Nuclear pore complex processing

The recycling process of Ran between its GDP and GTP complexed state is supported by another class of Ran-binding proteins, which support GTP-hydro-lysis by RanGAP and are either soluble proteins [ 143] or part of the nuclear pore complex [144]. 

Ions, small metabolites, and globular proteins up to 60 kDa can diffuse through a water-filled channel in the nuclear pore complex these channels behave as if they are -0.9 nm in diameter. However, large proteins and ribonucleoprotein complexes cannot diffuse in and out of the nucleus. Rather, these macromolecules are selectively transported in and out of the nucleus with the assistance of soluble transporter proteins that bind macromolecules and also Interact with certain nucleoporlns. The principles underlying macromolecular transport through nuclear pore complexes were first determined for the Import of individual proteins into the nucleus, which we discuss first before turning to the question of how fully processed mRNAs are transported into the c rt oplasm. 

After pre-tRNAs are processed In the nucleoplasm, the mature tRNAs are transported to the cytoplasm through nuclear pore complexes by exportin-t, as discussed previously. In the cytoplasm, tRNAs are passed between aminoacyl-tRNA synthetases, elongation factors, and ribosomes during protein synthesis (Chapter 4). Thus tRNAs generally are associated with proteins and spend little time free in the cell, as Is also the case for mRNAs and rRNAs. 

Fig. 10.21. The nuclear pore complex. The approximately 100 different polypeptide chains of the nuclear pore complex form an assembly of 8 spokes attached to two ring structures (a cytoplasmic ring in the outer nuclear membrane and a nuclear ring through the inner membrane) with a transporter plug in the center. Small molecules, ions, and proteins with less than a 50-kDa mass passively diffuse through the pore in either direction. However, RNAs and most proteins are too large to diffuse through, and are actively transported in a process that requires energy, is selective for the molecule transported, is unidirectional, and can be regulated.

The genetic information of mammalian cells is almost exclusively located in the nucleus (Figure 17.1). This cell organelle is bounded by a double membrane which contains nuclear pore complexes as the connection to the cytoplasm and it is studded with ribosomes [12]. Replication and transcription of DNA are cellular processes that take place in the nucleus. DNA replication occurs during the mammalian cell cycle, which can be divided into different phases. The gap between mitosis and the initiation of DNA replication is termed G -phase, while the gap between DNA synthesis (S-phase) and mitosis (M-phase) is called G2-phase [13]. 

Akey, C. W., and Goldfarb, D. S. (1989). Protein import through the nuclear pore complex is a multistep process. J. Cell Biol. 109, 971-982. 

Miao, L., 8c Schulten, K. (2009). Transport-related structures and processes of the nuclear pore complex studied through molecular dynamics. Structure, 17, 449. 

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