Major conformational change in the complex SF3b upon integration into the spliceosomal U11/U12 di-snRNP as revealed by electron cryomicroscopy.

In some eukaryotes, a minor class of introns is removed by the U12-dependent spliceosome, which contains the small nuclear ribonucleoprotein (snRNP) heterodimer U11/U12. The U11/U12 di-snRNP forms a molecular bridge that functionally pairs the intron ends of the pre-mRNA. We have determined the three-dimensional (3D) structure of the human U11/U12 di-snRNP by single particle electron cryomicroscopy using angular reconstitution and random conical tilt. SF3b, a heteromeric protein complex functionally important for branch site recognition, was located in the U11/U12 di-snRNP by antibody labeling and by identification of structural domains of SF3b155, SF3b49, and p14. The conformation of SF3b bound to the U11/U12 di-snRNP differs from that of isolated SF3b: upon integration into the di-snRNP, SF3b rearranges into a more open form. The manner in which SF3b is integrated in the U11/U12 di-snRNP has important implications for branch site recognition. Furthermore, a putative model of the pre-mRNA binding to the U11/U12 di-snRNP is proposed.