The phosphoprotein (P) and L binding sites reside in the N-terminus of the L subunit of the measles virus RNA polymerase.

Abstract Measles virus encodes an RNA-dependent RNA polymerase composed of the L and P proteins. Recent studies have shown that the L proteins of both Sendai virus and parainfluenza virus 3 form an L–L complex [Cevik, B., Smallwood, S., Moyer, S.A., 2003. The oligomerization domain resides at the very Nterminus of the Sendai virus L RNA polymerase protein. Virology 313, 525–536.; Smallwood, S., Moyer, S.A., 2004. The L polymerase protein of parainfluenza virus 3 forms anoligomer and can interact with the heterologous Sendai virus L, P and C proteins. Virology 318, 439–450.; Smallwood, S., Cevik, B., Moyer, S.A., 2002. Intragenic complementation and oligomerization of the L subunit of the Sendai virus RNA polymerase. Virology 304, 235–245.]. Using differentially tagged L proteins, we show here that measles L also forms an oligomer and the L–L binding site resides in the N-terminal 408 amino acids overlapping the P binding site in the same region of L. To identify amino acids important for binding P and L, site-directed mutagenesis of the L-408 protein was performed. Seven of twelve mutants in L-408 were unable to form a complex with measles P while the remainder did bind at least some P. In contrast, all of the mutants retained the ability to form the L–L complex, so different amino acids are involved in the L and P binding sites on L. Four of the 408 mutations defective in P binding were inserted into the full-length measles L protein and all retained L–L complex formation, but did not bind P. Full-length L mutants that did not bind P were also inactive in viral RNA synthesis, showing a direct correlation between P–L complex formation and activity.