Role of Ca⁺⁺ in virus-induced membrane fusion. Ca⁺⁺ accumulation and ultrastructural changes induced by Sendai virus in chicken erythrocytes

Some of the ultrastructural (freeze-etching technique), morphological, and biochemical effects of Sendai virus interaction with chicken erythrocytes have been studied under fusogenic (in the presence of CaCl₂) and nonfusogenic (in the presence of ethyleneglycol-bis-N,N'-tetraacetic acid [EGTA]) conditions. The following phenomena occur, irrespective of the presence of CaCl₂ or EGTA: binding of iodinated virus particles to chicken erythrocytes at 4°C and their partial release from the cells at 37°C; gradual incorporation of the viral envelope and viral M-protein into plasma membrane, as visualized in the protoplasmic and exoplasmic fracture (P and E, respectively) faces of the membrane; and virus-dependent transient clustering of intramembrane particles at 4°C, which is reversible after transferring the cells back to 37°C. The following virus-induced phenomena occur only in the presence of CaCl₂: rounding of cells followed by their fusion; transient decrease in the density of intramembrane particles; and the virus-induced uptake of ⁴⁵CaCl₂ by chicken erythrocytes. The uptake is specific as it is inhibited by LaCl₃, and no accumulation of [¹⁴C]glucose-1-phosphate ([¹⁴C]G-1-P) could be observed under the ⁴⁵CaCl₂ uptake conditions. The data show that fusion of virus with plasma membrane is a Ca⁺⁺-independent process and, as such, it should be distinguished from the virus-induced membrane-membrane and cell fusion processes. The latter is absolutely dependent on the rise of intracellular Ca⁺⁺, as reflected by the fact that Ca⁺⁺-induced rounding of chicken erythrocytes always precedes fusion.