Biochemical studies of the maturation of herpesvirus nucleocapsid species.

Three distinct species of nucleocapsids of equine herpesvirus type-1, designated as either light (L), intermediate (I), or heavy (H) on the basis of their densities in Renografin-76 density gradients (Perdue et al. 1975), were characterized with respect to their amino acid content, DNA content, and role in the maturation process. Preparations of L and I nucleocapsids, shown previously to lack a densely staining core within the capsid, exhibited virtually identical amino acid compositions. Preparations of H nucleocapsids, which possess densely staining cores, contained significantly more lysine, glutamic acid, and serine than did L and I capsids and, in general, more closely resembled the enveloped virion in amino acid composition. The increased content of lysine and glutamic acid in H nucleocapsids indicates that polypeptide IVa (30,000 mol wt), which is present only in H nucleocapsids, is rich in these amino acids. These amino acids may be present as polylysine and polyglutamic acid which have been shown to collapse DNA and function in the DNA packaging event of bacterial viruses (Laemmli, 1975). DNA isolated from preparations of each of the three nucleocapsid species was characterized and shown to be identical to the DNA of enveloped virus in density (1.716 g/cm(3)) and size (50-55 S). The intranuclear production of the three nucleocapsid species was studied with respect to both the time and rate of synthesis of each and the relative amount of each species present during infection. All three species appeared at 6-8 hr postinfection and were produced in a logarithmic fashion until 15 hr postinfection. The ratio of L:I:H particles, based on the percentage that each species comprised of the total purified nucleocapsid population, remained constant and was approximately 45:45:10 at all times postinfection. Analysis of the fate of each of the three intranuclear nucleocapsid classes by pulse-chase experiments indicated that I and H species are removed from the nucleus by participation in the maturation process, but L capsids remain in the nucleus throughout infection and are defective by-products of virus assembly. Based on the evidence accumulated in this study and on previous data (Perdue et al. 1974, 1975), a model depicting equine herpesvirus maturation is proposed. This model proposes that the viral DNA is inserted into I nucleocapsids and that this process is accompanied by condensation of the internal protein resulting in the formation of a dense core and hence an H nucleocapsid.