Intramembrane changes occurring during maturation of herpes simplex virus type 1: freeze-fracture study.

During the maturation of two strains of herpes simplex virus type 1 (VR3 and Patton), intramembrane changes were detected with the freeze-fracture technique in the viral envelope and the infected cell plasma membrane, and these changes were compared with data obtained from thin sections. Regardless of the strain, the inner leaflet of the viral envelope of extracellular virions was characterized by a density of intramembrane particles (IMP) three times larger than the host nuclear and plasma membrane. Addition of IMP, which probably represent virus-coded proteins, was detected in the viral envelope only after budding from the nuclear membrane, whereas it occurred during envelopment of capsids at cytoplasmic vacuoles. Fused membranes also showed one of their fracture faces covered with a high density of IMP similar to that of the mature virion envelope. The internal side of the membrane leaflet bearing these numerous particles was always characterized by the presence of an electron-dense material in thin sections. In addition, the plasma membrane of fibroblasts and Vero cells showed strain-specific changes: patches of closely packed IMP were observed with the VR3 strain, whereas ridges almost devoid of IMP characterized the plasmalemma of cells infected with the Patton strain. These intramembrane changes, however, were not observed as early as herpes membrane antigens. Thus, application of the freeze-fracture technique to herpes simplex virus type 1-infected cells revealed striking structural differences between viral and uninfected cell membranes. These differences are probably related to insertion and clustering of virus-coded proteins in the hydrophobic part of the membrane bilayer.