Participates in the assembly of the viral procapsid in the cytoplasm. Forms first a 12S pre-assembly complex with protein H, and F and G pentamers, then twelve 12S complexes are joined by the D protein to form the procapsid. Internal scaffold protein B is released from the procapsid upon genome packaging. Autoproteolytic activity cleaves protein B and probably facilitates its removal through the pores of the procapsid. Component of the procapsid complex composed of 60 copies of the internally located B, 240 copies of the external scaffolding protein D, 60 copies of each of the viral structural proteins F and G proteins, and 12 copies of H. The proteolytic cleavage of the internal scaffolding protein B releases the scaffold protein in order to continue virion assembly. Belongs to the microviridae B protein family.