Figure 1From: The HAC1 gene from Pichia pastoris: characterization and effect of its overexpression on the production of secreted, surface displayed and membrane proteinsThe unfolded protein response in the yeast S. cerevisiae. Under normal conditions Ire1p is present in the ER membrane as a monomer in association with Kar2/Bip. Upon ER stress, in a first step, Kar2 dissociates from Ire1p which causes clustering of Ire1p in the ER membrane. In a second step, direct interaction of the unfolded protein with a stress sensing region of the Ire1p orients the cytosolic effector domains [4]. Clustering causes transautophosphorylation of the kinase domain (K) and simultaneous activation of the endoribonuclease (R) activity. Activation of Ire1p initiates an unconventional mRNA splicing reaction, which removes an intron from a unique mRNA species, HAC1, which encodes for an active transcription factor. Hac1p activates target genes coding for chaperones, foldases, lipid synthesis etc.Back to article page