Separable roles in vivo for the two RNA binding domains of a Drosophila A1-hnRNP homolog

We analyzed the roles of the three domains of a Drosophila hnRNP A1 homolog by expression of wild-type and mutant versions of HRB87F/hrp36 in Drosophila melanogaster. HRB87F/hrp36 is one of two Drosophila proteins that is most similar to mammalian A1 hnRNP, and like A1, consists of two copies of the RNA-binding domain (RBD) motif followed by a glycine-rich domain (GRD). The role of the domains in nuclear localization and RNA binding to polytene chromosomal sites was determined. RBD-1 and the GRD were largely responsible for both the cellular location of the protein and for the typical chromosomal distribution pattern of the protein at sites of PolII transcription. RBD-1 also provided a role in the exon-skipping activity of the protein that was not provided by RBD-2. On the other hand, RBD-2 and the GRD were responsible for the very limited chromosomal distribution pattern seen upon heat shock, when HRB87F/hrp36 is sequestered at heat-shock puff 93D, which encodes a long nucleus-restricted RNA. Thus, these studies indicate that the two RBDs function independently of each other but in concert with the GRD. In addition, the self-association property of the GRD was strikingly evident in these overexpressed proteins.