Identification of Gbp2 as a novel poly(A)+ RNA-binding protein involved in the cytoplasmic delivery of messenger RNAs in yeast.

Important progress in understanding messenger RNA export from the nucleus could be achieved by increasing the list of proteins that are involved in this process. Here, we present the identification of Gbp2 as a novel shuttling RNA-binding protein in Saccharomyces cerevisiae. Nuclear import of Gbp2 is dependent on the receptor Mtr10 and the serine/arginine-specific protein kinase Sky1. Deletion of the genes encoding both of these proteins or disruption of two of the arginine/serine repeats each shifts the steady-state localization of Gbp2 to the cytoplasm. Interestingly, deletion of MTR10 only also causes an increase in poly(A)(+) RNA binding by Gbp2, suggesting a role of Mtr10 in the dissociation of Gbp2 from mRNA in the cytoplasm. The nuclear export of Gbp2 is always coupled to mRNA export and is dependent on continuous RNA polymerase II transcription and mRNA-export factors. Although GBP2 is not essential for normal cell growth, overexpression of this gene is toxic and causes a nuclear retention of bulk poly(A)(+) RNA. Together, our findings clearly show an involvement of Gbp2 in mRNA transport. In addition, as a non-essential protein, Gbp2 also has the interesting potential to be spatially or temporally regulated.