Dal81 Enhances Stp1- and Stp2-Dependent Transcription Necessitating Negative Modulation by Inner Nuclear Membrane Protein Asi1 in Saccharomyces cerevisiae

Mirta Boban and Per O. Ljungdahl¹

Ludwig Institute for Cancer Research, Box 240, S-171 77 Stockholm, Sweden

^¹ Corresponding author: Stockholm University, Wenner-Gren Institute, SE-10691 Stockholm, Sweden.
E-mail: plju{at}wgi.su.se

The yeast transcription factors Stp1 and Stp2 are synthesizedas latent cytoplasmic precursors. In response to extracellularamino acids, the plasma membrane SPS sensor endoproteolyticallyexcises the N-terminal domains that mediate cytoplasmic retention,enabling the processed forms to efficiently enter the nucleusand induce gene expression. Cytoplasmic retention is not absolute,low levels of full-length Stp1 and Stp2 "leak" into the nucleus,and the concerted action of inner nuclear membrane proteinsAsi1, Asi2, and Asi3 restricts their promoter access. In cellslacking Asi function, the precursor forms bind promoters andconstitutively induce gene expression. To understand the requirementof Asi-dependent repression, spontaneous mutations in Requiredfor Latent Stp1/2-mediated transcription (RLS) genes that abolishthe constitutive expression of SPS sensor-regulated genes inan asi1 ${Delta}$ strain were selected. A single gene, allelic with DAL81,was identified. We show that Dal81 indiscriminately amplifiesthe transactivation potential of both full-length and processedStp1 and Stp2 by facilitating promoter binding. In dal81 ${Delta}$ mutants,the repressing activity of the Asi proteins is dispensable,demonstrating that without amplification, the levels of full-lengthStp1 and Stp2 that escape cytoplasmic retention are insufficientto activate transcription. Conversely, the high levels of processedStp1 and Stp2 that accumulate in the nucleus of induced cellsactivate transcription in the absence of Dal81.