Targeting to the Golgi apparatus

Proteins translated on the RER generally fold and assemble into subimits in the ER before being transferred to the Golgi apparatus. Other proteins fold in the cytoplasm. Molecular chaperones (proteins such as calnexin and BiP) assist in this process of protein folding. Proteins that are misfolded are targeted for destruction by ubiquitin and digested in cytoplasmic protein-digesting complexes called proteasomes. 

Most proteins targeted to the ER by a signal peptide are directed to the Golgi apparatus for further sorting. Proteins leave the Golgi apparatus in membranecoated vesicles and are destined for insertion in the plasma membrane, to become lysosomes or to fuse with the plasma membrane and release their contents as secreted proteins. 

Transport of membrane lipids from the site of synthesis to the target membranes has been postulated to be mediated by membrane vesicles, by lipid transport proteins and/or to occur at contact sites between membranes [1-3]. Vesicular-mediated lipid transport between plant membranes has been reconstituted in cell-free systems. The results suggest that transport of lipids from endoplasmic reticulum (ER) to the Golgi apparatus [4,5] or to the chloroplast envelope (Sandelius and Rantfors, unpublished) and from the envelope to the thylakoid [6] is metabolically regulated, as shown by temperature dependence and requirement of and/or stimulation by ATP and cytosol or stroma. 

P.A. Gleeson, Targeting of proteins to the Golgi apparatus, Histochem Cell Biol, 1998, 109, 517-532. 

From the Golgi apparatus, the proteins are transported by vesicles to various targets in the cells—e.g., to lysosomes (4), the plasma membrane (6), and secretory vesicles (5) that release their contents into the extracellular space by fusion with the plasma membrane (exocytosis see p. 228). Protein transport can either proceed continuously (constitutive), or it can be regulated by chemical signals. The decision regarding which pathway a protein... 

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