| The vLIP protein, a secreted molecule encoded by the Marek's disease herpesvirus (MDV), contains a region homologus to the alpha/beta hydrolase domain of pancreatic lipases. Homologs of vLIP are present among the other 2 herpesviruses comprising the Marek's disease genus, as well as among several avian adenoviruses. We have shown that vLIP is among the gamma class of herpesvirus transcripts, and thus is likely expressed during the assembly of progeny virions following DNA replication. The vLIP protein and its homologs among other viruses each strictly conserve the serine nucleophile of the catalytic triad, which is responsible for the initiation of substrate hydrolysis in cellular enzymes. These viral lipase homologs also conserve residues involved in forming the oxyanion hole, which stabilizes tetrahedral reaction intermediates. However, the acid and histidine (His) positions of the catalytic triad, though present in some of the viral homologs, are not strictly conserved. The acid position is substituted for Asn in the cases of MDV-1 and HVT, while MDV-2 and the adenoviruses appear to encode appropriate residues for catalytic function (Asp or Glu). The His residue, when present at all, is located in a region of poor homology to cellular lipases. Moreover, recombinant vLIP protein lacked activity on both phospholipid and triacylglyceride substrates in lipase assays. However, during expression in the baculovirus system, vLIP was observed to associate with baculovirus particles. Interestingly, when the serine nucleophile was mutated to alanine, the association of vLIP with baculovirus particles was diminished by almost 12-fold. Since baculovirus particles contain phospholipid bilayer envelopes, the serine dependent association with such particles may indicate that vLIP, though inactive as an enzyme, still forms covalent contacts with lipids. It is known that a stable acyl enzyme intermediate is formed during enzymatic activity in alpha/beta hydrolases. In the case of alpha/beta fold lipases, a fatty acid chain from the substrate becomes covalently bound to the serine nucleophile during each cycle of enzyme activity. Although more evidence is needed, we postulate that vLIP may employ the alpha/beta hydrolase fold in a novel mechanism to covalently bond fatty acids. |
