The U(L)3 Protein of Herpes Simplex Virus 1 Is Translated Predominantly from the Second In-Frame Methionine Codon and Is Subject to at Least Two Posttranslational Modifications

The U(L)3 open reading frame (ORF) has the coding capacity of 235 codons. The proteins reacting with the anti-U(L)3 antibody form in denaturing polyacrylamide gel bands with apparent M(r)s of 34,000, 35,000, 38,000, 40,000, 41,000, and 42,000 and designated 1 to 6, respectively. Studies on their origins revealed the following. (i) The U(L)3 proteins forming all six bands were present in lysates of cells infected with wild-type virus and treated with tunicamycin or monensin or in cells infected with the mutant lacking the gene encoding the U(S)3 protein kinase. (ii) The proteins contained in the slower-migrating bands were absent from cells infected with the mutant lacking the U(L)13 protein kinase. Bands 1 and 3, however were phosphorylated in cells infected with this mutant. (iii) Band 2 protein was absent from cells transfected with a plasmid carrying a substitution of the predicted first methionine codon of the U(L)3 ORF and superinfected with the U(L)3(−) mutant. (iv) Band 1 and 3 proteins were absent from lysates of cells transfected with a plasmid carrying a substitution of the second (M12) methionine codon of the U(L)3 ORF and superinfected with the U(L)3(−) mutant. (v) Cells superinfected with mutants lacking both methionine codons did not accumulate any of the proteins contained in the six bands. (vi) In vitro transcription-translation studies indicated that the translation of band 1 protein was initiated from the second (M12) methionine codon and that band 3 protein represented a U(L)13-independent, posttranslationally processed form of these proteins. The results indicate that the U(L)3 protein of herpes simplex virus 1 is translated predominantly from the second in-frame methionine codon and is subject to at least two posttranslational modifications.