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Chapter 4

Protein Sorting and
Vesicle Traffic

CHAPTER OUTLINE
Introduction
4.1 The machinery of protein sorting
4.2 Targeting proteins to the plastids
4.3 Transport into mitochondria and peroxisomes
4.4 Transport in and out of the nucleus
4.5 The role of ER in protein sorting and assembly
4.6 Vacuolar targeting and secretion
4.7 Protein modification in the Golgi
4.8 Endocytosis

Natasha Raikhel
Maarten J. Chrispeels

 

 

 

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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