12.3Transport of Proteins Synthesized by Free Polysomes
Free polysomes synthesize proteins that are transported to organelles such as the nucleus, mitochondria, chloroplasts, and peroxisomes, as well as enzymes and structural proteins that function in the cytoplasm. Many of the synthesized proteins migrate to the target organelle by diffusion through the cytoplasm. Furthermore, each protein is
accurately absorbed into the target organelle. This is because of the specific signal sequence for selective uptake into the target organelle in the synthesized protein.
Transportation to the Nucleus
Proteins required inside the nucleus (e.g., ribosomal proteins, proteins involved in transcription and DNA replication, and proteins that constitute chromosomes) are selectively transported into the nucleus after synthesis by a free polysome in the cytoplasm. On the other hand, most RNA transcribed inside the nucleus is transported into the cytoplasm. This type of material transport between the cytoplasm and nucleus is performed by nuclear membrane pores formed in the nuclear membrane. A huge structure called the nuclear pore complex forms the passageway of these nuclear pores (Fig. 12-11A).
Free transportation through the pores is made possible for any protein with a molecular weight less than 20,000, when small pores (the pores have a diameter of approximately 9 nm) open in the central part of the nuclear pore complex. However, in reality, some molecules that vastly exceed this limit still manage to easily pass through the nuclear pore complex for transport between the cytoplasm and nucleus. Moreover, a certain directionality for transport is observed in proteins that are transported through the nuclear pore complex. For example, proteins required in the nucleus after being synthesized in the cytoplasm are not selectively transported into the nucleus. The signal sequence also plays a very important role here. These include the nuclear localization signal (NLS), which is found in proteins that are to be transported from the cytoplasm into the nucleus, and the nuclear export signal (NES), which is found in proteins that are to be transported from the nucleus into the cytoplasm.
There are two types of transport proteins that recognize and bind NLS and NES and travel back and forth between the cytoplasm and nucleus while transporting cargos in their prescribed directions (Fig. 12-11B). The first protein is called importin, which binds NLS and transports that protein from the cytoplasm into the nucleus, while the second is called exportin, which binds NES and transports that protein from the nucleus into the cytoplasm. These two types of transport proteins have many intermediates, each of which function to transport different types of cargo.
Fig. 12-11 Nuclear pores
(A) An electron microscope photograph of nuclear pores and a model showing the structure of the nuclear pore complex. (B) A model for protein transport through the nuclear pore complex. Importin is a transport protein that transports proteins having NLS into the nucleus. Exportin is a protein that transports proteins having NES out of the nucleus into the cytoplasm. G proteins either bind the cargo to a transport protein or separate the cargo from the transport protein.
Transportation to the Mitochondria, Chloroplasts, and Peroxisomes
Proteins absorbed into the mitochondria, chloroplasts, and peroxisomes also contain signal sequences (transit sequences) that indicate their respective destinations (Fig. 12-12A). Through the selective binding of these transit sequences and their receptors, mistakes in protein destination are prevented, and thus, the protein can be absorbed by the target organelle. The receptor can be found in either a free state within the cytoplasm or in the membrane of the organelle. Receptors found in a free state within the cytoplasm also have their own receptors to which they bind in the membrane of the target organelle. Therefore, in all cases, each protein can selectively bind to a receptor found in the membrane of the target organelle. Furthermore, a protein bound to the membrane receptor is guided to a translocon, which is a channel found in the membrane for proteins, for uptake into the organelle (Fig. 12-12B). After uptake, in case of mitochondria and chloroplasts, the transit sequence is cleaved by signal peptidase.
Fig. 12-12 Transport signalling for each organelle
(A) The N ends of proteins transported to the mitochondria and chloroplasts have transit sequences. The transit sequences of proteins for transport to the peroxisome are also found on the C end. Enzymatic or structure proteins for distribution into the cytoplasm do not have this type of transit sequence. (B) Proteins destined for the mitochondria, chloroplasts, and peroxisomes selectively bind to a receptor found on the membrane of the respective organelle and absorbed into its interior. Here the steric structure of the protein is unwound so that it can pass through the translocon. Then, after being absorbed inside, it again forms the steric structure. In almost all cases, after being absorbed into the target organelle, the transit sequence is cleaved.