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modifications in the ER and in the Golgi apparatus. The finished proteins are packed into vesicles and sent to the lysosomes, endosomes, or cytoplasmic membrane. There the vesicle fuses with the membranes of the organelles or the cell, and the content of the vesicle is released through exocytosis.
Figure 5.1 Schematic overview of protein transport inside a cell.
The selectivity of protein transport is based on recognition signals that proteins must carry. If a protein does not have a signal, it remains in the cytoplasm. All other proteins contain address labels that determine the designated location. They are either coherent signal sequences, with 15–60 amino acids, or signal patches, which are only recognizable in a three‐dimensional state and are made up of signal sequences from many protein domains. The signal sequences are very conserved in their structure. Important examples are shown in Table 5.1. Signal sequences are usually found on the N‐ or C‐terminal of a protein. They are usually removed by signal peptidases as soon as a protein has reached its destination.
Table 5.1 Examples of typical recognition sequences.
| Targeted compartment | Sequence |
|---|---|
| Nuclear import | ‐Pro‐Pro‐Lys‐Lys‐Lys‐Arg‐Lys‐Val‐ |
| Nuclear export | ‐Met‐Glu‐Glu‐Leu‐Ser‐Gln‐Ala‐Leu‐Ala‐Ser‐Ser‐Phe‐ ‐ |
| Mitochondria | H3N+‐Met‐Leu‐Ser‐Leu‐Arg‐Gln‐Ser‐Ile‐Arg‐Phe‐Phe‐Lys‐Pro‐Ala‐Thr‐ |
| Arg‐Thr‐Leu‐Cys‐Ser‐Ser‐Arg‐Tyr‐Leu‐Leu‐ | |
| Plastids | H3N+‐Met‐Val‐Ala‐Met‐Ala‐Met‐Ala‐Ser‐Leu‐Gln‐Ser‐Ser‐Met‐Ser‐Ser‐ |
| Leu‐Ser‐Leu‐Ser‐Ser‐Asn‐Ser‐Phe‐Leu‐Gly‐Gln‐Pro‐Leu‐Ser‐Pro‐Ile‐Thr‐ Leu‐Ser‐Pro‐Phe‐Leu‐Gln‐Gly‐ | |
| Peroxisomes | ‐Ser‐Lys‐Leu‐COO– |
| ER import | H3N+‐Met‐Met‐Ser‐Phe‐Val‐Ser‐Leu‐Leu‐Leu‐Val‐Gly‐Ile‐Leu‐Phe‐Trp‐ |
| Ala‐Thr‐Glu‐Ala‐Glu‐Gln‐Leu‐Thr‐Lys‐Cys‐Glu‐Val‐Phe‐Gln‐ | |
| ER retention | ‐Lys‐Asp‐Glu‐Leu‐COO–‐ |
Amino acids printed in bold are especially important for the signal sequence.
5.1 Import and Export of Proteins via the Nuclear Pore
The cell nucleus is enclosed by a nuclear envelope consisting of two concentric membranes (see Section 3.1.2). Every nuclear envelope contains over 3000–4000 NPCs. The NPC in animals has a molecular weight of 125 million Da and is made up of 30 proteins, which are termed nucleoporins. An NPC consists of 500–1000 individual proteins. NPCs are able to import (e.g. histone proteins, DNA polymerases, RNA polymerases, transcriptional regulators, telomerase, RNA processing enzymes) or export (e.g. the subunits of the ribosomes that are assembled in the nucleolus, all RNAs) a large number of proteins in a short time. About 1000 macromolecules per second pass a single NPC. The nuclear pores are filled with water and allow substances smaller than 5000 Da to pass through unhindered. For larger molecules (>60 000 Da), they are highly selective. Cargo proteins must bear the correct signal sequence (Table 5.1). The structure of a nuclear pore is schematically represented in Figure 5.2.
Figure 5.2 Structure of a nuclear pore (reconstructed from electron microscopy images). The nuclear pore complex contains about 30 different proteins. Inner diameter = 9 nm. The upper side is oriented toward the cytosol.
Source: Nigg (1997). Reproduced with permission of Springer Nature.
For the import or export, mobile nuclear import receptors (importins) and export receptors (exportins) are required. These receptors must, on the one hand, recognize the recognition signal of the protein to be transported (cargo protein; Table 5.1) and, on the other hand, interact with the nucleoporins of the nuclear pores. Nuclear import and export are shown schematically in Figure 5.3. First, a cargo protein and nuclear import receptor complex are formed. As soon as a cargo protein/import receptor complex has arrived at the inner side of the nuclear membrane, a guanosine triphosphate(GTP)‐binding protein (Ran‐GTP) binds to the import receptor. A conformational change occurs, and a cargo protein is released. The complex of Ran‐GTP and the import receptor binds to nucleoporin and transports it through the pore in the direction of cytosol. Once it arrives, Ran‐GTP is dephosphorylated and dissociated from the import receptor as Ran‐guanosine diphosphate(GDP), whereby the receptor is reactivated. Export out of the nucleus occurs with a similar principle (Figure 5.3). The change from Ran‐GTP to Ran‐GDP is catalyzed by a GTPase‐activating protein (GAP); the exchange of GDP to GTP in the nucleus is assisted by a guanine exchange factor(GEF). The export of cargo from the nucleus to the cytosol follows a similar logic (Figure 5.3).