CHAPTER 12Intracellular Organization and Protein Sorting

The many thousands of macromolecules and their associated biochemical activities inside a cell are spatially segregated to different regions. Intracellular organization is a particularly prominent feature of eukaryotic cells, which unlike bacteria are elaborately subdivided into functionally distinct, membrane-enclosed compartments. Many of these compartments define the cell’s major organelles such as the endoplasmic reticulum, Golgi apparatus, lysosome, plastid, and mitochondrion, the last two of which are still further subcompartmentalized by internal membranes. Other subsets of the cell’s macromolecules can organize into dynamic and reversible assemblies, called biomolecular condensates, that can serve as specialized biochemical factories or temporary storage depots. To understand the eukaryotic cell, it is essential to know how the cell creates and maintains its complex intracellular organization.

An animal cell contains about 10 billion (10¹⁰) protein molecules of perhaps 10,000 kinds, and the synthesis of almost all of them begins in the cytosol, the space of the cytoplasm outside the membrane-enclosed organelles. Each newly synthesized protein is then delivered specifically to the organelle that requires it. The unique protein and lipid composition on the surface of each organelle is used as a cue to direct new deliveries of proteins and lipids to sustain that organelle’s identity. The characteristic set of proteins and other specialized molecules define each organelle’s structural and functional properties. They catalyze the reactions that occur there and selectively transport molecules into and out of the organelle. The intracellular transport of proteins is the central theme of both this chapter and the next. By tracing the protein traffic from one part of the cell to another, one can begin to make sense of the otherwise bewildering maze of intracellular membranes and other subcellular structures.