Figure 6.

Matchmaker model of the gBP21-mediated RNA/RNA annealing reaction. Schematic representation of the basic steps of the gBP21-mediated annealing reaction. First, a kinetically and thermodynamically stable 1 + 1 gBP21/gRNA complex is formed. One binding determinant is the binding site for the 3′ hairpin of gRNAs (23,35), which has been shown to be sufficient for the high affinity interaction (14). Secondly, the 5′ end of the gRNA is unfolded in order to present the anchor region for the annealing reaction. The identified six ionic bonds might participate in this step in order to reduce the negative surface charge of the gRNA. This should increase the collision frequency with the pre-mRNA. gBP21 stays bound to the structurally altered gRNA, which is characteristic of a matchmaker-type protein. Thirdly, the anchor sequences of the two RNAs hybridize, forming a partially dsRNA product, which probably triggers the dissociation of gBP21 from the complex.