Poly(A)-binding protein

Poly(A)-binding protein (PAB or PABP) is a RNA-binding protein which binds to the poly(A) tail of mRNA. The poly(A) tail is located on the 3' end of mRNA and is 200-250 nucleotides long. The binding protein is also involved in mRNA precursors by helping Polyadenylate polymerase add the Poly(A) nucleotide tail to the pre-mRNA before translation. The nuclear isoform selectively binds to around 50 nucleotides and stimulates the activity of Polyadenylate polymerase by increasing its affinity towards RNA. Poly(A)-binding protein is also present during stages of mRNA metabolism including Nonsense-mediated decay and nucleocytoplasmic trafficking. The poly(A)-binding protein may also protect the tail from degradation and regulate mRNA production. Without these two proteins in-tandem, then the poly(A) tail would not be added and the RNA would degrade quickly.

Cytosolic Poly-A Binding Protein (PABPC) is made up of four RNA recognition motifs (RRMs) and a C-terminal region known as the PABC domain. RRM is the most common motifs for RNA recognition and is usually made up of 90-100 amino acids. Previous solution NMR and X-ray crystallography studies have shown that RRMs are globular domains, each composed of 4 anti-parallel β sheets that are backed by 2 α-helices. The central two β-strands, connected by a short linker, of each RRM forms a trough-like surface that’s thought to be responsible for binding to the poly(A) oligonucleotides. The polyadenylate RNA adopts an extended conformation running the length of the molecular trough. Adenine recognition is primarily mediated by contacts with conserved residues found in the RNP motifs of the two RRMs. In vitro studies have shown the binding affinities to be on the order of 2-7nM, while affinity for poly(U), poly(G), and poly(C) were reportedly lower or undetectable in comparison. This shows that the poly(A)-binding protein is specific to poly(A) oligonucleotides and not others. Since the two central β-strands are used for poly(A) oligonucleotide binding, the other face of the protein is free for protein-protein interactions.

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