A genome-linked copy of the NS-1 polypeptide is located on the outside of infectious parvovirus particles

Abstract

The 5' ends of all newly synthesized single-stranded (s1) DNA genomes of the autonomous parvovirus minute virus of mice are covalently linked to the major virally coded nonstructural protein NS-1, but later in infection this association is disrupted, giving rise to an abbreviated form of single-stranded DNA designated s2. Both s1 and s2 forms are encapsidated and migrate in velocity gradients as 110S particles, and, as such, both appear to be infectious. Most virions are released from A9 cells as s1 particles, but the NS-1 molecules are located on the outside of the virion where they are accessible to both antibodies and enzymes. These polypeptides are cleaved from the encapsidated DNA by nucleolytic or proteolytic digestion, which can occur either in the culture medium or upon subsequent entry into further host cells. Since the s1 to s2 cleavage can be minimized by blocking viral reentry, it is likely that most of the processing occurs after entry into the host cell. Incoming virus is rapidly converted to the s2 form when it is used to infect new host cells, but in vitro removal of the NS-1 molecules with proteases or nucleases fails to influence the infectivity of s1 particles under normal culture conditions. Limited proteolysis of s1 particles with trypsin demonstrates that NS-1 is linked to the DNA via its amino-terminal domain. Analysis of the 5' ends of s1 and s2 forms indicates that there are approximately 24 externally located nucleotides linking the NS-1 molecules to the 5.1-kilobase nuclease-resistant DNA core of the virion.