Composition of alphavirus-like replication complexes: involvement of virus and host encoded proteins.

The RNA-dependent RNA polymerases (RdRp's) of plus-strand RNA viruses contain one or more viral proteins. These proteins contain polymerase (POL) motifs corresponding to the active sites of the polymerase proteins. Additionally, domains corresponding to RNA helicase (HEL), methyltransferase (MT) or proteinase activity of the RdRp may be present. Comparison of available sequence data showed that each class of domain can be subdivided into two or three subclasses, and resulted in the classification of plus-strand RNA viruses into three supergroups. Here, we review our current knowledge on the composition of the RdRp's of alpha-like viruses from supergroup III. The strategy for the expression of viral replicase proteins, their stoichiometry in the enzyme complex, their mutual interactions and their possible functions in RNA synthesis are discussed. Moreover, the review covers host proteins that have been identified in viral RdRp's and their possible role in RNA synthesis.