RNA or protein ligands for metal co-factors in the spliceosome? Case closed … sort of

During the course of the meetings which occurred prior to this blog’s inception, it was agreed that one feature we’d like to regularly publish would be ‘digested versions’ of important or interesting research papers. We’ve only had one so far and, given that this is a blog written by doctoral students at a scientific research institute, I’d say it should be quite high on our priorities list. There are also going to be some problems pitching this – we’re largely a plant science institute with a few groups working on slightly different problems, but what about our readership? At this stage, it won’t extend much further than those who follow the JIC on Twitter, so mainly plant scientists and microbiologists. So is it our goal with this blog to preach to the converted or to make a fist of widening the net? I’d say we should do our best to attract readers from all branches of science and the wider public.

So, with that in mind, I’ve decided to summarise a paper I read recently which really couldn’t get more general and, well, important within the field of molecular biology as a whole. It’s a paper which aims to dissect, biochemically, one of the central questions at the heart of what many call our ‘central dogma’ of the flow of genetic information from DNA to RNA to Protein.

Biochemistry: Metal ghosts in the splicing machine

So, for the benefit of our non-molecular biologist readers, present or future, allow me to elaborate on this: Your body relies on proteins for everything. From the catalysis of chemical reactions, to responding to external stimuli, right through to forming the structural basis of connective tissues like bone and cartilage, as well as holding together our cells. But how are these proteins manufactured? Well, I have little doubt that you’ve probably heard of deoxyribonucleic acid (DNA). DNA is the structure within our cells which carries the instructions for making proteins. DNA is turned into Ribonucleic Acid (RNA) and the RNA acts as a messenger molecule for producing the protein. There are several types of RNA at play here, the most important being mRNA, tRNA, rRNA and snRNA. mRNA stands for ‘messenger’ RNA, which passes information from one place, to another. tRNA is ‘transfer’ RNA, this molecule acts as a physical link between the genetic code and the amino acids which are the building blocks of proteins. rRNA is ‘ribosomal’ RNA and its job is to form the molecular machinery (known as a ribosome) responsible for linking the amino acids together into chains called ‘peptides’. This is pretty easy to conceptualise. We’ll take a factory as an analogy. An order comes in to the factory which we’ll think of as a letter or e-mail containing a blueprint of the design the customer requires, and is represented by our mRNA. The workers, or tRNAs as we’ll call them, must put the right pieces in the right order to make the product but simply putting them next to each other won’t do anything. For example, metal parts may need welding together, which requires machinery or, in our case, the ribosome, which is composed of rRNA.

You’ll note that I’ve missed out one of the RNAs I mentioned earlier: snRNA, which stands for small nuclear RNA. The primary role of snRNA is to process the mRNA. The code required to make a protein is not encoded by DNA in a linear fashion; the regions which mean something (exons) are interspersed with regions of code which do not go on to make a protein (introns) and something must be done about this. Well, snRNAs in complex with a highly heterogenous and large protein complex known as the spliceosome ‘splice’ the pre-mRNA and ligate (join) the exons back together to form a mature mRNA. So to go back to our analogy, imagine the initial plans were drawn up by the work experience kid and before they’re sent out, the boss has to go over them and cut bits out. That’s the role of the snRNA containing spliceosome.

So, onto the paper I read recently which caught my interest entitled ‘RNA catalyses nuclear pre-mRNA splicing.’1 The researchers from the University of Chicago set out to determine which part of the aforementioned large, complex metalloenzyme known as the spliceosome was responsible for catalysis of the splicing reaction. For those unsure of what a metalloenzyme is, it means it requires metal co-factors (magnesium ions, in this case) in order to carry out its function. It was hitherto unknown whether the active site ligands for these magnesium ions were provided by the snRNA or by one of the many proteins which make up the rest of the complex.

However, using a technique known as metal specificity switch, the researchers could pinpoint a group of phosphate oxygens which appear to be crucial for Mg2+ binding and subsequent catalysis of splicing. Single oxygen atoms in snRNA U6 were sequentially replaced with sulphur (magnesium does not bind particularly well to sulphur relative to oxygen) and subsequently tested for splicing efficiency. Five of the positions appeared to result in decreased efficiency. In addition to this, the sulphurous snRNA containing spliceosomes were tested in the presence of metals known to bind sulphur with higher efficiency (e.g. manganese and cadmium). It was found that these sulphur binding metals were able to re-establish splicing efficiency where oxygen had been replaced with sulphur. These findings tell us that snRNA does indeed provide ligands for the catalytically crucial metal ions involved in the processing of pre-mRNA and that this is dependent on binding specificity of the metal co-factors. Further support for this comes from the spatial similarity of the metal ion binding sites of U6 snRNA to those found in group II introns, a class of intron which is able to self-splice without any input from the spliceosome but through exactly the same two phopshoryl-transfer reaction normally catalysed by the spliceosome2.

However, none of this actually excludes the possibility that one or more of the proteins involved in the spliceosome also provides ligands for metal co-factors. The same approach used on U6 snRNA was employed to assess the potential of Prp8 to provide ligands for the co-factors and no impaired splicing was observed. Prp8 had previously been imagined to be catalytically important for the spliceosome. What of other proteins then? Well, as previously stated, the spliceosome is a very heterogeneous and dynamic complex with proteins arriving and departing throughout the splicing reaction so de-convoluting each protein’s role in the reaction appears to be a daunting process. Crystal structures of the spliceosome at each stage of the splicing reaction, enabling observation of proteins close to the active site and their likelihood of binding metal co-factors, would certainly speed up the process. However, expressing and crystallising such a large, dynamic protein complex presents an enormous set of challenges in itself.

For now, however, it seems more likely that most of the proteins are simply regulators of the process or acting as chaperones, holding U6 snRNA in the correct conformation to catalyse the reaction and that the catalytically important part of the complex is, indeed, snRNA.

[1] Fica, S. M. et al. Nature 503, 229-234, (2013).

[2] Marcia, M. & Pyle, A. M. Cell 159, 497-507, (2012).

By Ben Hall – a second year PhD student in Mark Banfield’s group.

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