Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 242

Movement of molecules larger than 500 kDa does not occur by passive diffusion, because the cytoplasm is crowded with organelles, high concentrations of proteins, and the cytoskeleton. Rather, viral particles and their components are transported via the cytoskeleton. Such movement can be visualized in live cells by using fluorescently labeled viral proteins (Chapter 2).

The cytoskeleton is a dynamic network of protein filaments that extends throughout the cytoplasm. It is composed of microtubules and actin filaments. Microtubules are organized in a polarized manner, with minus ends situated at the microtubule-organizing center near the nucleus, and plus ends located at the cell periphery (Fig. 5.11). This arrangement permits directed movement of cellular and viral components over long distances. Actin filaments typically assist in virus movement close to the plasma membrane. Techniques to follow the movement of virus particles after entry continue to improve. For example, a combination of technologies such as real-time quantum dots-based single particle tracking with biochemical assays was used to track reovirus particles that enter cells via clathrin-mediated endocytosis. Following internalization, movement of individual particles was slow and dependent on actin, while movement became faster toward the cell interior and dependent on microtubules.

Transport along actin filaments is accomplished by myosin motors, and movement on microtubules is via kinesin and dynein motors. Hydrolysis of adenosine triphosphate (ATP) provides the energy for the motors to move their cargo along cytoskeletal tracks. There are two basic ways for viral or subviral particles to travel within the cell: within a membrane vesicle such as an endosome, which interacts with the cytoskeletal transport machinery; or directly (Fig. 5.11). In the latter case, some form of the virus particle must bind to the transport machinery. After leaving endosomes, the subviral particles derived from adenoviruses and parvoviruses are transported along microtubules to the nucleus. Although adenovirus particles exhibit bidirectional plus- and minus-end-directed microtubule movement, their net movement is toward the nucleus. Adenovirus binding to cells activates two different signal transduction pathways that increase the net velocity of minus-end-directed motility. The signaling pathways are therefore required for efficient delivery of the viral genome to the nucleus. Adenovirus subviral particles are loaded onto microtubules by interaction of the capsid protein, hexon, with dynein. The particles move toward the centrosome and are then released and dock onto nuclear pores, prior to viral genome entry into the nucleus.

A number of different viruses enter the peripheral nervous system and spread to the central nervous system via axons. As no viral genome encodes the molecular motors or cytoskeletal structures needed for long-distance axonal transport, viral adapter proteins are required to allow movement within nerves. An example is the axonal transport of alphaherpesvirus subviral particles. After fusion at the plasma membrane, the viral nucleocapsid is carried by retrograde transport to the neuronal cell body. Such transport is accomplished by the interaction of a major component of the tegument, viral protein VP1/2, with minus-end-directed dynein motors. In contrast, other virus particles are carried to the nerve cell body within endocytic vesicles. For example, after endocytosis of poliovirus, virus particles remain attached to the cellular receptor CD155. The cytoplasmic domain of the receptor engages the dynein light chain TCTEX-1 to allow retrograde transport of virus-containing vesicles.