Figure 5. Ypt1 negatively regulates HAC1u RNA expression in the absence of ER stress.

Under normal growth conditions (left panel). Ypt1 regulates HAC1 RNA expression by destabilizing the transcript. This is likely accomplished by recruiting HAC1 away from the ER and in proximity to decay factors. Ire1 and Ada5 are necessary for the interaction and could play an active role in recruiting HAC1 to the ER, thus facilitating the Ypt1-HAC1 association. Under UPR-induced conditions (right panel). The GTPase no longer interacts with HAC1. Instead, HAC1u is spliced by Ire1 and the mature HAC1i mRNA is translated to activate the ER stress response cascade. Dotted line denotes putative recruitment of HAC1u by Ire1; solid lines show established interactions. HAC1u = unspliced HAC1; HAC1i = spliced HAC1; ER = endoplasmic reticulum; UPR = unfolded protein response.