Overexpression of the vaccinia virus A38L integral membrane protein promotes Ca2+ influx into infected cells.

The vaccinia virus Western Reserve A38L protein is a hydrophobic integral membrane glycoprotein with amino acid similarity to mammalian integrin-associated protein. The protein has an N-terminal immunoglobulin superfamily domain, followed by five membrane-spanning domains and a short cytoplasmic tail. Deletion of the protein reduces virus plaque size but does not affect virus virulence (J. E. Parkinson, C. M. Sanderson, and G. L. Smith, Virology, in press). In this study, we have used a recombinant vaccinia virus in which the A38L gene may be inducibly overexpressed by addition of isopropyl-beta-D-thiogalactopyranoside (IPTG), to demonstrate that overexpression of the vaccinia virus A38L gene produces drastic changes in the morphology, permeability, and adhesion of infected cells. In particular, A38L overexpression caused swelling of cells, marginalization of nuclear chromatin, and vacuolization of the endoplasmic reticulum, features characteristic of cell necrosis. By 18 h postinfection, cells become permeable and lytic as defined by the free entry of propidium iodide and loss of the cytoplasmic enzyme lactate dehydrogenase. Chelation of extracellular Ca2+ 22 h postinfection inhibited further release of lactate dehydrogenase, showing that Ca2+ influx was required for A38L-induced lysis. Direct measurement of 45Ca2+ influx showed that the rate of Ca2+ uptake was directly related to the period of A38L induction. The A38L protein, therefore, promotes the formation of pores within the plasma membrane of cells, and these pores facilitate Ca2+ entry and induce necrosis. Addition of rifampin inhibited virus assembly but not the ability of A38L to induce necrosis, indicating that pore formation is independent of viral morphogenesis. Finally, overexpression of the A38L protein resulted in a reduced plaque size and a threefold decrease in production of infective particles in vitro. The A38L protein represents the first example of a virus protein which directly or indirectly promotes the influx of extracellular Ca2+.