BackgroundMembrane protein research is frequently hampered by the low natural abundance of these proteins in cells and typically relies on recombinant gene expression. Different expression systems, like mammalian cells, insect cells, bacteria and yeast are being used, but very few research efforts have been directed towards specific host cell customization for enhanced expression of membrane proteins. Here we show that by increasing the intracellular membrane production by interfering with a key enzymatic step of lipid synthesis, enhanced expression of membrane proteins in yeast is achieved.ResultsWe engineered the oleotrophic yeast, Yarrowia lipolytica, by deleting the phosphatidic acid phosphatase, PAH1, which led to massive proliferation of endoplasmic reticulum (ER) membranes. For all eight tested representatives of different integral membrane protein families, we obtained enhanced protein accumulation levels and in some cases enhanced proteolytic integrity in the ∆pah1 strain. We analysed the adenosine A2AR G-protein coupled receptor case in more detail and found that concomitant induction of the unfolded protein response in the ∆pah1 strain enhanced the specific ligand binding activity of the receptor. These data indicate an improved quality control mechanism for membrane proteins accumulating in yeast cells with proliferated ER.ConclusionsWe conclude that redirecting the metabolic flux of fatty acids away from triacylglycerol- and sterylester-storage towards membrane phospholipid synthesis by PAH1 gene inactivation, provides a valuable approach to enhance eukaryotic membrane protein production. Complementary to this improvement in membrane protein quantity, UPR co-induction further enhances the quality of the membrane protein in terms of its proper folding and biological activity. Importantly, since these pathways are conserved in all eukaryotes, it will be of interest to investigate similar engineering approaches in other cell types of biotechnological interest, such as insect cells and mammalian cells.
Protein production in Pichia pastoris is often based on the methanol‐inducible P AOX1 promoter which drives the expression of the target gene. The use of methanol has major drawbacks, so there is a demand for alternative promoters with good induction properties such as the glucose‐regulated P GTH1 promoter which we reported recently. To further increase its potential, we investigated its regulation in more details by the screening of promoter variants harboring deletions and mutations. Thereby we could identify the main regulatory region and important putative transcription factor binding sites of P GTH1. Concluding from that, yeast metabolic regulators, monomeric Gal4‐class motifs, carbon source‐responsive elements, and yeast GC‐box proteins likely contribute to the regulation of the promoter. We engineered a P GTH1 variant with greatly enhanced induction properties compared with that of the wild‐type promoter. Based on that, a model‐based bioprocess design for high volumetric productivity in a limited time was developed for the P GTH1 variant, to employ a glucose fed‐batch strategy that clearly outperformed a classical methanol fed‐batch of a P AOX1 strain in terms of titer and process performance.
Membrane protein research is still hampered by the generally very low levels at which these proteins are naturally expressed, necessitating heterologous expression. Protein degradation, folding problems, and undesired post-translational modifications often occur, together resulting in low expression levels of heterogeneous protein products that are unsuitable for structural studies. We here demonstrate how the integration of multiple engineering modules in Pichia pastoris can be used to increase both the quality and the quantity of overexpressed integral membrane proteins, with the human CXCR4 G-protein coupled receptor as an example. The combination of reduced proteolysis, enhanced ER folding capacity, GlycoDelete-based N-Glycan trimming, and nanobody-based fold stabilization improved the expression of this GPCR in P. pastoris from a low expression level of a heterogeneously glycosylated, proteolyzed product to substantial quantities (2-3 mg/L shake flask culture) of a nonproteolyzed, homogeneously glycosylated proteoform. We expect that this set of tools will contribute to successful expression of more membrane proteins in a quantity and quality suitable for functional and structural studies.
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