The 464-amino
acid baculovirus LEF4
protein is a bifunctional
mRNA capping enzyme with triphosphatase and
guanylyltransferase activities. The N-terminal half of LEF4 constitutes an autonomous triphosphatase catalytic domain. The LEF4 triphosphatase belongs to a family of
metal-dependent
phosphohydrolases, which includes the
RNA triphosphatases of fungi, protozoa, Chlorella virus and poxviruses. The family is defined by two
glutamate-containing motifs (A and C), which form a
metal-binding site. Most of the family members resemble the fungal and Chlorella virus
enzymes, which have a complex active site located within the hydrophilic interior of a topologically closed eight stranded beta barrel (the so-called '
triphosphate tunnel'). Here we probed whether baculovirus LEF4 is a member of the tunnel subfamily, via mutational mapping of
amino acids required for triphosphatase activity. We identified four new essential side chains in LEF4 via
alanine scanning and illuminated structure-activity relationships by conservative substitutions. Our results, together with previous mutational data, highlight five acidic and four
basic amino acids that are likely to comprise the LEF4 triphosphatase active site (Glu9, Glu11, Arg51, Arg53, Glu97, Lys126, Arg179, Glu181 and Glu183). These nine essential residues are conserved in LEF4 orthologs from all strains of baculoviruses. We discerned no pattern of clustering of the catalytic residues of the baculovirus triphosphatase that would suggest structural similarity to the tunnel
proteins (exclusive of motifs A and C). However, there is similarity to the active site of
vaccinia RNA triphosphatase. We infer that the baculovirus and poxvirus triphosphatases are a distinct lineage within the
metal-dependent
RNA triphosphatase family. Synergistic activation of the LEF4 triphosphatase by
manganese and
magnesium suggests a two-
metal mechanism of gamma
phosphate hydrolysis.