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

Heterogeneous nuclear ribonucleoprotein (hnRNP) A2 binds a 21-nucleotide my elin 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 i n a biosensor to explore interactions with A2RE and the cognate oligodeoxyr ibonucleotide. Both proteins have a single site that bound oligonucleotides with markedly different sequences but did not bind in the presence of hepa rin. 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 n ear 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 traffickin g in oligodendrocytes, had dissociation constants above 5 muM for this prot ein. The two concatenated RNA recognition motifs (RRMs), but not the indivi dual RRMs, mimicked the binding behavior of hnRNP A2. These data highlight the specificity of the interaction of A2RE with these hnRNPs and suggest th at the sequence-specific A2RE-binding site on hnRNP A2 is formed by both RR Ms acting in cis.