Maturation of parvovirus LuIII in a subcellular system. I. Optimal conditions for in vitro synthesis and encapsidation of viral DNA.

The development of an in vitro system prepared by lysis of LuIII virus-infected cells with Brij-58 has enabled the study of the assembly pathway of a parvovirus. Under optimal conditions, radioactive precursors are incorporated both into viral replicative form double-stranded DNA and into progeny viral DNA (vDNA) during pulses as short as 30 s. Labelled 110S particles can be isolated at the end of such pulses. Therefore, synthesis and encapsidation of progeny viral DNA into pre-existing empty viral capsids appear to be closely related processes. Up to about 10 min after incorporation of vDNA, the 110S particles band at 1.44 g/ml and are relatively unstable in 3.5 M-CsCl. Moreover, newly synthesized vDNA molecules show an abnormal electrophoretic behaviour probably due to the presence of a covalently linked terminal protein. This so far uncharacterized alkali-stable polypeptide is lost (cleaved off?) concomitant with the maturation of the 110S virus particles. Maturation is reflected by a change in the virus stability in CsCl and a shift in density from 1.44 to 1.41 g/ml around 10 to 15 min after encapsidation of progeny vDNA.