Association of U2, U4, U5, and U6 small nuclear ribonucleoproteins in a spliceosome-type complex in absence of precursor RNA.

Small nuclear ribonucleoprotein particles (snRNPs) associate to form multi-snRNP complexes during splicing of mRNA precursors. A vast majority of the three snRNPs U4, U5, and U6 are present in a nuclear extract in a single complex, while U1 and U2 snRNPs exist as separate particles. Under conditions optimal for splicing in vitro the U4-U5-U6 (U4/5/6) complex dissociates to release free snRNPs, suggesting that the interactions between its components are dynamic. Several forms of splicing complexes assemble on precursor RNA during splicing in vitro. One of these forms, spliceosome B, contains U2, U4, U5, and U6 snRNPs bound to the precursor RNA. This same set of snRNPs associates efficiently in the absence of precursor RNA during incubation of the extract at high salt concentration. Formation of this U2-U4-U5-U6 (U2/4/5/6) complex, the pseudospliceosome, suggests that the basic structure of the spliceosome is specified by snRNP-snRNP interactions.