Major (U2-type) spliceosome in Drosophila melanogaster

The major spliceosome removes U2-type introns from nuclear pre-mRNAs. It is assembled stepwise on each intron from the U1, U2, U4/U6 and U5 small nuclear ribonucleoprotein particles (snRNPs) and many non-snRNP proteins. Each snRNP contains a small nuclear RNA; the U1, U2, U4 and U5 snRNAs carry a heptameric ring of Sm proteins, whereas U6 snRNA binds the related LSm proteins. U1 snRNP recognizes the 5' splice site, and the U2 snRNP auxiliary factor (U2AF) together with SF1 recognizes the 3' splice site region, allowing U2 snRNP to pair with the branch site (complex A). Joining of the U4/U6.U5 tri-snRNP forms the precatalytic complex B; release of U1 and U4 and recruitment of the Prp19 complex and associated proteins create the catalytic spliceosome, which carries out the two transesterification steps (complex C) and then releases the spliced mRNA (post-catalytic complex P) with an exon junction complex deposited upstream of the exon-exon junction.

Decomposition. FlyBase splits the spliceosome two ways: by snRNP (U1, U2, U4-U6, U5, U4-U6-U5) and by assembly stage (SPL-C-A, SPL-C-B, SPL-C-C, SLP-C-P), with the snRNP genes re-listed in each stage group. Here the snRNPs, the Sm rings and U2AF are parts, and each stage part contains only the non-snRNP proteins first listed at that stage (complex A, then B, then C). Complex P adds no protein absent from complex C; it lacks Bin1, CG10466, CG3225, CG8833 and Prp18. Assignment of the NTC core (Prp19, Cdc5, Tango4 [PLRG1], BCAS2) and EJC core (mago, tsu [Y14], CG7483 [eIF4AIII]) follows orthology to the human complexes; other stage factors are kept as stage sets without finer sub-structure. Lsm10 and Lsm11 enter through FlyBase's Sm protein group (U7-SM) and appear only in the U7 Sm ring variant; the U7 snRNP itself (with snRNA:U7) belongs to dmel_histone_mrna_3prime_processing, and the 7SK snRNP, a sibling in FlyBase's snRNP group, to dmel_rnapii_elongation_complexes. The minor (U12-type) spliceosome and splicing regulators (SR proteins, other hnRNPs) are outside this module unless FlyBase lists them in a spliceosome stage. No molecular function is assigned in this version. The catalytic centre of the spliceosome is formed by the U2 and U6 snRNAs, and this generated decomposition models snRNP and assembly-stage membership only; the enzymatic protein subunits (for example the DExD/H-box helicases such as Brr2 and Prp5, the Prp19 ubiquitin ligase and the cyclophilins) are not yet given annoton functions. FlyBase groups realized by this module: SPL-C (FBgg0000519), U2AF (FBgg0000538), SNURPS (FBgg0000631), SM-ALL (FBgg0000632). Subgroups folded into it: SNRNP, U1, U2, U4-U6, U5, U4-U6-U5, SM, SPL-C-A, SPL-C-C, SPL-INT, SPL-C-B, SLP-C-P, U7-SM. Participants were generated from the FlyBase gene-group membership and mapped to UniProtKB by projects/FLYBASE_GENE_GROUPS/generate_modules.py from module_specs/dmel_spliceosome.yaml. Genes without a usable UniProtKB entry are grounded to their FlyBase gene ids: snRNA:U1:21D, snRNA:U1:82Eb, snRNA:U1:95Ca, snRNA:U1:95Cb, snRNA:U1:95Cc, snRNA:U2:14B, snRNA:U2:34ABa, snRNA:U2:34ABb, snRNA:U2:34ABc, snRNA:U2:38ABa, snRNA:U2:38ABb, snRNA:U4:25F, snRNA:U4:38AB, snRNA:U4:39B, snRNA:U5:14B, snRNA:U5:23D, snRNA:U5:34A, snRNA:U5:35D, snRNA:U5:38ABa, snRNA:U5:38ABb, snRNA:U5:63BC, snRNA:U6:96Aa, snRNA:U6:96Ab, snRNA:U6:96Ac. None of the member genes has a gene review in this repository yet.