The Yeast hnRNP-like Protein Hrp1/Nab4 Accumulates in the Cytoplasm after Hyperosmotic Stress: A Novel Fps1-dependent Response

The Hrp1/Nab4 shuttling protein belongs to a family of RNA binding proteins that bind to nascent RNA polymerase II transcripts and form hnRNP complexes. Members of this family function in a staggering array of cellular activities, ranging from transcription and pre-mRNA processing in the nucleus to cytoplasmic mRNA translation and turnover. It has recently been recognized that the yeast stress response can include alterations in hnRNP-mediated mRNA export. We now report that the steady-state localization of Hrp1p rapidly shifts from the nucleus to the cytoplasm in response to osmotic stress. In contrast to a general stress response resulting in a transient relocation, Hrp1p redistribution is specific to hyperosmotic stress and is only reversed after stress removal. Hrp1p relocalization requires both the CRM1/XPO1 exportin and the FPS1 glycerol transporter genes but is independent of ongoing RNA transcription and protein arginine methylation. However, mutations in the high osmolarity glycerol and protein kinase C osmosensing pathways do not impact the Hrp1p hyperosmotic response. We present a working model for the cytoplasmic accumulation of Hrp1 and discuss the implications of this relocalization on Hrp1p function.