Two GTP binding sites exist in this protein [1].

1: Identification of two nucleotide-binding domains on the PB1 subunit of influenza virus RNA polymerase. Asano Y, Ishihama A; J Biochem (Tokyo) 1997;122:627-634. PMID:9348094

2: Identification of two separate domains in the influenza virus PB1 protein involved in the interaction with the PB2 and PA subunits: a model for the viral RNA polymerase structure. Gonzalez S, Zurcher T, Ortin J; Nucleic Acids Res 1996;24:4456-4463. PMID:8948635