Properties of Mouse Leukemia Viruses X V . Electron Microscopic Studies on the Organization of Friend Leukemia Virus and Other Mammalian C-Type Viruses

Abstract

Further investigations on the structure of Friend murine leukemia virus (FLV) revealed that the transition from the immature (now termed native) to the structurally less organized mature (now termed collapsed) form occurs mainly as a result of the preparation for EM. A short pretreatment of virus with the detergent NP40 prior to negative staining with uranyl acetate is able to preserve the native structure of a high percentage of virus particles from standard preparations. Treatment with conventional fixatives was ineffective. Using this preparation technique, a more detailed EM analysis of the viral internal organization became possible. A thin layer designated inner coat was newly detected in close apposition to the viral membrane between the viral envelope and core. Removal of the unit membrane by more intensive detergent treatment suggests the existence of material extending between the viral surface knobs and the viral interior. The icosahedral core shell has an opening at 1 side and this opening matches with the hole in the apparently beehive-like arranged ribonucleoprotein (RNP) strand which represents the innermost structure. A comparative study of a series of representative mammalian C-type viruses with the same technique indicated a close similarity to Friend virus in fine structure, although differences in stability with observed. Based on these and earlier findings a model of the structure of mammalian C-type viruses is presented.