Evidence for functional significance of the permuted C motif in CO ²⁺-stimulated RNA-dependent RNA polymerase of infectious bursal disease virus

Segment B of bisegmented infectious bursal disease virus (IBDV) encodes virus protein 1 (VP1), possessing RNA-dependent RNA polymerase (RdRp) activity. This multidomain protein includes an RdRp domain with a non-canonical order of three sequence motifs forming the active site: C-A-B. The A-B-C order of the motifs, as found in RdRps of the majority of viruses, was converted by relocation (permutation) of motif C to a C-A-B order. Due to the unusual location and unproven significance, the motif was named 'C,?'. This motif includes an Ala-Asp-Asn tripeptide that replaces the C motif Gly-Asp-Asp sequence, widely considered a hallmark of RdRps. In this study, functional significance of the C? motif was investigated by using purified His-tagged VP1 mutants with either a double replacement (ADN to GDD) or two single-site mutants (ADD or GDN). All mutants showed a significant reduction of RdRp activity in vitro, in comparison to that of VP1. Only the least-affected GDN mutant gave rise to viable, albeit partially impaired, progeny using a reverse-genetics system. Experiments performed to investigate whether the C motif was implicated in the control of metal dependence revealed that, compared with Mn2+ and Mg2+, Co2+ stimulated RdRp unconventionally. No activity was observed in the presence of several divalent cations. Of two Co2+ salts with Cl- and SO42- anions, the former was a stronger stimulant for RdRp. When cell-culture medium was supplemented with 50 mu M Co2+, an increase in IBDV progeny yield was observed. The obtained results provide evidence that the unusual CO2+ dependence of the IBDV RdRp might be linked to the permuted organization of the motif