X-RAY CRYSTALLOGRAPHY REVEALS A LARGE CONFORMATIONAL CHANGE DURING GUANYL TRANSFER BY MESSENGER-RNA CAPPING ENZYMES

We have solved the crystal structure of an mRNA capping enzyme at 2.5 Angstrom resolution. The enzyme comprises two domains with a deep, but narrow, cleft between them. The two molecules in the crystallographic asymmetric unit adopt very different conformations; both contain a bo und GTP, but one protein molecule is in an open conformation while the other is in a closed conformation. Only in the closed conformation is the enzyme able to bind manganese ions and undergo catalysis within t he crystals to yield the covalent guanylated enzyme intermediate. Thes e structures provide direct evidence for a mechanism that involves a s ignificant conformational change in the enzyme during catalysis.