An association between actin and nucleocapsid polypeptides in isolated murine retroviral particles.

Mammalian cells infected with retroviruses frequently display virus particles budding at the tip of cellular projections resembling microvilli and filopodia. In normal and infected cells these cellular projections contain actin microfilaments and in specialized retrovirus-tipped projections from the P815 cell, a direct association between actin filaments and the apical virus particle could be demonstrated (Mortara and Koch, 1986). Here we confirm and extend these observations using a murine macrophage cell line chronically infected with a C-type retrovirus. Immunochemical and biochemical methods were used to identify actin-associated and actin-binding components among the retroviral polypeptides. The results show that Pr65gag and its p15 N-terminal domain can bind to actin in vitro and may be major binding sites for actin filaments on the retroviral nucleocapsid.