RNA-binding proteins (RBPs) are involved in all steps of gene expression, having a large impact.
RBPs are highly expressed and abundant proteins, with around 10% of a cell’s proteome interacting with its RNA in vivo. This includes localising RNAs, such as with zip code binding proteins, helping degrade RNAs (as occurs when poly(A) binding protein is unable to bind to the 3’ poly(A) UTR tail after sufficient deadenylation), and RNA processing. In eukaryotes, RNA processing is one of the most important post-translational modifications, allowing the removal of introns with small nucleolar ribonucleoproteins (snRNPs, or snurps). This is possible due to the ability of the snRNPs to bind specifically to the pre-mRNA, allowing introns to be both recognised and excised by the spliceosome (a large complex of U-rich snRNPs).
Allowing the localisation of proteins is important in regulating protein expression. Many mRNA transcripts have a zip code sequence within their 3’ UTR. This allows the binding of a protein, which can either catalyse the transcripts degradation if negative regulation is employed by the cell, or move the protein to a desired region. In Saccharomyces cerevisiae, transcripts for the gene ASH1 bind to a She2 and She3 complex, which in turn bind to Myo4, binding to actin cables. This protein binding allows the localisation of transcripts to the daughter cell, controlling the site of expression. mRNA localisation has a large impact on protein expression, also allowing the development of the anterior-posterior axis in larger organisms during embryogenesis.