IDENTIFICATION OF THE SEQUENCES IN HMG-COA REDUCTASE REQUIRED FOR KARMELLAE ASSEMBLY

In all eukaryotic cells that have been examined, specific membrane arr ays are induced in response to increased levels of the ER membrane pro tein, HMG-CoA reductase. Analysis of these inducible membranes has the potential to reveal basic insights into general membrane assembly. Ye ast express two HMG-CoA reductase isozymes, and each isozyme induces a morphologically distinct proliferation of the endoplasmic reticulum. The isozyme encoded by HMG1 induces karmellae, which are long stacks o f membranes that partially enclose the nucleus. In contrast, the isozy me encoded by HMG2 induces short stacks of membrane that may be associ ated with the nucleus, but are frequently present at the cell peripher y. To understand the molecular nature of the different cellular respon ses to Hmg1p and Hmg2p, we mapped the region of Hmg1p that is needed f or karmellae assembly. For this analysis, a series of exchange alleles was examined in which a portion of the Hmg2p membrane domain was repl aced with the corresponding Hmg1p sequences. Results of this analysis indicated that the ER lumenal loop between predicted transmembrane dom ains 6 and 7 was both necessary and sufficient for karmellae assembly, when present in the context of an HMG-CoA reductase membrane domain. Immunoblotting experiments ruled out the simple possibility that diffe rences in the amounts of the various chimeric HMG-CoA reductase protei ns was responsible for the altered cellular responses. Our results are consistent with the hypothesis that each yeast isozyme induces or org anizes a qualitatively different organization of ER membrane.