Self-assembles to form an icosahedral capsid with a pseudo T=2 symmetry, about 60 nm in diameter, and consisting of 120 VP3 subunits. Inner capsid (core) component. Displays NTPase, RNA 5'-triphosphatase (RTPase) and RNA helicase activities and probably participates in transcription of the viral genome. Helicase activity might be involved in unwinding or reannealing dsRNA during RNA synthesis. RTPase enzymatic activity represents the first step in RNA capping, which yields a 5'-diphosphorylated plus-strand RNA (By similarity). ATP + H2O = ADP + H(+) + phosphate Interacts with VP6. Belongs to the reoviridae major inner capsid protein family.