Dynamic association of proteins with the pre-mRNA branch region

A U1/U4/U5 snRNP complex induced by a 2'-O-methyl-oligoribonucleotide complementary to U5 snRNA

Nuclear messenger RNA splicing involves multiple interactions between the five spliceosomal small nuclear ribonucleoprotein particles (snRNPs) U1, U2, U4, U5, and U6 and numerous spliceosomal proteins. Here it is shown that binding of a 2'-O-methyl-oligoribonucleotide complementary to U5 small nuclear RNA (snRNA) nucleotides 68 to 88 (BU5Ae) disrupts the initial U4/U5/U6 tri-snRNP complex, enhances the U2/U6 interaction, and induces a Ul/U4/U5 snRNP complex. The Ul/U4/U5 snRNP complex interacts specifically with an RNA oligonucleotide containing the 5' splice site sequence and may therefore represent a transitional stage in the displacement of U1 from the 5' splice site by U5 snRNP.

Initial splice-site recognition and pairing during pre-mRNA splicing

Advances over the past year have provided new insights into the mechanisms involved in the initial recognition and pairing of the 5' and 3' splice sites in complex metazoan pre-mRNAs. Highlights include the demonstration that exonic enhancers can promote trans splicing and that an excess of the serine and arginine rich family of splicing proteins can obviate the requirement for U1 small nuclear ribonucleoprotein particle in splicing.

A discrete 3' region of U6 small nuclear RNA modulates the phosphorylation cycle of the C1 heterogeneous nuclear ribonucleoprotein particle protein

The C heterogeneous ribonucleoprotein particle (hnRNP) protein bind to nascent pre-mRNA and may participate in assembly of the early prespliceosome. Ser/Thr phosphorylation of the C1 hnRNP protein in HeLa nuclear extracts regulates its binding to pre-mRNA (S. H. Mayrand, P. Dwen, and T. Pederson, Proc. Natl. Acad. Sci. USA 90:7764-7768, 1993). We have now further investigated the phosphorylation cycle of the C1 hnRNP protein, with emphasis on its regulation. Pretreatment of nuclear extracts with micrococcal nuclease eliminated the phosphorylation of C1 hnRNP protein, but pretreatment with DNase did not, suggesting a dependence on RNA. Oligodeoxynucleotide-targeted RNase H cleavage of U1, U2, and U4 small nuclear RNAs did not affect the phosphorylation of C1 hnRNP protein. However, cleavage of nucleotides 78 to 95, but not other regions, of U6 small nuclear RNA resulted in an inhibition of the dephosphorylation step of the C1 hnRNP protein phosphorylation cycle. This inhibition was as pronounced as that seen with the serine/threonine protein phosphatase inhibitor okadaic acid. C1 hnRNP protein dephosphorylation could be completely restored by the addition of intact U6 RNA. Add-back experiments with mutant RNAs further delineated the minimal region essential for C1 protein dephosphorylation as residing in nucleotides 85 to 92 of U6 RNA. These results illuminate a hitherto unanticipated function of U6 RNA: the modulation of a phosphorylation-dephosphorylation cycle of C1 hnRNP protein that influences the binding affinity of this protein for pre-mRNA. This newly revealed function of U6 RNA is likely to play a very early role in the prespliceosome assembly pathway, prior to U6 RNA's entry into the mature spliceosome's active center.

Evidence that sequence-independent binding of highly conserved U2 snRNP proteins upstream of the branch site is required for assembly of spliceosomal complex A

A critical step in the pre-mRNA splicing reaction is the stable binding of U2 snRNP to the branchpoint sequence (BPS) to form the A complex. The multimeric U2 snRNP protein complexes SF3a and SF3b are required for A complex assembly, but their specific roles in this process are not known. Saccharomyces cerevisiae homologs of all of the SF3a, but none of the SF3b, subunits have been identified. Here we report the isolation of a cDNA encoding the mammalian SF3b subunit SAP 145 and the identification of its probable yeast homolog (29% identity). This first indication that the homology between yeast and metazoan A complex proteins can be extended to SF3b adds strong new evidence that the mechanism of A complex assembly is highly conserved. To investigate this mechanism in the mammalian system we analyzed proteins that cross-link to 32P-site-specifically labeled pre-mRNA in the A complex. This analysis revealed that SAP 145, together with four other SF3a/SF3b subunits, UV cross-links to pre-mRNA in a 20-nucleotide region upstream of the BPS. Mutation of this region, which we have designated the anchoring site, has no apparent effect on U2 snRNP binding. In contrast, when a 2'O methyl oligonucleotide complementary to the anchoring site is added to the spliceosome assembly reaction, A complex assembly and cross-linking of the SF3a/SF3b subunits are blocked. These data indicate that sequence-independent binding of the highly conserved SF3a/SF3b subunits upstream of the branch site is essential for anchoring U2 snRNP to pre-mRNA.

Accumulation of a novel spliceosomal complex on pre-mRNAs containing branch site mutations

Pre-mRNA assembles into spliceosomal complexes in the stepwise pathway E-->A-->B-->C. We show that mutations in the metazoan branchpoint sequence (BPS) have no apparent effect on E complex formation but block the assembly of the A complex and the UV cross-linking of U2 small nuclear ribonucleoprotein particle (snRNP) proteins. Unexpectedly, a novel complex, designated E*, assembles on pre-mRNAs containing BPS mutations. Unlike the E complex, the E* complex accumulates in the presence of ATP. U1 snRNP and U2AF, which are tightly bound to pre-mRNA in the E complex, are not tightly bound in the E* complex. Significantly, previous work showed that U1 snRNP and U2AF become destabilized from pre-mRNA after E complex assembly on normal pre-mRNAs. Thus, our data are consistent with a model in which there are two steps in the transition from the E complex to the A complex (E-->E*-->A). In the first step, U1 snRNP and U2AF are destabilized in an ATP-dependent, BPS-independent reaction. In the second step, the stable binding of U2 snRNP occurs in a BPS-dependent reaction.

Synthesis and biochemical evaluation of RNA containing an intrahelical disulfide crosslink

BACKGROUND

Several factors impede the elucidation of RNA structure and function by X-ray and NMR methods, including the complexity of folded RNA motifs, the tendency of RNA to aggregate, and its ability to fold into multiple isomeric structures. The ability to constrain the process of RNA folding to give a single, homogeneous product would assist these investigations. We therefore set out to develop a synthetic procedure for the site-specific insertion of a disulfide crosslink into oligoribonucleotides. We also examined the ability of a crosslinked species to serve as a substrate for ricin, an RNA glycosylase.

RESULTS

A convertible nucleoside derivative (C) suitable for the site-specific introduction of N4-alkylcytidine residues into RNA has been developed. The corresponding C phosphoramidite was employed in the synthesis of an 8-mer oligonucleotide, 5'-CGGA-GACG-3', which was then efficiently converted to an 8-mer containing two S-protected N4-(2-thioethyl)C residues. Upon deprotection and air oxidation, the 8-mer efficiently formed an intramolecular disulfide bond, yielding a GAGA tetraloop presented on a two-base-pair CpG disulfide crosslinked ministem. We show that this ministem-loop is an excellent substrate for ricin. Control 8-mers lacking the disulfide crosslink were substantially poorer substrates for ricin.

CONCLUSIONS

The nucleoside chemistry described here should be generally useful for the site-specific introduction of a range of non-native functional groups into RNA. We have used this chemistry to constrain an RNA ministem through introduction of an intrahelical disulfide crosslink. That this tetraloop substrate linked to a two base-pair ministem is efficiently processed by ricin is clear evidence that ricin makes all of its energetically favorable contacts to the extreme end of the stem-loop structure, and that the two base pairs of the stem abutting the loop remain intact during recognition and processing by ricin.

In vitro splicing deficiency induced by a C to T mutation at position -3 in the intron 10 acceptor site of the phenylalanine hydroxylase gene in a patient with phenylketonuria

A previous study has identified a C-->U mutation at position -3 in the 3' splice site of intron 10 of the phenylalanine hydroxylase pre-mRNA in a patient with phenylketonuria. In vivo, this mutation induces the skipping of the downstream exon. This result is puzzling because both CAG and UAG have been reported to function equally as 3' splice sites. In this report, we show that the C-->U mutation affects predominantly the first step of the splicing reaction and that it blocks spliceosome assembly at an early stage. The 3' region of the phenylalanine hydroxylase intron 10 has two unusual characteristic features: multiple potential branch sites and a series of four guanosine residues, which interrupt the polypyrimidine tract at positions -8 to -11 from the 3' splice site. We show that the mutation precludes the use of the proximal branch site, while having no effect on the remote one. We also show that in the UAG transcript, the four guanosine residues inhibit the splicing of intron 10. The substitution of these purine residues by one cytosine residue, regardless of the position, increases the splicing efficiency of the mutant UAG precursor while having no effect on the wild-type CAG precursor. Substituting the four purine residues by four pyrimidines relieves the inhibition and rescues the use of the proximal branch site. These results demonstrate that according to the context, the C and U nucleotides preceding the AG are not equivalent for the splicing reaction.

The nuclear matrix phosphoprotein p255 associates with splicing complexes as part of the [U4/U6.U5] tri-snRNP particle

The monoclonal antibody CC3 recognizes a phosphorylated epitope present on an interphase protein of 255 kDa. Previous work has shown that p255 is localized mainly to nuclear speckles and remains associated with the nuclear matrix scaffold following extraction with non-ionic detergents, nucleases and high salt. The association of p255 with splicing complexes is suggested by the finding that mAb CC3 can inhibit in vitro splicing and immunoprecipitate pre-messenger RNA and splicing products. Small nuclear RNA immunoprecipitation assays show that p255 is a component of the U5 small nuclear ribonucleoprotein (snRNP) and the [U4/U6.U5] tri-snRNP complex. In RNase protection assays, mAb CC3 immunoprecipitates fragments containing branch site and 3' splice site sequences. As predicted for a [U4/U6.U5]-associated component, the recovery of the branch site-protected fragment requires binding of U2 snRNP and is inhibited by EDTA. p255 may correspond to the previously identified p220 protein, the mammalian analogue of the yeast PRP8 protein. Our results suggest that changes in the phosphorylation of p255 may be part of control mechanisms that interface splicing activity with nuclear organization.