The Core Proteins A2 and B1 Exist as (A2)₃B1 Tetramers in 40S Nuclear Ribonucleoprotein Particles

Abstract

The six "core" proteins of HeLa cell 40S nuclear ribonucleoprotein particles (hnRNP particles) package 700-nucleotide lengths of pre-mRNA into a repeating array of regular particles. We have previously shown that the C proteins exist as anisotropic tetramers of (C1)₃C2 in 40S hnRNP particles and that each particle probably contains three such tetramers. We report here that proteins A2 and Bl also exist in monoparticles as (A2)₃B1 tetramers and that each monoparticle contains at least three such tetramers. Proteins A2 and B1 dissociate from isolated monoparticles as a stable tetramer upon nuclease digestion. In low-salt gradients, the tetramers sediment at 6.8S, which is consistent with a mass of 145 kDa. In 200 mM salt, the concentration which dissociates these proteins from RNA, only 4.2S dimers exist in solution. Tetramers of (A2)₃B1 possess the ability to package multiples of 700 nucleotides of RNA in vitro into an array of regular, 22.5-nm 43S particles. Unlike the in vitro assembly of intact 40S hnRNP, the (A2)₃B1 tetramers assemble by means of a highly cooperative process. These findings indicate that the (A2)₃B1 tetramers play a major role in hnRNP assembly and they further support the contention that 40S monoparticles are regular structures composed of three copies of three different tetramers, i.e., 3[(A1)₃B2, (A2)₃B1, (C1)₃C2].