Formation of a CovalentNϵ2-Guanylylhistidyl Reaction Intermediate by the GTP:GTP Guanylyltransferase from the Brine ShrimpArtemia

The chemical nature of the enzyme–nucleotide phosphoramidate reaction intermediate employed by the unique GTP:GTP guanylyltransferase from yolk platelets ofArtemia franciscanacysts to synthesize diguanosine tetraphosphate (Gp4G) has been investigated. Labeling of the enzyme with [α-32P]GTP followed by isolation of the labeled phosphoamino acid by periodate treatment and alkaline hydrolysis and comparison of the product with phosphoamino acid standards by thin-layer and ion-exchange chromatography showed that the linkage involves the Nϵ2 ring nitrogen of an enzyme histidyl residue. Thus, this enzyme is distinct from the mRNA capping enzymes which can also synthesize Gp4G but which employ a lysyl–nucleotide intermediate. Based on its reaction mechanism and substrate specificity, GTP:GTP guanylyltransferase may belong to the GAFH superfamily which includes the histidine triad proteins, Ap4A phosphorylases, and galactose-1-phosphate uridylyltransferase.