2 Cellular compartments and traffic
2.1 Introduction
This section reminds you of the numerous specialised intracellular compartments of the eukaryotic cell, with how molecules are moved rapidly and specifically between them in eukaryotic cells.
What are the principal membrane-bound compartments of the cell and the trafficking pathways that connect them?
Early and late endosomes, lysosomes, peroxisomes, the endoplasmic reticulum, the cis, medial and trans Golgi network, secretory vesicles, the nucleus, mitochondrion, and chloroplast.
On average, these compartments occupy about half of the volume of the cell. Their associated membranes in a liver cell (hepatocyte), for example, constitute about 50 times more of the total area than the plasma membrane (Table 2). The extent of the intracellular compartments varies between cell types, but the contemporary picture of a cell is a labyrinth of membrane-bound intracellular compartments, endosomes and organelles, with a highly developed interconnecting system of transport vesicles.
Table 2 Relative amounts of membrane in two eukaryotic cells (% of total membrane, by surface area).
| Membrane type | Hepatocyte | Pancreatic exocrine cell |
|---|---|---|
| plasma membrane | 2% | 5% |
| rough ER | 35% | 60% |
| smooth ER | 16% | <1% |
| Golgi apparatus | 7% | 10% |
| mitochondrial innermembrane | 32% | 17% |
| mitochondrial outermembrane | 7% | 4% |
| nuclear membrane | 0.2% | 0.7% |
| lysosomes, peroxisomes, endosomes, secretory vesicles | <2% | <4% |
Vesicular transport is the mechanism by which molecules are ferried in membrane-bound vesicles between the membrane-bound compartments listed above, or between these compartments and the plasma membrane. Small transport vesicles pinch off from one compartment, and diffuse, or are more often actively transported, to another compartment, where they fuse and discharge their cargo (Figure 9). The size and shape of the vesicles, their cargo, their packaging and their means of transportation vary depending on the trafficking pathway involved. The cargo itself may be soluble within the compartment (e.g. a secreted hormone) or may be associated with membrane proteins. Topologically, the extracellular space, the ER, the Golgi network, the endosomes, secretory vesicles and transport vesicles are all equivalent spaces, each separated from the cytosol by one phospholipid bilayer. For molecules to move from the cytosol into one of these compartments they have to cross one membrane. In this chapter we are concerned only with transport between these topologically equivalent compartments mediated by the budding and fusion of vesicles.
Transport of proteins to the correct cellular compartment depends on signal sequences in the polypeptide chain.
Signal sequences at the N-terminus of newly synthesised polypeptides are recognised by a signal recognition protein, which directs them to sites on the ER, where they are transported into the lumen of the ER as they are translated. The signal sequence is enzymatically removed from most proteins after translation. For multipass membrane proteins, internal stop-transfer and start-transfer signals are required to produce the correct looping across the membrane.
Proteins destined for specific cellular compartments also contain signal sequences, although these may be non-contiguous within the polypeptide chain, and are referred to as signal patches. Unlike the N-terminal signal sequences, signal patches are integral to the final protein and are not cleaved off. The signal patches serve to sort the proteins for their appropriate compartment and/or ensure that they remain in that compartment. Proteins called sorting receptors recognise the signal patches and group the proteins in appropriate vesicles for transport. We shall start by looking at how particular proteins are selectively localised in particular compartments.