Chapter 13: Intracellular Membrane Traffic

Vesicle formation is described in terms of protein coats—COPII for ER-to-Golgi transport, COPI for Golgi-to-ER retrograde transport, and clathrin for endocytic and trans-Golgi network (TGN)-to-endosome pathways. Cargo selection is driven by adaptor proteins that recognize sorting signals in the cytoplasmic tails of cargo proteins, and small GTPases such as Sar1 and ARF are crucial for coat assembly and disassembly. Once vesicles bud, their specificity for target membranes is guided by Rab proteins and tethering complexes, while membrane fusion is mediated by v-SNARE and t-SNARE proteins that ensure correct docking and content delivery. The chapter explores the dynamics of the Golgi apparatus, including its cisternal maturation model, and how proteins are modified and sorted for different destinations. Lysosome biogenesis, endosome maturation, and receptor-mediated endocytosis are also discussed, with a focus on mannose 6-phosphate–tagged enzymes and clathrin-coated vesicles. The role of multivesicular bodies (MVBs) in sorting membrane proteins for degradation and the involvement of ESCRT complexes are covered in detail. Throughout, the chapter emphasizes the highly orchestrated nature of vesicle-mediated trafficking and its essential role in maintaining cellular organization, signaling, and homeostasis.