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Schematic illustration of a classical nuclear import pathway. Data from Yang Q et al. 1998. Mol Cell 1:223–234.

Different groups of proteins are imported by specific nuclear transport receptor complexes. In what is known as the “classical system” of import, cargo proteins containing basic nuclear localization signals (NLS) bind to the cytoplasmic nuclear localization signal receptor protein importin-α (step 1 in the figure). This complex then binds importin-β1, which mediates docking with the nuclear pore complex by binding to members of a family of nucleoporins (step 2). It is likely that initial association involves nucleoporins present in the cytoplasmic filaments of the nuclear pore. Importin-β1 also interacts with RAN, a small RAS-related nucleotide-binding protein. RAN GTPase (step 3) is required for translocation of the complex into the nucleus through the central channel of the nuclear pore (step 4).

A single translocation through the nuclear pore complex does not require energy consumption. However, maintenance of a gradient of the guanosine nucleotide-bound forms of RAN is absolutely essential for continued transport. A RAN-specific guanine nucleotide exchange protein named RCC1 (regulator of chromosome condensation 1) resides in the nucleus and promotes the exchange of GDP to GTP. In contrast, a RAN-GTPase-activating protein (RANGAP-1) localized in the cytoplasm promotes hydrolysis of GTP. The nuclear pool of RAN-GDP is replenished by the action of nuclear transport factor 2 (NTF2), which transports RAN-GDP from the cytoplasm to the nucleus efficiently (step 5), where it can be converted to RAN-GTP. The asymmetric distribution of RCC1 and RANGAP-1 allows for the formation of a gradient of RAN-GTP/RAN-GDP. This gradient provides the driving force and directionality for nuclear transport.

Importin-β1 has a higher affinity for RAN-GTP, which is more abundant in the nucleus, than for RAN-GDP. Therefore, following import into the nucleus, importin-β1 binds to RAN-GTP and the complex disassembles, eventually releasing the cargo protein. The importin-β1 recycles to the cytoplasm bound to RAN-GTP (step 6). There, it is displaced by the action of two high-affinity RAN-GTP-binding proteins, RANBP1 and RANBP2 (or NUP358). This enables conversion of RAN-GTP to RAN-GDP and binding of importin-β1 to new substrates.

Figure 5.26 Different strategies for entering the nucleus. (A) Each segment of the influenza virus genome is small enough to be transported through the pore complex. (B) The herpes simplex virus 1 capsid docks onto the nuclear pore complex and is minimally disassembled to allow transit of the viral DNA into the nucleus. (C) The adenovirus subviral particle is dismantled at the nuclear pore, allowing transport of the viral DNA with core protein VII into the nucleus. (D) The capsids of some viruses (parvovirus and hepadnavirus) are small enough to enter the nuclear pore complex without disassembly but do not enter by this route. These virus particles bind the nuclear pore complex, which induces local disruption of the nuclear envelope, allowing nuclear entry.