Regulated targeting of the monotopic hairpin membrane protein Erg1 requires the GET pathway

The constitutively active form of a key cholesterol synthesis enzyme is lipid droplet-localized and upregulated in endometrial cancer tissues

Cholesterol is essential for both normal cell viability and cancer cell proliferation. Aberrant activity of squalene monooxygenase (SM, also known as squalene epoxidase), the rate-limiting enzyme of the committed cholesterol synthesis pathway, is accordingly implicated in a growing list of cancers. We previously reported that hypoxia triggers the truncation of SM to a constitutively active form, thus preserving sterol synthesis during oxygen shortfalls. Here, we show SM truncation is upregulated and correlates with the magnitude of hypoxia in endometrial cancer tissues, supporting the in vivo relevance of our earlier work. To further investigate the pathophysiological consequences of SM truncation, we examined its lipid droplet-localized pool using complementary immunofluorescence and cell fractionation approaches and found that it exclusively comprises the truncated enzyme. This partitioning is facilitated by the loss of an endoplasmic reticulum-embedded region at the SM N terminus, whereas the catalytic domain containing membrane-associated C-terminal helices is spared. Moreover, we determined multiple amphipathic helices contribute to the lipid droplet localization of truncated SM. Taken together, our results expand on the striking differences between the two forms of SM and suggest upregulated truncation may contribute to SM-related oncogenesis.

Efficient Plant Triterpenoids Synthesis in Saccharomyces cerevisiae: from Mechanisms to Engineering Strategies

Triterpenoids possess a range of biological activities and are extensively utilized in the pharmaceutical, food, cosmetic, and chemical industries. Traditionally, they are acquired through chemical synthesis and plant extraction. However, these methods have drawbacks, including high energy consumption, environmental pollution, and being time-consuming. Recently, the de novo synthesis of triterpenoids in microbial cell factories has been achieved. This represents a promising and environmentally friendly alternative to traditional supply methods. Saccharomyces cerevisiae, known for its robustness, safety, and ample precursor supply, stands out as an ideal candidate for triterpenoid biosynthesis. However, challenges persist in industrial production and economic feasibility of triterpenoid biosynthesis. Consequently, metabolic engineering approaches have been applied to improve the triterpenoid yield, leading to substantial progress. This review explores triterpenoids biosynthesis mechanisms in S. cerevisiae and strategies for efficient production. Finally, the review also discusses current challenges and proposes potential solutions, offering insights for future engineering.

Quantitative Mass Spectrometry Characterizes Client Spectra of Components for Targeting of Membrane Proteins to and Their Insertion into the Membrane of the Human ER

To elucidate the redundancy in the components for the targeting of membrane proteins to the endoplasmic reticulum (ER) and/or their insertion into the ER membrane under physiological conditions, we previously analyzed different human cells by label-free quantitative mass spectrometry. The HeLa and HEK293 cells had been depleted of a certain component by siRNA or CRISPR/Cas9 treatment or were deficient patient fibroblasts and compared to the respective control cells by differential protein abundance analysis. In addition to clients of the SRP and Sec61 complex, we identified membrane protein clients of components of the TRC/GET, SND, and PEX3 pathways for ER targeting, and Sec62, Sec63, TRAM1, and TRAP as putative auxiliary components of the Sec61 complex. Here, a comprehensive evaluation of these previously described differential protein abundance analyses, as well as similar analyses on the Sec61-co-operating EMC and the characteristics of the topogenic sequences of the various membrane protein clients, i.e., the client spectra of the components, are reported. As expected, the analysis characterized membrane protein precursors with cleavable amino-terminal signal peptides or amino-terminal transmembrane helices as predominant clients of SRP, as well as the Sec61 complex, while precursors with more central or even carboxy-terminal ones were found to dominate the client spectra of the SND and TRC/GET pathways for membrane targeting. For membrane protein insertion, the auxiliary Sec61 channel components indeed share the client spectra of the Sec61 complex to a large extent. However, we also detected some unexpected differences, particularly related to EMC, TRAP, and TRAM1. The possible mechanistic implications for membrane protein biogenesis at the human ER are discussed and can be expected to eventually advance our understanding of the mechanisms that are involved in the so-called Sec61-channelopathies, resulting from deficient ER protein import.

Lipid saturation induces degradation of squalene epoxidase for sterol homeostasis and cell survival

A fluid membrane containing a mix of unsaturated and saturated lipids is essential for life. However, it is unclear how lipid saturation might affect lipid homeostasis, membrane-associated proteins, and membrane organelles. Here, we generate temperature-sensitive mutants of the sole fatty acid desaturase gene OLE1 in the budding yeast Saccharomyces cerevisiae Using these mutants, we show that lipid saturation triggers the endoplasmic reticulum-associated degradation (ERAD) of squalene epoxidase Erg1, a rate-limiting enzyme in sterol biosynthesis, via the E3 ligase Doa10-Ubc7 complex. We identify the P469L mutation that abolishes the lipid saturation-induced ERAD of Erg1. Overexpressed WT or stable Erg1 mutants all mislocalize into foci in the ole1 mutant, whereas the stable Erg1 causes aberrant ER and severely compromises the growth of ole1, which are recapitulated by doa10 deletion. The toxicity of the stable Erg1 and doa10 deletion is due to the accumulation of lanosterol and misfolded proteins in ole1 Our study identifies Erg1 as a novel lipid saturation-regulated ERAD target, manifesting a close link between lipid homeostasis and proteostasis that maintains sterol homeostasis under the lipid saturation condition for cell survival.

Proteomics Identifies Substrates and a Novel Component in hSnd2-Dependent ER Protein Targeting

Importing proteins into the endoplasmic reticulum (ER) is essential for about 30% of the human proteome. It involves the targeting of precursor proteins to the ER and their insertion into or translocation across the ER membrane. Furthermore, it relies on signals in the precursor polypeptides and components, which read the signals and facilitate their targeting to a protein-conducting channel in the ER membrane, the Sec61 complex. Compared to the SRP- and TRC-dependent pathways, little is known about the SRP-independent/SND pathway. Our aim was to identify additional components and characterize the client spectrum of the human SND pathway. The established strategy of combining the depletion of the central hSnd2 component from HeLa cells with proteomic and differential protein abundance analysis was used. The SRP and TRC targeting pathways were analyzed in comparison. TMEM109 was characterized as hSnd3. Unlike SRP but similar to TRC, the SND clients are predominantly membrane proteins with N-terminal, central, or C-terminal targeting signals.

From guide to guard-activation mechanism of the stress-sensing chaperone Get3

Oxidative stress conditions can cause ATP depletion, oxidative protein unfolding, and potentially toxic protein aggregation. To alleviate this proteotoxic stress, the highly conserved yeast protein, Get3, switches from its guiding function as an ATP-dependent targeting factor for tail-anchored proteins to its guarding function as an ATP-independent molecular chaperone that prevents irreversible protein aggregation. Here, we demonstrate that activation of Get3's chaperone function follows a tightly orchestrated multi-step process, centered around the redox status of two conserved cysteines, whose reactivity is directly controlled by Get3's nucleotide-binding state. Thiol oxidation causes local unfolding and the transition into chaperone-active oligomers. Vice versa, inactivation requires the reduction of Get3's cysteines followed by ATP-binding, which allows the transfer of bound client proteins to ATP-dependent chaperone systems for their effective refolding. Manipulating this fine-tuned cycle of activation and inactivation in yeast impairs oxidative stress resistance and growth, illustrating the necessity to tightly control Get3's intrinsic chaperone function.