SUMMARY PARAGRAPH for PRP19
About the NineTeen Complex
The nineteen complex (for prp NineTeen associated Complex and also referred to as the NTC) is a non-snRNA containing protein complex involved in splicing of nuclear RNAs via the spliceosome. It was originally isolated as a complex containing Prp19p and at least eight other proteins that complemented the splicing defect of extracts from prp19 mutant cells (7). Subsequent work has identified the genes encoding the additional members of the complex: NTC20 (8), SNT309 (9), ISY1 (8), SYF2 (10, 11), CWC2 (12), PRP46 (12), CLF1 (11, 12), CEF1 (13), and SYF1 (11, 12). The complex appears to be conserved as mammalian cells contain a functional equivalent called the Prp19/CDC5 complex composed of a similar, though not identical, set of proteins (14). The nineteen complex associates with the assembling splicesosome during or after the dissociation of the U4 snRNA, stabilizes the U5 and U6 snRNAs in the activated spliceosomal complex that is catalytic for the first step of splicing, and remains through the second step of splicing (15, 16). Following disassembly of the spliceosome, members of the nineteen complex have been found in association with the excised intron (17, 18).
The nineteen complex also appears to be involved in control of fidelity and efficiency of splicing. Mutations in isy1 suppress the relaxed fidelity of recognition of the conserved branchpoint sequence conferred by mutations in prp16, an ATP-dependent RNA helicase required for the second step of splicing, and an isy1 null mutation decreases accuracy of 3'-splice site usage (19). In addition, cells with mutations in prp45, a protein found in association with NineTeen complex members, are defective in splicing of introns with non-canonical sequences at the branchpoint or the 5' or 3' splice sites (20). Splicing efficiency of various transcripts is differentially affected by mutations in spliceosomal components, such as PRP19, suggesting that the spliceosome can distinguish between individual transcripts and possibly use these differences to specifically regulate gene expression via control of splicing (21). Interestingly, the Drosophila crooked neck protein (the homolog of CLF1) regulates glial cell differentiation by facilitating the splicing of specific target genes (22).
Mutational and genetic analysis of several nineteen complex subunits has suggested involvement in other cellular processes in addition to splicing, such as cell cycle regulation, cytoskeletal structure, DNA repair, and vesicular transport (23, 18, 24, 10, 25, 25 26). In most cases it appears that the primary defect is in splicing and the other defects are the result of failure to remove an intron from the transcript of a gene involved in that process. However Clf1p, and possibly also Prp19p, may have other direct roles in addition to splicing.
Prp19p is an essential protein and a core component of the nineteen complex that is essential for spliceosomal splicing of nuclear mRNAs (1, 2). It tetramerizes via a central coiled-coil domain and thus forms a central structural element of the nineteen complex to which several other proteins, such as Cef1p, Snt309p, and Cwc2p, bind (5). The four C-terminal WD40 segments form globular domains that may form a typical beta-propeller structure held together by a central stalk formed by the coiled-coil domains (5). The amino terminus contains a U-box domain that has E3 ubiquitin ligase activity in vitro (4). Mutations which disrupt tetramer formation or the E3 ubiquitin ligase activity of Prp19p are both lethal (5, 4).
Originally identified in a screen for mutations conferring sensitivity to DNA damaging agents, it has long been proposed that Prp19p plays a direct role in repair of DNA damage, both in yeast and in other species (27, 28, and references therein). However, in S. cerevisiae, decreased efficiency of splicing of intron-containing genes involved in recombination (MEI4, MER2/REC107, REC114, and DMC1), repair (e.g. MMS2, RFA2, RAD14, and KIN28), cell cycle or chromosome segregation (e.g. UBC9, GLC7, HOP2, CIN2, and MOB1) appears to be a sufficient explanation for the cell cycle and DNA repair defects of prp19 mutant cells, though a role for Prp19p as a nuclear scaffold protein remains possible (29).
PRP19 is conserved across many species, with homologs found in H. sapiens, M. musculus, C. elegans, D. melanogaster, A. thaliana, S. pombe, and even P. falciparum (30, 31). As a member of the Prp19/CDC5 complex, the role of Prp19p in splicing is conserved in mammalian cells (14). The human protein (called hPso4) localizes to the nuclear matrix (30), interacts with a regulatory subunit of the 26S proteasome (32; 31), and may have roles in replicative lifespan (33), DNA repair, and apoptosis (34).
Last updated: 2009-12-11