Studies on the virus-cell interactions have proven valuable in elucidating vital cellular processes. Interestingly, certain virus-host membrane interactions found in eukaryotic systems seem also to operate in prokaryotes (Bamford, D.H., M. Romantschuk, and P. J. Somerharju, 1987. EMBO (Eur. Mol. Biol. Organ.) J. 6:1467-1473; Romantschuk, M., V.M. Olkkonen, and D.H. Bamford. 1988. EMBO (Eur. Mol. Biol. Organ.) J. 7:1821-1829). straight phi6 is an enveloped double-stranded RNA virus infecting a gram-negative bacterium. The viral entry is initiated by fusion between the virus membrane and host outer membrane, followed by delivery of the viral nucleocapsid (RNA polymerase complex covered with a protein shell) into the host cytosol via an endocytic-like route. In this study, we analyze the interaction of the nucleocapsid with the host plasma membrane and demonstrate a novel approach for dissecting the early events of the nucleocapsid entry process. The initial binding of the nucleocapsid to the plasma membrane is independent of membrane voltage (DeltaPsi) and the K(+) and H(+) gradients. However, the following internalization is dependent on plasma membrane voltage (DeltaPsi), but does not require a high ATP level or K(+) and H(+) gradients. Moreover, the nucleocapsid shell protein, P8, is the viral component mediating the membrane-nucleocapsid interaction.