The plant endomembrane system
A plant cell contains upto 10,000 different kinds of proteins. Each of these proteins must be localized to the precise intracellular membrane, organelle or directed to the cell surface for its proper functioning. The plant endomembrane system is a complex system of organelles specialized for the synthesis, transport, modification and secretion of proteins and other macromolecules. This system is composed of several functionally distinct membrane compartments: the endoplasmic reticulum (ER), the Golgi apparatus including the trans-Golgi network (TGN), secretory vesicles, the vacuole/lysosome andendosomes. Notably, the membranes of mitochondria and chloroplasts do not belong to the endomembrane system (Vitale and Galili, 2001; Dacks et al., 2009).
However, the synthesis of the majority of proteins of a eukaryotic cell occurs in the cytosol, and from there proteins are migrated to reach their final destination. These proteins thus contain the information necessary to be transported to the correct target compartment. Targeting to the cell secretory pathway, in particular, has been proposed to improve the stability and yield of several proteins (Ma et al., 2003; Yoshida et al., 2004; Vitale and Pedrazzini, 2005). In theabsence of any specific targeting signals, a protein entering the endomembrane system will follow the default secretory pathway and will be secreted to the cell exterior.