Partitioning of the mevalonate synthesizing enzymes at ER‐lysosome contacts influences their catalytic efficiency

Abstract

The last decade of molecular biology is hallmarked by an extraordinary increase in our understanding of previously unappreciated spatial scales in biology. Enzymes and metabolites are partitioned into distinct membrane-bound organelles to spatially regulate metabolism and signaling. More recent discoveries have uncovered that enzymes can also inducibly or permanently compartmentalize within sub-organelle domains such as inter-organelle contact sites (1). However, a key knowledge gap is understanding role that enzyme partitioning plays on regulating enzyme function or pathway flux. Here, we performed a GFP-based screen in Saccharomyces cerevisiae to identify metabolic enzymes that may enrich at sub-organelle domains. We found that during Acute Glucose Restriction (AGR) the rate-limiting enzymes of sterol biogenesis, Hmg1 and Hmg2, partition at the Nucleus-Vacuole Junction (NVJ), an inter-organelle contact site between the nuclear envelope and yeast vacuole. NVJ partitioning of Hmg1/2 correlates with increased de-novo synthesis of sterol-esters. Additionally, Hmg1/2 partitioning can be blocked by deletion of the NVJ tether protein Nvj1. Remarkably, Hmg1/2 is less catalytically efficient in Nvj1 knock-out cells, suggesting a potential link between its spatial compartmentalization and increased enzymatic activity. We further demonstrate that Nvj1-mediated partitioning of Hmg1/2 during AGR contributes to cellular fitness, and coincides with ability to resume growth following glucose starvation. Overall, we have uncovered a previously underappreciated level of spatial regulation for Hmg1/2, and also identified a novel role for inter-organelle contact sites in the regulation of mevalonate metabolism.