The phosphoprotein (P) binding site resides in the N terminus of the L polymerase subunit of sendai virus.

Sendai virus encodes an RNA-dependent RNA polymerase which is composed of the L and P proteins. Site-directed mutagenesis of the N terminus of L has identified amino acids important for binding P. Seven of nine mutants in amino acids 1 to 350 of Sendai L lost the ability to bind to Sendai P, although they were still able to bind the viral C protein. Loss of P binding correlated with the loss of all RNA synthesis activities. Two L mutants gave limited P-L complex formation and limited viral transcription and replication.