CRYSTAL-STRUCTURE OF THE MS2 COAT PROTEIN DIMER - IMPLICATIONS FOR RNA-BINDING AND VIRUS ASSEMBLY

Background: The coat protein in RNA bacteriophages binds and encapsida tes viral RNA, and also acts as translational repressor of viral repli case by binding to an RNA hairpin in the RNA genome. Because of its du al function, the MS2 coat protein is an interesting candidate for stru ctural studies of protein-RNA interactions and protein-protein interac tions, In this study, unassembled MS2 coat protein dimers were selecte d to analyze repressor activity and virus assembly. Results: The cryst al structure of a mutant MS2 coat protein that is defective in viral a ssembly yet retains repressor activity has been determined at 2.0 Angs trom resolution. The unassembled dimer is stabilized by interdigitatio n of alpha-helices, and the formation of a 10-stranded antiparallel be ta-sheet across the interface between monomers. The substitution of ar ginine for tryptophan at residue 82 results in the formation of two ne w inter-subunit hydrogen bonds that further stabilize the dimer. Resid ues that recognition, identified by molecular genetics, were located a cross the beta-sheet. Two of these residues (Tyr85 and Asn87) are disp laced in the unliganded dimer and are located in the same beta-strand as the Trp-->Arg mutation. Conclusions: When compared with the structu re of the coat protein in the assembled virus, differences in orientat ion of residues 85 and 87 suggest conformational adjustment on binding RNA in the first step of viral assembly. The substitution at residue 82 may affect virus assembly by imposing conformational restriction on the loop that makes critical inter-subunit contacts in the capsid.