LAUSR

Creation of the splicing catalytic center The catalytic center of the fully assembled, precatalytic pre–B spliceosome is not yet formed, even though all the pieces are present in this complex. Charenton et al. solved the structures of the human pre–B complex, providing detailed mechanistic insights into how the helicase Prp28 transfers the 5′ splice site of the messenger RNA precursor from U1 small nuclear ribonucleoprotein (snRNP) to U6 snRNA. The pairing of the 5′ splice site with U6 induces a series of conformational changes that triggers the relocation of Brr2 helicase and loading of U4 snRNA to the helicase active site. This leads to unwinding of the U4/U6 snRNA duplex and allows U6 to fold and pair with U2 to create the spliceosome active site. Science, this issue p. 362 The structure of a fully assembled precatalytic human spliceosome explains catalytic center formation. The prespliceosome, comprising U1 and U2 small nuclear ribonucleoproteins (snRNPs) bound to the precursor messenger RNA 5ʹ splice site (5ʹSS) and branch point sequence, associates with the U4/U6.U5 tri-snRNP to form the fully assembled precatalytic pre–B spliceosome. Here, we report cryo–electron microscopy structures of the human pre–B complex captured before U1 snRNP dissociation at 3.3-angstrom core resolution and the human tri-snRNP at 2.9-angstrom resolution. U1 snRNP inserts the 5ʹSS–U1 snRNA helix between the two RecA domains of the Prp28 DEAD-box helicase. Adenosine 5ʹ-triphosphate–dependent closure of the Prp28 RecA domains releases the 5ʹSS to pair with the nearby U6 ACAGAGA-box sequence presented as a mobile loop. The structures suggest that formation of the 5ʹSS-ACAGAGA helix triggers remodeling of an intricate protein-RNA network to induce Brr2 helicase relocation to its loading sequence in U4 snRNA, enabling Brr2 to unwind the U4/U6 snRNA duplex to allow U6 snRNA to form the catalytic center of the spliceosome.