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Some enveloped viruses vary considerably in size and shape. For example, the particles of paramyxoviruses, such as measles and Sendai viruses, range in size from 120 to up to 540 nm in diameter and may contain multiple copies of the (−) strand RNA genome in helical nucleocapsids of different pitch. Influenza A virus particles exhibit even more extreme pleomorphism: they appear spherical, elliptical, or filamentous, and all forms come in a wide range of sizes (see the figure). Clinical isolates are primarily filamentous but adopt the spherical morphologies when adapted to propagation in the laboratory, particularly in chicken eggs.

Several lines of evidence indicate that the filamentous phenotype is genetically determined. For example, the particles of some influenza A virus isolates are primarily filamentous, whereas those of other isolates are not. Furthermore, genetic experiments have identified specific residues in the matrix proteins (M1 and M2) required for assembly of filamentous particles. Deletion of the internal domain of the NA glycoprotein also induces formation of elongated particles, a phenotype exacerbated by concurrent removal of the cytoplasmic tail of the major viral glycoprotein HA. These observations imply that matrix-glycoprotein interactions during assembly govern the morphology of influenza A virus particles. However, the mechanism underlying the “choice” between assembly of filamentous versus spherical particles is not well understood and the influence of host cell components remains obscure. Furthermore, the physiological significance of the filamentous particles is not known, despite their predominance in clinical isolates. It has been speculated that these forms might facilitate cell-to-cell transmission of virus particles through the respiratory mucosa of infected hosts.

 Badham MD, Rossman JS. 2016. Filamentous influenza viruses. Curr Clin Microbiol Rep 3:155–161.

Cryo-electron tomogram sections of influenza A virus particles (strain PR8). Bar = 50 nm. Reprinted from Nayak DB et al. 2009. Virus Res 143:147–161, with permission. Courtesy of D.B. Nayak, University of California, Los Angeles.

In contrast to the head, the ~100-nm-long tail, which comprises two protein layers, exhibits helical symmetry (Fig. 4.26A). The outer layer is a contractile sheath that functions in injection of the viral genome into host cells. The tail is connected to the head via a hexameric ring and at its other end to a complex, dome-shaped structure termed the baseplate, where it carries the cell-puncturing spike. Both long and short tail fibers project from the baseplate. The former, which are bent, are the primary receptor-binding structures of bacteriophage T4. As discussed in Chapter 5, remarkable conformational changes induced upon receptor binding by the tips of the long fibers are transmitted via the baseplate to initiate injection of the DNA genome.