Identification of Critical Residues in Spliceosomal Protein Dib1
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In eukaryotes, when DNA is transcribed into pre-messenger RNA, multiple processing events must occur before translation into functional protein occurs. One important processing event is known as splicing. Splicing is the removal of non-protein coding regions, known as introns, from pre-messenger RNA and the ligation of the protein coding regions, known as exons, to form a mature mRNA. This process is extremely important. Recurring inaccuracies in splicing by just a single nucleotide can lead to the shift in a reading frame of a protein coding sequence, which then leads to nonfunctional protein or disease in some cases.
Splicing is facilitated by a large macromolecular complex known as the spliceosome. The spliceosome is composed of five snRNPs (U1, U2, U4, U5, and U6) and many associated proteins which help orient the pre-mRNA in such a way that catalyzes the splicing reactions. Of the associated proteins, Dib1 in S. cerevisiae is amongst those whose biochemical function still remains unknown.
Dib1 is a 15 kDa protein with a thioredoxin-like fold that is associated with U5 in the spliceosome. Dib1 is highly conserved from S. cerevisiae to H. sapiens and is necessary for cell viability. Previous studies have shown that Dib1 and the S. pombe homolog Dim1 are necessary for splicing, potentially have peptidase activity, and that many splicing factors interact with Dim1. In this study, the goal was to identify critical residues in Dib1 to help elucidate its biochemical function. In order to do this, mutational analysis, protein purification, and CD spectroscopy were performed.