Chapter 7: Golgi Complex: Protein Sorting

Detailed attention is given to the organelle's structural polarity, distinguishing between the cis face, which receives vesicles from the ER, the medial cisternae, and the trans face, where final sorting occurs. A significant portion of the chapter is dedicated to the biochemical functions of the Golgi, specifically the post-translational modifications of proteins, including the sequential processing of N-linked oligosaccharides, O-linked glycosylation, sulfation of extracellular matrix components, and the proteolytic cleavage of prohormones such as proinsulin into their active forms. The mechanisms of intracellular transport are rigorously explored, describing how coat proteins (COPs and clathrin) facilitate vesicle budding and how specific fusion proteins (NSF and SNAREs) and GTP-hydrolyzing enzymes mediate the delivery of cargo between compartments. The chapter elucidates the critical sorting pathways, such as the mannose-6-phosphate receptor system for targeting hydrolases to lysosomes—illustrated by the pathology of I-cell disease—and differentiates between the constitutive secretory pathway and the regulated secretion observed in specialized cells like pancreatic acinar cells and neurons. Furthermore, the discussion extends to plant cell biology, highlighting the Golgi's role in synthesizing complex cell wall polysaccharides like hemicelluloses and pectins. The chapter concludes with an analysis of exocytosis and membrane dynamics, covering the calcium-dependent fusion of secretory vesicles with the plasma membrane, the recycling of membrane lipids and proteins via endocytosis, and complex trafficking processes such as transcytosis in polarized epithelial cells.