The role of the 3-hydroxy 3-methylglutaryl coenzyme A reductase cytosolic domain in karmellae biogenesis

In all cells examined, specific endoplasmic reticulum (ER) membrane arrays are induced in response to increased levels of the ER membrane protein 3-hy droxy 3-methylglutaryl coenzyme A (HMG-CoA) reductase. In yeast, expression of Hmg1p, one of two yeast HMG-CoA reductase isozymes, induces assembly of nuclear-associated ER stacks called karmellae. Understanding the features of HMG-CoA reductase that signal karmellae biogenesis would provide useful insights into the regulation of membrane biogenesis. The HMG-CoA reductase protein consists of two domains, a multitopic membrane domain and a cytosol ic catalytic domain. Previous studies had indicated that the HMG-CoA reduct ase membrane domain was exclusively responsible for generation of ER membra ne proliferations. Surprisingly, we discovered that this conclusion was inc orrect: sequences at the carboxyl terminus of HMG-CoA reductase can profoun dly affect karmellae biogenesis. Specifically, truncations of Hmg1p that re moved or shortened the carboxyl terminus were unable to induce karmellae as sembly. This result indicated that the membrane domain of Hmg1p was not suf ficient to signal for karmellae assembly. Using P-galactosidase fusions, we demonstrated that the carboxyl terminus was unlikely to simply serve as an oligomerization domain. Our working hypothesis is that a truncated or misf olded cytosolic domain prevents proper signaling for karmellae by interferi ng with the required tertiary structure of the membrane domain.