Chapter 12: Intracellular Organization and Protein Sorting

Intracellular Organization and Protein Sorting explores three major protein transport mechanisms: gated transport (e.g., nuclear pore complexes), transmembrane transport (e.g., protein translocation into the ER, mitochondria, or peroxisomes), and vesicular transport (e.g., movement between ER and Golgi or to the plasma membrane). The chapter explains how signal sequences in polypeptides determine their cellular destinations, and how signal recognition particles (SRPs) and translocons guide nascent proteins into the ER. Mitochondrial and peroxisomal import pathways are also detailed, including the use of chaperones and membrane transport complexes. The chapter highlights the importance of protein folding, glycosylation, and quality control within the ER, and discusses how misfolded proteins are retained or degraded via ER-associated degradation (ERAD). Vesicular transport pathways are introduced through the formation and targeting of vesicles coated with clathrin, COPI, or COPII, as well as the roles of Rab proteins, SNAREs, and tethering complexes in vesicle docking and fusion. Endocytosis, retrograde transport, and organelle biogenesis are also addressed. Overall, the chapter illustrates how the orchestrated transport and localization of proteins underpin the spatial and functional complexity of eukaryotic cells.