Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 250

Proteins that reside within the nucleus are characterized by the presence of specific nuclear targeting sequences. Such nuclear localization signals are both necessary for nuclear entry of the proteins in which they are present and sufficient to direct heterologous, nonnuclear proteins to enter this organelle. Nuclear localization signals identified by these criteria share a number of common properties: they are generally fewer than 20 amino acids in length, and are usually rich in basic amino acids. Although no consensus nuclear localization sequence can be defined, most nuclear localization signals belong to one of two classes, simple or bipartite sequences (Fig. 5.25). A particularly well-characterized example of a simple nuclear localization signal is that of simian virus 40 large T antigen, which comprises five contiguous basic residues flanked by hydrophobic amino acids.

Figure 5.24 Structure and organization of the nuclear pore complex. (Bottom left) Cartoon depiction of the nuclear membrane, showing the topology of the nuclear pore complexes. The entire structure of the nuclear pore complex of Saccharomyces cerevisiae, comprising 552 proteins, was determined using cryo-electron tomography and modeling (see Kim SJ et al. 2018. Nature 555:475–482). The yeast and human nuclear pore complexes are highly conserved. (Top) Side view of the nuclear pore showing three spokes. Spokes are high-ordered subcomplexes formed by NUPs (nucleoporins). Spokes in turn assemble into larger complexes to form two outer and two inner rings flanking a membrane ring. The complex is completed by a nuclear basket composed of NUPs. The outer ring comprises export complexes (olive green) and connectors (tan) with the inner ring (magenta, pink, and green). The inner rings sandwich the membrane ring (light yellow). The nucleoplasmic outer ring links to the nuclear basket (only partly depicted as light blue filaments). (Bottom right) Top view of the nuclear pore from the cytoplasm showing a model of the central transporter (green). Structure reconstructions courtesy of Michael Rout, Rockefeller University, New York.

Figure 5.25 Nuclear localization signals. The general form and specific examples of simple and bipartite nuclear localization signals are shown in the one-letter amino acid code, where X is any residue. Bipartite nuclear targeting signals are defined by the presence of two clusters of positively charged amino acids separated by a spacer region of variable length and sequence. Both clusters of basic residues, which often resemble the simple targeting sequences of proteins like simian virus 40 T antigen, are required for efficient import of the proteins in which they are found. The subscript indicates either length (3–7) or composition (e.g., 3/5 means at least 3 residues out of 5 are basic). Gold particles with diameters as large as 26 nm are readily imported following their microinjection into the cytoplasm, as long as they are coated with proteins or peptides containing a nuclear localization signal.

Import of a protein into the nucleus via nuclear localization signals occurs in two distinct, and experimentally separable, steps (Box 5.5). A protein containing such a signal first binds to a soluble cytoplasmic receptor protein. This complex then engages with the cytoplasmic surface of the nuclear pore complex, a reaction often called docking, and is then translocated through the nuclear pore. In the nucleus, the complex is disassembled, releasing the protein cargo.