THE A34R GLYCOPROTEIN GENE IS REQUIRED FOR INDUCTION OF SPECIALIZED ACTIN-CONTAINING MICROVILLI AND EFFICIENT CELL-TO-CELL TRANSMISSION OF VACCINIA VIRUS

The mechanisms allowing vaccinia virus to spread from cell to cell are incompletely understood, The A34R gene of vaccinia virus encodes a gl ycoprotein that is localized in the outer membranes of extracellular v irions, The small-plaque phenotype of an A34R deletion mutant was simi lar to that of mutants,vith deletions in other envelope genes that fai l to produce extracellular vaccinia virions. Transmission electron mic roscopy, however, revealed that the A34R mutant produced numerous extr acellular particles that were labeled with antibodies to other outer-e nvelope proteins and with protein A-colloidal gold, Fluorescence and s canning electron microscopy indicated that expression of the A34R prot ein was necessary for detection of vaccinia virus-induced actin tails, which provide motility to the intracellular enveloped form of vaccini a virus, and of virus-tipped specialized microvilli that project from the cell, The ability of vaccinia virus-infected cells to form syncyti a after a brief exposure to a pH below 6, known as fusion from within, failed to occur in the absence of expression of the A34R protein; nev ertheless, purified A34R(-) virions were capable of mediating low-pH-i nduced fusion from without. The present study provides genetic and mic roscopic evidence for the involvement of a specific viral protein in t he formation or stability of actin-containing microvilli and for a rol e of these structures in cell-to-cell spread rather than in formation of extracellular virions.