Curr Genet (2008) 53:337–346 DOI 10.1007/s00294-008-0190-1 123 RESEARCH ARTICLE Characterization of Ire1 in the yeast Yarrowia lipolytica reveals an important role for the Sls1 nucleotide exchange factor in unfolded protein response regulation Anna Babour · Mehdi Kabani · Anita Boisramé · Jean-Marie Beckerich Received: 22 December 2007 / Revised: 4 April 2008 / Accepted: 4 April 2008 / Published online: 18 April 2008  Springer-Verlag 2008 Abstract Following endoplasmic reticulum (ER) stress, eukaryotic cells trigger a conserved signal transduction pathway called the unfolded protein response (UPR) that regulates the ER’s capacity to perform protein folding according to cellular demand. In Saccharomyces cerevi- siae, the UPR is initiated by Ire1, a type I transmembrane serine/threonine kinase/endoribonuclease, that senses unfolded protein levels within the ER in collaboration with the ER Hsp70-family member, BiP/Kar2. Here, we report on the characterization of the Yarrowia lipolytica Ire1 ortholog. Our results show that Sls1, a nucleotide exchange factor for BiP, has important functions in regulating ER stress and the interaction of BiP and Ire1. They suggest that Sls1 regulates this interaction, by stimulating the conver- sion of BiP from the ADP-bound to the ATP-bound state, which favors its interaction with Ire1. Moreover, we identi- Wed known and new partners for Ire1 using the Tandem AYnity PuriWcation (TAP) approach. Keywords Sls1 · BiP · IRE1 · UPR · Yarrowia lipolytica Introduction The endoplasmic reticulum (ER) constitutes a major pro- tein-folding compartment of the eukaryotic cell and is the Wrst compartment of the secretory pathway where proteins fold and assemble before being transported to their Wnal destination. Changes in the ER that interfere with their cor- rect maturation initiate the unfolding protein response (UPR) pathway (Patil and Walter 2001; Schroder and Kauf- man 2005), which culminates in the induction of genes encoding chaperones and other proteins that prevent poly- peptide aggregation and participate in protein folding, covalent modiWcation, post-translational assembly of pro- tein complexes, protein degradation (Travers et al. 2000). Ire1 is an ER resident type I transmembrane protein that was shown to be the key actor of this pathway in the yeast S. cerevisiae (Cox et al. 1993; Mori et al. 1993). ER stress induces Ire1 to dimerize, which is believed to promote upregulation of the serine/threonine kinase and RNase activities located in the cytosolic domain of the protein (Shamu and Walter 1996). Upon activation, Ire1 promotes unconventional splicing of a precursor form of an mRNA encoded by HAC1 (Cox and Walter 1996) producing a mature form that is translated into a transcription factor for induction of various genes as previously cited. Two mam- malian homologs of Ire1 have been identiWed and found to have a similar function in the ER stress signaling (Tiraso- phon et al. 1998; Wang et al. 1998). Moreover, in higher eukaryotes, the UPR also comprises signals initiated by the Communicated by K. Kuchler. Electronic supplementary material The online version of this article (doi:10.1007/s00294-008-0190-1) contains supplementary material, which is available to authorized users. Present Address: A. Babour (&) University of California, San Diego, NSB 5315 9500 Gilman Drive, La Jolla, CA 92093, USA e-mail: ababour@biomail.ucsd.edu A. Babour · M. Kabani · A. Boisramé · J.-M. Beckerich Laboratoire de Microbiologie et Génétique moléculaire, INRA, CNRS, Institut National Agronomique Paris-Grignon, 78850 Thiverval-Grignon, France Present Address: M. Kabani Laboratoire d’Enzymologie et Biochimie Structurales (LEBS), CNRS, 91198 Gif-sur-Yvette, France