Prp8 in a Reduced Spliceosome Lacks a Conserved Toggle that Correlates with Splicing Complexity across Diverse Taxa.

Conformational rearrangements are critical to regulating the assembly and activity of the spliceosome. The spliceosomal protein Prp8 undergoes multiple conformational changes during the course of spliceosome assembly, activation, and catalytic activity. Most of these rearrangements of Prp8 involve the disposition of the C-terminal Jab-MPN and RH domains with respect to the core of Prp8. Here we use x-ray structural analysis to show that a previously characterized and highly conserved ß-hairpin structure in the RH domain that acts as a toggle in the spliceosome is absent in Prp8 from the reduced spliceosome of the red alga Cyanidioschyzon merolae. Using comparative sequence analysis, we show that the presence or absence of this hairpin corresponds to the presence or absence of protein partners that interact with this hairpin as observed by x-ray and cryo-EM studies. The presence of the toggle correlates with increasing intron number suggesting a role in the regulation of splicing.

Conserved structure of Snu13 from the highly reduced spliceosome of Cyanidioschyzon merolae.

Structural and functional analysis of proteins involved in pre-mRNA splicing is challenging because of the complexity of the splicing machinery, known as the spliceosome. Bioinformatic, proteomic, and biochemical analyses have identified a minimal spliceosome in the red alga Cyanidioschyzon merolae. This spliceosome consists of only 40 core proteins, compared to ∼ 70 in S. cerevisiae (yeast) and ∼ 150 in humans. We report the X-ray crystallographic analysis of C. merolae Snu13 (CmSnu13), a key component of the assembling spliceosome, and present evidence for conservation of Snu13 function in this algal splicing pathway. The near identity of CmSnu13's three-dimensional structure to yeast and human Snu13 suggests that C. merolae should be an excellent model system for investigating the structure and function of the conserved core of the spliceosome.

A novel U2 and U11/U12 snRNP protein that associates with the pre-mRNA branch site.

Previous UV cross-linking studies demonstrated that, upon integration of the U2 snRNP into the spliceosome, a 14 kDa protein (p14) interacts directly with the branch adenosine, the nucleophile for the first transesterification step of splicing. We have identified the cDNA encoding this protein by microsequencing a 14 kDa protein isolated from U2-type spliceosomes. This protein contains an RNA recognition motif and is highly conserved across species. Antibodies raised against this cDNA-encoded protein precipitated the 14 kDa protein cross-linked to the branch adenosine, confirming the identity of the p14 cDNA. A combination of immunoblotting, protein microsequencing and immunoprecipitation revealed that p14 is a component of both 17S U2 and 18S U11/U12 snRNPs, suggesting that it contributes to the interaction of these snRNPs with the branch sites of U2- and U12-type pre-mRNAs, respectively. p14 was also shown to be a subunit of the heteromeric splicing factor SF3b and to interact directly with SF3b155. Immuno precipitations indicated that p14 is present in U12-type spliceosomes, consistent with the idea that branch point selection is similar in the major and minor spliceosomes.

Kinetic analysis of the M1 RNA folding pathway.

The biological activity of large RNAs is dependent on the formation of complex folded structures that determine function. Typically the creation of such structures requires divalent magnesium and in many cases the folding process takes place over the course of several minutes. It has been proposed that the folding paths of large RNAs proceed through discrete intermediates but the nature of these intermediates is not known in most cases. Here, we describe our studies on the folding of the M1 RNA sub-unit of Escherichia coli RNase P. We performed kinetic footprinting studies of M1 RNA folding with the chemical footprinting reagent peroxynitrous acid to provide a detailed description of the folding pathway of RNase P RNA. Our results indicate that, in contrast to the Group I ribozyme, the M1 RNA folds into its catalytically active structure through the formation of two separately folded domains and that the folding of each proceeds through a discrete series of intermediates. Similar rates of folding were observed for regions believed to form the interface between the two domains. This observation is consistent with a kinetic trap which occurs by interaction of the domains during folding.

Studies of RNA cleavage by photolysis of N-hydroxypyridine-2(1H)-thione. A new photochemical footprinting method.

N-Hydroxypyridine-2(1H)-thione (N-HPT) has been studied as a photochemical source of hydroxyl radicals for use in photoinitiated nucleic acid footprinting experiments. Steady-state photolysis of dilute aqueous solutions of N-HPT at 350 nm in the presence of a 385 nucleotide (32)P-labeled RNA, the Tetrahymena L-21 ribozyme, resulted in cleavage of the RNA at nucleotide resolution. No cleavage of the RNA occurred in the absence of light or in the absence of N-HPT. Photolysis of the analogous pyridine lacking the N-hydroxyl group did not result in detectable amounts of RNA cleavage. The addition of RNA to preirradiated solutions of N-HPT gave no apparent RNA cleavage products, suggesting that the photoproducts of N-HPT do not result in RNA modification. Cleavage of RNA, upon photolysis in the presence of N-HPT, occurred in a sequence-independent fashion with double-stranded RNA being cleaved as efficiently as single-stranded RNA. Based on these observations, we conclude that photochemically generated diffusable hydroxyl radicals are responsible for the RNA cleavage. Experiments involving the photolysis of N-HPT in the presence of the Tetrahymena ribozyme and magnesium showed a magnesium-dependent protection from RNA cleavage due to formation of a folded RNA tertiary structure. The locations and amount of protection were identical to those observed in footprinting experiments performed with other hydroxyl radical sources. The presence of N-HPT had no effect on either the rate of folding or the catalytic activity of the folded RNA, indicating that this reagent does not disrupt RNA tertiary structure or otherwise affect activity. Thus, N-HPT is established as a new reagent for use in photoinitiated RNA footprinting experiments.

Template-directed interference footprinting for RNA based on inosine-specific cleavage.

We report here the development of a Template-directed Interference (TDI) footprinting assay for RNA. The TDI nucleotide analogue inosine (I) lacks the exocyclic amine of G and is a suitable probe for the role of this group in RNA structure and function. Using an I-specific cleavage protocol we identified three functionally significant G residues in the Tetrahymena ribozyme. These residues are proximal to the active site of the folded intron and likely contribute to the positioning of substrates at the catalytic core.

Evidence for helical unwinding of an RNA substrate by the RNA enzyme RNase P: use of an interstrand disulfide crosslink in substrate.

To gain an understanding of structural changes induced in substrates by Escherichia coli ribonuclease P (RNase P), we have incorporated an interstrand disulfide crosslink proximal to the cleavage site in a model substrate. RNase P is able to process the reduced, non-crosslinked form of this substrate as well as a substrate in which the free thiol molecules have been alkylated with iodoacetamide. However, the oxidized, crosslinked form is cleaved at a significantly lower rate. Therefore, helical unwinding of the analog of the aminoacyl stem of the substrate near its site of cleavage may be necessary for efficient processing by E. coli RNase P.

Characterization of the Tetrahymena ribozyme folding pathway using the kinetic footprinting reagent peroxynitrous acid.

Large RNAs fold into complex structures which determine their biological activities. A full understanding of both RNA structure and dynamics will include the description of the pathways by which these structures are formed. Kinetic footprinting [Sclavi, B., et al. (1997) J. Mol. Biol. 266, 144-159] has been shown to be a powerful method for the study of dynamic processes involving RNA. Here we describe the use of a readily available reagent, peroxynitrous acid, as a kinetic footprinting tool for the study of RNA folding. Hydroxyl radicals generated from this reagent were used to footprint the Tetrahymena ribozyme during its magnesium-dependent folding-in agreement with synchroton X-ray footprinting [Sclavi, B., et al. (1998) Science 279, 1940-1943] and oligonucleotide/hybridization cleavage experiments [Zarrinkar, P. P., and Williamson, J. R. (1994) Science 265, 918-924], this work suggests an ordered, hierarchical folding pathway for the ribozyme. Several slow steps in the folding pathway were observed in the peroxynitrous acid footprinting, but none of these corresponded to the rate-determining step of folding. This suggests that the formation of the global, protected structure is followed by one or more slow local rearrangements to yield the final active structure. These studies illustrate the utility of peroxynitrous acid as a reagent for the elucidation of RNA folding pathways and the study of RNA dynamics.

Caged RNA: photo-control of a ribozyme reaction.

We report here the first photo-chemical control of a ribozyme reaction by the site-specific modification of the 2'-hydroxyl nucleophile in the hammerhead system with a caging functionality. Rapid laser photolysis of the O-(2-nitrobenzyl) caging group initiates an efficient and accurate hammerhead-catalyzed cleavage of substrate RNA under native conditions. RNAs in which reactive functionalities or recognition elements are caged in this manner will be useful tools to probe RNA reactivity and dynamics.

SC35-mediated reconstitution of splicing in U2AF-depleted nuclear extract.

Assembly of the mammalian spliceosome is known to proceed in an ordered fashion through several discrete complexes, but the mechanism of this assembly process may not be universal. In an early step, pre-mRNAs are committed to the splicing pathway through association with U1 small nuclear ribonucleoprotein (snRNP) and non-snRNP splicing factors, including U2AF and members of the SR protein family. As a means of studying the steps of spliceosome assembly, we have prepared HeLa nuclear extracts specifically depleted of the splicing factor U2AF. Surprisingly, the SR protein SC35 can functionally substitute for U2AF65 in the reconstitution of pre-mRNA splicing in U2AF-depleted extracts. This reconstitution is substrate-specific and is reminiscent of the SC35-mediated reconstitution of splicing in extracts depleted of U1 snRNP. However, SC35 reconstitution of splicing in U2AF-depleted extracts is dependent on the presence of functional U1 snRNP. These observations suggest that there are at least three distinguishable mechanisms for the binding of U2 snRNP to the pre-mRNA, including U2AF-dependent and -independent pathways.

Dynamic association of proteins with the pre-mRNA branch region.

The association of proteins with the branch site region during pre-mRNA splicing was probed using a novel methodology to site-specifically modify the pre-mRNA with the photo-reagent benzophenone. Three sets of proteins were distinguished by the kinetics of their associations with pre-mRNAs, by their association with discrete splicing complexes, and by their differing factor requirements. An early U1 snRNP-dependent cross-link of the branch region to a p80 species was followed by cross-links to p14, p35, and p150 polypeptides associated with the U2 snRNP-pre-mRNA complex. Concomitant with formation of the spliceosome, a rearrangement of protein factors about the branch region occurred, in which the p35 and p150 cross-links were replaced by p220 and p70 species. These results establish that the branch region is recognized in a dynamic fashion by multiple distinct proteins during the course of spliceosomal assembly.

Direct identification of the active-site nucleophile in a DNA (cytosine-5)-methyltransferase.

The overproduction, purification, and determination of the active-site catalytic nucleophile of the DNA (cytosine-5)-methyltransferase (DCMtase) enzyme M.HaeIII are reported. Incubation of purified M.HaeIII with an oligodeoxynucleotide specifically modified with the mechanism-based inhibitor 5-fluoro-2'-deoxycytidine [Osterman, D. G., et al. (1988) Biochemistry 27, 5204-5210], in the presence of the cofactor S-adenosyl-L-methionine (AdoMet), resulted in the formation of a covalent DNA-M.HaeIII complex, which was purified to homogeneity. Characterization of the intact complex showed it to consist of one molecule of the FdC-containing duplex oligonucleotide, one molecule of M.HaeIII, and one methyl group derived from AdoMet. Exhaustive proteolysis, reduction, and alkylation of the DNA-M.HaeIII complex led to the isolation of two DNA-bound peptides--one each from treatment with Pronase or trypsin--which were subjected to peptide sequencing in order to identify the DNA attachment site. Both peptides were derived from the region of M.HaeIII containing a Pro-Cys sequence that is conserved in all known DCMtases. At the position of this conserved Cys residue (Cys71), in the sequence of each peptide, was found an unidentified amino acid residue; all other amino acid residues were in accord with the known sequence. It is thus concluded that Cys71 of M.HaeIII forms a covalent bond to DNA during catalytic methyl transfer. This finding represents a direct experimental verification for the hypothesis that the conserved Cys residue of DCMtases is the catalytic nucleophile [Wu, J. C., & Santi, D. V. (1987) J. Biol. Chem. 262, 4778-4786].(ABSTRACT TRUNCATED AT 250 WORDS)