cytosol, vacuole, cell wall, mitochondrial
matrix, chloroplast stroma, thylakoid lumen, peroxisome/glyoxysome,
lumen of the endoplasmic reticulum (ER), cisternae
of the Golgi apparatus, and nuclear sap. Membrane-bound
proteins occur in more than a dozen different
lipid bilayers that delimit these compartments
(e.g., the vacuolar membrane or tonoplast, the
plasma membrane, ER, Golgi membranes, outer membrane
of the chloroplast envelope), including membranes
that lie within organelles (e.g., thylakoids).
Some proteins are unique to a particular structure,
compartment, or membrane. Alternatively, very
similar proteins with comparable amino acid sequences,
structures, and functions can occur in more than
one compartment. For example, acid invertases
occur in the vacuole and cell wall (see Chapter
13), and water-channel proteins (aquaporins) are
found in the tonoplast and plasma membrane (see
Chapter 3). Cells therefore require the necessary
machinery to sort each protein and direct it to
its proper destination.
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