Binding of an RNA trafficking response element to heterogeneous nuclear ribonucleoproteins A1 and A2

Heterogeneous nuclear ribonucleoprotein (hnRNP) A2 binds a 21-nucleotide myelin basic protein mRNA response element, the A2RE, and A2RE-like sequences in other localized mRNAs, and is a trans-acting factor in oligodendrocyte cytoplasmic RNA trafficking. Recombinant human hnRNPs A1 and A2 were used in a biosensor to explore interactions with A2RE and the cognate oligodeoxyribonucleotide. Both proteins have a single site that bound oligonucleotides with markedly different sequences but did not bind in the presence of heparin. Both also possess a second, specific site that bound only A2RE and was unaffected by heparin, hnRNP A2 bound A2RE in the latter site with a K-d near 50 nM, whereas the K-d for hnRNP A1 was above 10 muM. UV cross-linking assays led to a similar conclusion. Mutant A2RE sequences, that in earlier qualitative studies appeared not to bind hnRNP A2 or support RNA trafficking in oligodendrocytes, had dissociation constants above 5 muM for this protein. The two concatenated RNA recognition motifs (RRMs), but not the individual RRMs, mimicked the binding behavior of hnRNP A2. These data highlight the specificity of the interaction of A2RE with these hnRNPs and suggest that the sequence-specific A2RE-binding site on hnRNP A2 is formed by both RRMs acting in cis

hnRNP A2 selectively binds the cytoplasmic transport sequence of myelin basic protein mRNA

Segregation of mRNAs in the cytoplasm of polar cells has been demonstrated for proteins involved in Xenopus and Drosophila oogenesis, and for some proteins in somatic cells. It is assumed that vectorial transport of the messages is generally responsible for this localization. The mRNA encoding the basic protein of central nervous system myelin is selectively transported to the distal ends of the processes of oligodendrocytes, where it is anchored to the myelin membrane and translated. This transport is dependent on a 21-nucleotide cis-acting segment of the 3'-untranslated region (RTS). Proteins that bind to this cis-acting segment have now been isolated from extracts of rat brain. A group of six 35-42-kDa proteins bind to a 35-base oligoribonucleotide incorporating the RTS, but not to several oligoribonucleotides with the same composition but randomized sequences, thus establishing specificity for the base sequence in the RTS. The most abundant of these proteins has been identified, by Edman sequencing of tryptic peptides and mass spectroscopy, as heterogeneous nuclear ribonucleoprotein (hnRNP) A2, a 36-kDa member of a family of proteins that are primarily, but not solely, intranuclear. This protein was most abundant in samples from rat brain and testis, with lower amounts in other tissues. It was separated from the other polypeptides by using reverse-phase HPLC and shown to retain preferential association with the RTS. In cultured oligodendrocytes, hnRNP A2 was demonstrated by confocal microscopy to be distributed throughout the nucleus, cell soma, and processes