Virus Structure and Properties

CMV has icosahedral particles 29 nm in diameter, which sediment as a single component with an S20,w of 98.6— 104c (c being virus concentration in mgml-1). Particles are built of 180 capsid protein subunits arranged with T = 3 quasisymmetry, contain about 18% RNA, and have an extinction coefficient at 260nm (1 mgml-1, 1cm light path) of 5.0. RNA1 and RNA2 are encapsidated in different particles, whereas RNA3 and RNA4 are probably packaged together in the same particle, some particles may contain three molecules of RNA4. Thus, virus preparations contain at least three different types of particles, but with similar morphology and sedimentation properties. Virus particles also contain low levels of the RNA species designated RNA4A, RNA5, and RNA6. There is a limit to the size of encapsidated RNAs, those larger than RNA1 are not encapsidated in vivo. CMV particles are stabilized by RNA-protein interactions, and no empty particles are formed. Particles disrupt at high neutral chloride salt concentrations or at low sodium dodecyl sulfate concentrations; biologically active particles can be reassembled by lowering the salt concentration or removing the sodium dodecyl sulfate. Particles are stable at pH 9.0 and do not swell at pH 7.0, an important difference with bromoviral particles.

The structure of CMV particles has been resolved at 3.2 A by X-ray crystallography. The T = 3 lattice of capsids is composed of 60 copies of three conformation-ally distinct subunits designated A, B, and C that form trimers with quasi-threefold symmetry. There are 20 hex-americ capsomers of B and C subunits with quasi-sixfold symmetry and 12 pentameric capsomers formed by A subunits with fivefold symmetry. The exterior radius along the quasi-sixfold axes is 144 A, the RNA is tightly packaged against the protein shell and leaves a hollow core of about 110 A along the threefold axes. The protein subunit has a b-barrel structure, with the long axis of the b-barrel domain oriented roughly in a radial direction. The N-terminal 22 amino acids ofthe capsid protein are needed for particle assembly. This region is positively charged and, in the B and C subunits, forms amphipatic helices that run parallel to the quasi-sixfold axes. There is an external region of negative electrostatic potential that surrounds the fivefold and quasi-sixfold axes and locates above regions of positive potential which extend to cover nearly homogeneously the inner surface of capsids, where interaction with encapsidated RNA occurs. Electrostatic distributions in CMV particles explain the physicochem-ical conditions required for particle stability.

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