Electron microscopy of the nucleocapsid from disrupted Moloney murine leukemia virus and of associated type VI collagen-like filaments

Abstract

To analyze the constituents of retroviruses, the Moloney murine leukemia virus was disrupted and observed by dark-field electron microscopy. Virus disruption was achieved by several methods: osmotic shock, freezing-thawing cycles, and exposure to urea up to 4 M, to NaCl up to 1 M, and to Triton X-100. Several components associated with broken Moloney murine leukemia virus were repeatedly found in preparations. These components have been described as rings, thick filaments, chain-like filaments, threads covered with proteins, threads with buckles, and naked threads. A quantitative analysis of the occurrence of these components has been carried out. Among them, the thick filaments composed of a compact helical arrangement of small beads 5 nm in diameter were considered to represent the nucleocapsid. The protease-sensitive buckles found on some threads could be a compact form of the viral RNA associated to the nucleocapsid protein NCp10. The RNase-sensitive naked threads are interpreted as the deproteinized viral RNA itself. The ubiquitous chain-like filaments possess a periodic structure identical to that of polymerized type VI collagen. It is proposed that this adhesive protein is associated with the viral envelope taken from the cell membrane during the budding process of retroviruses.