Mechanism of the mRNA guanylyltransferase reaction: isolation of N epsilon‐phospholysine and GMP (5′ leads to N epsilon) lysine from the guanylyl‐enzyme intermediate.

The mRNA capping reaction catalyzed by rat liver mRNA guanylyltransferase proceeds through an enzyme‐GMP intermediate in which GMP is linked to the enzyme by a phosphoamide linkage. The studies described here show that GMP is bound to the epsilon‐amino group of lysine of rat liver guanylyltransferase. The enzyme‐[32P]GMP intermediate was digested with pronase to a [32P]GMP‐peptide which was then converted to [32P]phosphoryl‐peptide through periodate oxidation followed by beta‐elimination. After alkaline hydrolysis of the [32P]phosphoryl‐peptide, the major radioactive product co‐electrophoresed with the authentic N epsilon‐phospholysine on DEAE‐cellulose paper. Neither [32P]Nimid‐phosphohistidine nor Nguanido‐phosphoarginine was detected in the hydrolysates. Furthermore, formation of N epsilon‐guanylyl‐lysine linkage on the enzyme was more directly shown by isolation of [32P]GMP(5′ leads to N epsilon)lysine when the steps of periodate oxidation and beta‐elimination were omitted. The results indicate that the nucleophile in the guanylyltransferase to which the guanylyl residue is linked is the epsilon‐amino group of a lysine residue. [32P]Phosphoryl‐lysine was also isolated from the vaccinia virus capping enzyme‐[32P]GMP intermediate. Guanylyltransferase from HeLa cells, wheat germ, Artemia salina and yeast also formed the enzyme‐GMP complex and, from the stability of the complex, the linkage between the enzyme and GMP was suggested to be a phosphoamide.