A2RE-mediated RNA transport

Directed intracellular transport of mRNA is one mechanism by which cells are able to establish asymmetry. The ability to target mRNA has been described in many cell types ranging from sperm to oligodendrocytes and in organisms ranging from humans to plants. One of the best characterised examples of RNA trafficking is the localisation of the mRNA encoding myelin basic protein (MBP). MBP is the major component of central nervous system myelin and is produced by highly polarised cells known as oligodendrocytes. Almost 20 years ago MBP mRNA was found to be enriched in the myelin fraction of brain tissue extracts. Later, in situ hybridisation and RT-PCR experiments confirmed that MBP mRNA is enriched in distal oligodendrocyte processes, at the site of myelination.MBP has a high affinity for membranes, making transport of the protein from the nucleus to where it is required at distal processes difficult. Directed transport of the mRNA allows localised translation and direct incorporation of the protein into the myelin membrane. This transport process is directed by a 21-nucleotide element in the 3rUTR of the message, known as the RNA translocation signal (RTS). This exacting transport signal is specifically recognised by trans-acting factors that then facilitate transport of this message. Through use of an RTS-RNA affinity matrix, a group of six proteins were isolated that interact with this signal in a sequence specific manner. This group of proteins predominantly includes heterogeneous nuclear ribonucleoprotein (hnRNP) A2 and up to 4 hnRNP A3-like molecules. hnRNP proteins are known to participate in a wide range of RNA processing events and A type hnRNPs are known to shuttle rapidly between the nucleus and the cytoplasm. hnRNP A2 was shown to interact in a sequence-specific manner with the RTS in vitro and the cX-acting element was renamed the hnRNP A2 response element (A2RE).Further analysis revealed A2RE-like sequences were present in a number of other mRNAs which are known to be transported in neurons including MAP2A, Arc and GABAR(A). These A2RE-like sequences were able to bind the same proteins as the A2RE suggesting their localisation may be directed by the same group of trans-acting factors as MBP mRNA.Deletion analysis of the A2RE revealed that the first 11 nucleotides were just as effective as the full 21 nucleotides in recruiting the A2RE binding proteins. Subsequent mutational analysis revealed critical residues at positions 4, 5, 6, 8 and 9 which, if altered, abolished binding.nnnnnnnn