Fig. 4

Features of the 2'-specific and 3'-retaining OAS proteins. a Conformation of AMP donor and AMP acceptor shared by 2'-specific OAS. In this conformation, the 2'-OH of the AMP acceptor is in close proximity to the alpha-P of the AMP donor. b Structure alignment of pig OAS1 (pOAS1, representing 2'-specific OAS) and demosponge OAS (dOAS, representing 3'-retaining OAS). At the right-hand side of the AMP acceptor, pig OAS1 is flat and commodious while demosponge OAS harbors a bulge. c Comparative analysis of AMP acceptor pockets. In 2'-specific OAS protein (left), the adenosine base of the AMP acceptor is coordinated both by hydrogen bonds with residues T190 and hydrophobic interactions with T187 (sequence refer to pig OAS1). These two residues help to fix the bottom of the AMP acceptor and stabilize the conformation where the 2'-OH of the AMP acceptor attacks the alpha-P of the AMP donor. However, in 3'-retaining OAS proteins (right), residues being homologous to pig OAS1 T187 and T190 have been mutated. Without interaction of those two residues, the AMP acceptor might move within the pocket, giving both 2'-OH and 3'-OH of AMP acceptor the chance to attack the alpha-P of the AMP donor