INDUCIBLE MEMBRANES IN YEAST - RELATION TO THE UNFOLDED-PROTEIN-RESPONSE PATHWAY

Overproduction of an endoplasmic reticulum (ER)-resident membrane prot ein (cytochrome P450 52A3) and of a secretory protein (invertase) was used to study the regulation of the luminal ER protein Kar2p under con ditions that lead to ER proliferation and secretory overload, respecti vely. In both cases we found (i) a significant increase of Kar2 protei n and mRNA levels, (ii) a transcriptional regulation based on the func tion of the 22 bp unfolded-protein-response element of the KAR2 promot er and (iii) an essential role of the transmembrane kinase Irelp for u pregulation of KAR2 gene expression. These results show that the same mechanism operates when KARZ induction is triggered by overproduction of cytochrome P450 or invertase and that this mechanism shares the kno wn features of the unfolded-protein-response pathway. Disruption of th e IREI gene resulted in a marked decrease of the invertase protein lev els produced. In contrast, a functional IREI gene was not required to reach high-level production of the integral membrane protein cytochrom e P450 52A3. Moreover, IREI gene disruption did not cytochrome P450 52 A3 overproduction, a process which follows on ER proliferation, thereb y monitoring the increase of ER size and adjusting the level of Kar2p accordingly. (C) 1997 John Wiley & Sons, Ltd.