Regulated accumulation of 3-hydroxy-3-methylglutaryl CoA reductase protein in potato cell cultures: effects of calcium and enzyme inhibitors

The biosynthetic pathway leading to plant cytosolic isoprenoids begins with the 3-hydroxy-3-methylglutaryl CoA reductase (HMGR)-mediated production of mevalonate (MVA). We describe a transgenic potato cell culture system expressing an epitope-tagged HMGR and use this to directly observe levels of an isoform of the protein in response to chemical stimuli. Addition of calcium to cell cultures results in a decrease in HMGR protein accumulation, whereas EGTA, a chelator of calcium, causes an increase in levels of HMGR observed on immunoblots. The HMGR protein also accumulates following cell treatment with the enzyme-specific inhibitor mevastatin, although this effect is overcome by the addition of MVA. The mevastatin-induced HMGR increases are possibly explained by the inhibition of a feedback-regulated down-regulation due to lowered pools of MVA or its products. Transcripts for the transgene controlled by a constitutive cauliflower mosaic virus promoter, and an endogenous HMGR-encoding gene are coordinately regulated in response to these same treatments, suggesting that the gene promoters are not responsible for the observed changes in transcript levels. The data suggest that potato HMGR is subject to many of the same post-transcriptional regulatory mechanisms as have been shown in animal and yeast systems.