Mip6 binds directly to the Mex67 UBA domain to maintain low levels of Msn2/4 stress dependent mRNAs

RNA binding proteins (RBPs) participate in almost all steps of gene expression, underscoring their potential as key regulators of RNA homeostasis. We structurally and functionally characterized Mip6, a four-RNA Recognition Motif (RRM)-containing RBP, as a functional and physical interactor of the mRNA export factor Mex67. Mip6 directly interacts with the ubiquitin-associated (UBA) domain of Mex67 through tryptophan 442 of Mip6-RRM4, whose mutation leads to slow growth of yeast cells in vivo. Although Mip6-RRM4 binds to RNA in vitro, Mex67 UBA binding prevents Mip6-RRM4 interaction with RNA. Mip6 shuttles between the nucleus and the cytoplasm in a Mex67-dependent manner and concentrates in cytoplasmic foci under several stressors. Photoactivatable Ribonucleoside-Enhanced Crosslinking and Immunoprecipitation (PAR-CLIP) experiments showed preferential binding of Mip6 to mRNAs regulated by the stress-response Msn2/4 transcription factors. Consistent with this binding, MIP6 deletion affected their export and expression levels. These results reveal a novel role for Mip6 in the homeostasis of Msn2/4 dependent transcripts by its direct interaction with Mex67 UBA domain. Yeast Mip6 RNA binding at 30 ºC and after 39 ºC for 20 minutes using Illumina HI-Seq