Ultrastructure of lymphocystis disease virus (LDV) as compared to frog virus 3 (FV3) and chilo iridescent virus (CIV): effects of enzymatic digestions and detergent degradations

Summary Ultrastructure of fish lymphocystis disease virus (LDV), the largest of all known icosahedral viruses, has been studied under electron microscopy using enzymatic digestions and detergent degradations. LDV structure appeared roughly the same as those of frog virus 3 (FV₃) and chilo iridescent virus (CIV), two other well known viruses of the familyIridoviridae, although the great flexibility of its capsid as observed on negatively stained and shadow cast particles, and its three electron dense layers visualized in ultrathin sections, differed from observations made with the two other viruses. Specific degradation of the virions with enzymes or detergents revealed that the composition of the three iridoviruses was very much alike. In fact, their capsid was composed of two layers as observed in negative staining: an external one, which was removed following digestion with proteinase K, and an internal one which could be digested with phospholipase A₂. Thus, the outermost layer is probably made of surface protein units, more or less tightly bound to each other, while the internal one would be a lipoprotein membrane. Consequently, these three iridoviruses appeared structurally related.