Elucidating the Essential Role of the A14 Phosphoprotein in Vaccinia Virus Morphogenesis: Construction and Characterization of a Tetracycline-Inducible Recombinant

Vaccinia virus is a complex DNA virus characterized by a high degree of genetic and physical autonomy from the host cell. Among the functions of the 200 gene products encoded by the 192-kb genome are those required for the expression of three temporal classes of genes, the replication of viral DNA, and the morphogenesis of nascent virions (27). The last is a complex process which remains poorly understood but is the subject of intense study. Electron microscopy, in conjunction with pharmacological intervention and the study of conditionally lethal mutants, has been useful in generating models of virion assembly (7, 9, 12, 20, 24, 25, 28, 31, 41, 44, 47, 49, 53, 55, 58, 59, 62, 63). Within the cytoplasm, morphogenesis is concentrated around areas of increased electron density which have been shown to contain viral DNA and the proteins to be encapsidated. Membrane crescents, which appear to elongate and curve until they enclose the electron-dense material within oval immature virions (IV), are the first hallmark of morphogenesis. The origin of these membranes is still a matter of controversy. De novo biogenesis of these membranes to form an IV delimited by a single lipid bilayer was an early and provocative model (10). Recent measurements of the membrane surrounding the IV are also consistent with the single-bilayer model (22), although this work did not address the origin of the membrane in question. Other investigators have presented data suggesting that the virion membrane derives from the intermediate compartment (49), a transitional zone of tubulovesicular components that reflects anterograde and retrograde transport between the endoplasmic reticulum and the Golgi apparatus. Membrane crescents, derived from tubulovesicular components of the intermediate compartment, would therefore be comprised of two membranes that have become so tightly apposed that the lumenal space is essentially absent. Clarification of which features of these distinct models are correct must be a central goal for researchers in the field. IV formation is followed by a maturation process that involves morphological changes and a series of proteolytic processing events which generate infectious virions known as intracellular mature virons (IMV). A number of viral proteins have been shown to play a role in IV and IMV morphogenesis. Of great interest to our laboratory is the demonstration that nonpermissive infections performed with temperature-sensitive mutants carrying lesions within the F10 protein kinase arrest at the earliest stage of morphogenesis (53, 55). No crescents or immature or mature particles are seen during these infections, and no electron-dense virosomes appear. These data strongly suggest that protein phosphorylation plays an essential role in the membrane remodeling that initiates viral morphogenesis. Recent experiments in our laboratory have provided evidence supporting the characterization of F10 as a dual-specificity kinase that can phosphorylate serine, threonine, and tyrosine residues (13). The importance of protein phosphorylation as a regulator of the viral life cycle is underscored by the fact that vaccinia virus encodes another protein kinase (B1) (2, 34, 42, 52) as well as a dual-specificity protein phosphatase, the product of the H1 gene. Using an inducible viral recombinant in which expression of the H1 phosphatase can be regulated experimentally (vindH1), we were able to demonstrate that H1 synthesis is not required for virion morphogenesis but is necessary to ensure the infectivity and transcriptional competence of nascent virions (35). Making the assumption that it was the phosphatase activity of H1 that was essential, we concentrated on identifying which virion proteins were H1 substrates. To do so, we examined 32P-labeled wild-type (wt) and H1-deficient (H1−) virions in order to determine which proteins were hyperphosphorylated in the absence of H1. Three proteins with apparent molecular weights of 25,000 (25K), 16K, and 11K were identified. We have previously shown that the 11K protein is the DNA-binding protein encoded by the F18 (F17 in the Copenhagen strain) gene and that the 25K protein is the product of the A17 gene (13, 35). The identification of the 16K phosphoprotein as the product of the A14 gene and its subsequent characterization are the subjects of this paper. As part of our analysis of the structure and function of the A14 protein, we also developed an alternative inducible system in which expression of viral genes can be regulated by the inclusion of tetracycline (TET) in the culture medium.