Maturation of parvovirus LuIII in a subcellular system. II. Isolation and characterization of nucleoprotein intermediates.

Nucleoprotein (NP) intermediates in the assembly pathway of parvovirus LuIII were isolated. These structures consist of replicating viral DNA and of preformed viral capsids into which progeny viral DNA is encapsidated concomitant with synthesis. The NP complexes sediment at between 70S and 100S. They are unstable in CsCl and dissociate in the presence of 1 M-NaCl. After fixation with glutaraldehyde, however, they accumulate at a density of 1.37 g/ml in CsCl gradients. Electron microscopy of fixed complexes revealed structures consisting of DNA threads associated with capsid-like particles. A considerable proportion of the DNA labelled during short pulses in such complexes, in a subcellular in vitro system derived from infected nuclei, can be effectively chased into 110S virions. The resulting 110S particles apparently have to undergo a sequence of morphogenic events to acquire the physicochemical properties of the mature infectious parvovirus.