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Herpes simplex virus phosphoproteins. I. Phosphate cycles on and off some viral polypeptides and can alter their
Abstract:
We report on phosphorylation, the stability of the bound phosphate, and the properties of several phosphorylated infected-cell polypeptides (ICPs) synthesized in cells infected with herpes simplex virus 1 and 2. Our results and conclusions are as follows. (i) Phosphorylation of ICPs occurs by at least two different pathways. Thus, the 4a and 4c electrophoretic forms of ICP 4 were labeled with 32P during a pulse concurrently with their synthesis, whereas ICP 22 and ICP 27 were labeled with 32P only during a subsequent chase in the presence of unlabeled phosphate. (ii) Pulse chase studies with [35S]methionine and 32P indicate that whereas most polypeptides are stable, the bound phosphate with few exceptions cycles on and off. Of special interest is the observation that the phosphate bound to ICP 4a and 4c cycles on and off, whereas that bound to ICP 4b is stably associated. Similar cycling was observed for ICP 6, 11, 22, and 27. The observation that 4a and 4c can be phosphorylated as late as 24 h after infection, i.e., long after their synthesis ceases, suggests that all three forms may have defined functions that persist throughout the reproductive cycle. (iii) All three forms of ICP 4 can be the translational products of only one of two copies of the ICP 4 gene in the viral genome. (iv) Analyses of the distribution of the viral proteins within the cell indicate that phosphorylation is not a major determinant in the compartmentalization of most viral phosphoproteins. (v) Comparisons of the binding to DNA-cellulose of artificial mixtures of 32P- and [35S]methionine-labeled proteins from infected cells indicate that phosphorylation in some instances enhances (e.g., ICP 29) and in other instances decreases (e.g., ICP 6) binding affinity for DNA. In light of previous reports that some of the proteins identified as phosphoproteins have regulatory functions, the data suggest that phosphorylation may modify the activity of regulatory proteins in herpes simplex virus-infected cells.