Use of a cell-free system to identify the vaccinia virus L1R gene product as the major late myristylated virion protein M25

Abstract

A 25-kDa vaccinia virus (VV) virion protein, designated M25, is modified in vivo by covalent addition of myristic acid. The predicted amino acid sequences of all VV open reading frames which have been reported were searched for the sequence M-G-X-X-X-(S/T/A), which has been proposed to be the consensus recognition signal for cotranslational modification of proteins by N-myristyltransferase. This conserved signal was found at the amino terminus of a single locus, which corresponded to the leftmost rightward-reading open reading frame (L1R) initiating within the VV HindIII L DNA fragment. By using synthetic oligonucleotides in concert with polymerase chain reaction techniques, a chimeric gene consisting of open reading fram L1R fused to a bacteriophage T7 promoter was constructed and cloned into a plasmid vector. Transcripts derived from the wild-type expression plasmid (designated pL1G1) were translated in vitro in a wheat germ extract to yield a polypeptide with an apparent molecular mass of 25 kDa. This polypeptide was labeled with either [35S]methionine or [3H]myristic acid and comigrated with in vivo-labeled protein M25 on sodium dodecyl sulfate-polyacrylamide gels. Polyclonal antiserum generated in rabbits against a trpE:L1R fusion protein immunoprecipitated a 25-kDa protein labeled either in vitro (the L1R gene product, designated protein L1) or in vivo (from purified VV, protein M25), identifying the M25 protein as the gene product of open reading frame L1R. Chromatographic analysis of the protein L1-bound fatty acid moieties liberated after acid methanolysis resulted in recovery of greater than 99% of the fatty acid as myristate-associated label. Cell-free translation of proteins derived from a set of deletions from the carboxy terminus of the open reading frame L1R suggested that the site of myristylation maps near the amino terminus of protein L1. This hypothesis was supported by cell-free translation of mutant L1R transcripts in which the penultimate glycine codon had been altered by site-directed mutagenesis to encode either an aspartic acid (pL1D1) or alanine (pL1A1) residue. In both cases, the mutant transcripts were translated into a 25-kDa protein which could be labeled in vitro with [35S]methionine but not with [3H]myristic acid. These data demonstrate that VV open reading frame L1R encodes a myristylated protein and provide evidence that the site of modification of protein L1 is the amino-terminal glycine residue.