Cooperative-binding and splicing-repressive properties of hnRNP A1.

hnRNP A1 binds to RNA in a cooperative manner. Initial hnRNP A1 binding to an exonic splicing silencer at the 3' end of human immunodeficiency virus type 1 (HIV-1) tat exon 3, which is a high-affinity site, is followed by cooperative spreading in a 3'-to-5' direction. As hnRNP A1 propagates toward the 5' end of the exon, it antagonizes binding of a serine/arginine-rich (SR) protein to an exonic splicing enhancer, thereby inhibiting splicing at that exon's alternative 3' splice site. tat exon 3 and the preceding intron of HIV-1 pre-mRNA can fold into an elaborate RNA secondary structure in solution, which could potentially influence hnRNP A1 binding. We report here that hnRNP A1 binding and splicing repression can occur on an unstructured RNA. Moreover, hnRNP A1 can effectively unwind an RNA hairpin upon binding, displacing a bound protein. We further show that hnRNP A1 can also spread in a 5'-to-3' direction, although when initial binding takes place in the middle of an RNA, spreading preferentially proceeds in a 3'-to-5' direction. Finally, when two distant high-affinity sites are present on the same RNA, they facilitate cooperative spreading of hnRNP A1 between the two sites.