This is more challenging with a subset of the Taxol P450s that are predicted to have multiple transmembrane domains with an uncharacterized ER-binding mechanism. Modifying this interaction without disrupting the internal loop structures and catalytic activity of the protein could prove quite difficult. Another aspect of P450 function is often determined by peripheral protein-protein interactions in the native host. Many highly modified plant natural products are synthesized by a collection of enzymes in complex along the ER membrane called metabolons, with the intermediate substrates shuttled from one reaction center to the next. These highly optimized systems often involve non-catalytic “scaffold” proteins that act as a foundation for the binding and assembly of numerous enzymes. Examining the expression and localization dynamics of Taxol-associated P450s revealed a deficiency in ER localization when tested in their native form in yeast. Expression was seen for all the P450s investigated, but they localize to the cytosol. As mentioned earlier, functionality for the vast majority of P450s from plants are dependent on proper ER-anchoring and redox coupling with additional ER-bound enzymes. We sought to modify the localization of Taxol P450s in yeast by substituting the native ER-anchor region for a sequence previously shown to result in proper ER localization in yeast. Structure modeling and transmembrane domain prediction was carried out for six of the Taxol P450s using the PHYRE2 platform. In general, the first transmembrane domain of a P450 acts as the ER-anchor region. We therefore selected the first amino acid after the predicted transmembrane domain for each P450s as a truncation point. The functional ER-anchor sequence was used to replace the truncated region along with the same linker sequence 264 used for the fluorescent reporter protein fusions.
The linker sequence was added with the aim of improving ER localization by introducing flexibility at the fusion site. Unfortunately,rolling hydro tables even with the modified anchor region the P450s fail to properly localize to the ER . Another potentiality for improving the localization and function of multiple P450s is engineering protein scaffolds that promote assembly at the ER membrane. Membrane steroid binding proteins are a class of protein that are commonly involved with biosynthetic pathways in plants. Furthermore, these are often required for functional biosynthesis, exemplified by the lignin biosynthetic pathway involved in secondary cell wall formation. Biosynthesis of the lignin polymer is highly regulated, multi-branched, and includes several P450s requiring a MSBP for complex assembly. By identifying transmembrane domain anchors and MSBPs with a high affinity for interaction, it may be possible to generate pseudo-metabolons when engineering plant natural product biosynthetic pathways into yeast, specifically those that require numerous ER bound enzymes. To test the potential of this application, we selected a MSBP to test in yeast with a previously characterized fusion protein confirmed for functional ER localization . Fluorescent reporters were used to observe the expression and localization of both MSBP and ER-CslG simultaneously to examine co-localization patterns . ER-CslG:eGFP showed proper ER localization patterns in most of the cells observed , while MSBP:mCherry showed varied patterns of localization . It is important to note that there are potential unknown interactions between MSBP and the mCherry fusion protein. In the cells that showed high levels of co-localization though, robust accumulation of both MSBP and ER-CslG can be seen along the ER envelope of the nucleus . The pattern observed seems to represent a nucleation reaction, with co-localization initiation leading to the accumulation of both MSBP and CslG at high concentrations.
This observation exemplifies a prospective method to promote the accumulation and optimum localization of plant derived P450s, as well as providing a secondary scaffold for the “assembly” of multiple P450s simultaneously. While more optimizations would be needed to integrate numerous enzymes into this engineering scheme, the potential of forming pseudo-metabolons could be a favorable approach when engineering bio-synthetic pathways of plant natural products like Taxol in yeast. They are commonly found in plants from the nightshade family, genus Solanum, including tomato, eggplant, and potato. Yeast provides a relatively unexplored opportunity for the bio-production of steroidal alkaloids in a heterologous host. As yeast does not natively produce cholesterol, the primary substrate for steroidal alkaloids, very limited efforts have been made to explore this molecular space for bio-production. Interrupting ergosterol production and shunting zymosterol to cholesterol can be achieved by genetically exchanging ERG5 and ERG6 with DHCR7 and DHCR24267 . Additionally, the substitution of cholesterol for ergosterol in the cell membrane results in limited disruption on yeast growth and development, making this a plausible system to explore. This “humanized” yeast system has been used in studies of mammalian cell-surface receptors and transporters in the past but harnessing this chassis for the bio-production of plant natural products has untapped economic potential268 . Further modifications could be made to optimize cholesterol overproduction and bio-availability, to ultimately produce a S. cerevisiae strain engineered specifically for the biosynthesis of cholesterol derivatives with diverse structures and applications. To explore the potential of yeast as a platform for steroidal alkaloids production, we engineered a cholesterol producing strain using CEN.PK as our parent line. DHCR7 and DHCR24 from humans were selected based on previous work on cholesterol biosynthesis.
Sequential gene substitution of the native ERG5 and ERG6 genes was carried out via homologous recombination. pADH1-DHCR7 was used to replace ERG5 and its native promoter with pADH1-DCHR24 replacing ERG6 and its native promoter, gene substitutions were performed at the native loci. One deficiency of “humanized” yeast is the capacity for acid transport, as cholesterol substitution disrupts endogenous acid transporters. This was observed when performing counter selection for the URA auxotrophic marker used in cloning, which utilizes plating on 5-FOA . This acidic medium, in conjunction with the 5-FOA counter selection , caused a drastic lag in growth for the final strain with colonies appearing more than ten days after plating. Multiple colonies that cleared PCR and sequencing screens were then selected for GCMS analysis of cholesterol production against an analytical standard. Interestingly, two strains were confirmed for cholesterol production but have varied product profiles. Chl24-20 was the top producing strain in this initial screen, with cholesterol being one of two major products in the GCMS analysis. While Chl75-10 did produce cholesterol, it was a minor product, with two other major peaks present . These were not structurally identified but based on MS spectrum analysis are hypothesized to be structurally similar to cholesterol with variation in the number of C-C double bonds. Chl24-20 was selected as our production strain for downstream analysis. While final cell densities of experimental strains and the control strain were equivalent at 72 hours, there was an observable lag in growth for the experimental strains during the first 36 hours of production. This is most likely due to sub-optimum cholesterol production along with peripheral effects of sterol substitution in the developing membranes of dividing yeast. After confirmation of cholesterol production in Chl24-20 with an analytical standard, we used GCMS analysis of Chl24-20 culture extract and a series of five dilutions of cholesterol standard in ethyl acetate, which is the same final solvent used for extraction . These data were then used to generate a standard curve for cholesterol to quantify production in Chl24-20,vertical horticulture which was calculated to be ~128.4nM with our extraction method . Numerous steroidal alkaloids have implications in human health and research including cyclopamine, α-tomatine, and α-solanine. The interest in α-tomatine is due to its function as an antimicrobial that protects the plant against fungi as well as herbivores270. Its broad-range mechanism of action, which involves disruption of cellular membranes as well as inhibition of acetylcholinesterase, makes it a great candidate for further development of antibiotics. α-solanine, from potato, has a similar membrane disruption effect, but is different in its capacity to interact with mitochondrial membranes. It acts as a poison by disrupting the mitochondrial membrane potential leading to a flood of calcium ions into the cytoplasm causing cell damage/death. Cyclopamine is of particular interest due to its interaction with Hedgehog signaling, which is involved with an array of developmental processes including embryonic development and tumorigenesis. Much like Taxol, it has profound implications for its capacity to inhibit tumor growth as a treatment for a variety of cancers, though the mechanism of action is quite different. Cyclopamine is a steroidal alkaloid derived from cholesterol that does not require glycosylation, making it a great target to test the potential of our steroidal alkaloid yeast platform. Verazine is an intermediate in cyclopamine production that has a well characterized pathway requiring three P450s and an aminotransferase. The conversion of verazine to cyclopamine then only requires two additional reactions that are currently being charactered . What is exciting about this pathway is the potential to generate novel glycosylated steroidal alkaloids with the introduction of GTs from either saponin, tomatine, or solanine biosynthesis for the glycosylation of the 3C position .
Recent advances in engineering nucleotide sugar pathways into yeast, coupled with the promiscuous nature of plant derived GTs, provides an opportunity for the biosynthesis of glycosylated steroidal alkaloids. Solanaceous plants produce a variety of these molecules, with diverse physiological applications and effects and therefore have an abundance of genetic material that can be implemented in yeast. The potential to produce novel glycosylated molecules is especially promising, for example glycosylated cyclopamine derivatives, with the introduction of GTs that glycosylate similar structures such as saponins. These new molecules have a greater degree of structural diversity and may provide enhanced, altered, or even novel properties. This specifically demonstrates the utility of synthetic yeast biology as a tool to produce “new-to-nature” molecules not possible with classic organic chemistry or extraction from plants. While further optimizing the production of free cholesterol, we can begin to introduce key nucleotide sugar pathways that produce the substrate for GT reactions, allowing for pathways requiring glycosylation to be investigated. Engineering a yeast platform specifically for the bio-production of glycosylated steroidal alkaloids would be an excellent resource for future studies on these complex and derivatized molecules.While much of the work discussed is varied in its approach or function, its core follows the reductionist theme of synthetic biology, aiming to rebuild living systems for specific applications in biotechnology. Whether engineering a fungus or a plant, the concepts applied remain the same. The amazing diversity of biological systems present on earth are a canvas for synthetic biologists, providing an almost infinite source of genetic information that can be reengineered and reshaped to solve challenging problems. In my opinion, synthetic biology is still in its infancy, but as we collectively increase our knowledge and understanding of the core tenets of life, we will see the true power of synthetic biology emerge. Even with these limits on our current understanding, we are transforming the old industrial economic model into a bio-based economic model. Opportunities for ambitious and clever engineers are abundant, and I predict an exponential increase in the development of novel synthetic systems in the coming decade. So much untapped potential makes this an exciting era for biotechnology, with the prospect for application of bio-based solutions across all sectors. Eventually, the integration of biological systems with digital systems will exponentially expand the possibilities, as electrical and biological process can be intertwined to generate platforms we have yet to imagine, all made possible with synthetic biology. Observing patterns in retail prices is fund a mental for understanding the economics of any agricultural consumer product. The study of cannabis retail prices, like the study of other economic aspects of the cannabis industry, is fraught with difficulty, in part because cannabis remains a Schedule I narcotic under U.S. federal law. Consumer price indexes, tax records, commercial retail scanner data, industry association reports and other sources of data typically available for agricultural products such as wine, almonds and cut flowers are unavailable for cannabis. Cannabis retailers have limited access to banking services; most cannabis retail transactions are conducted in cash; and cannabis businesses are under standably reluctant to share their financial data. There is a need for better information about all aspects of the cannabis industry, including prices and price patterns. In this article, we aim to contribute to the scant literature on cannabis retail prices by describing the basic patterns of price ranges at retailers in California over a 21-month time span during which the industry under went a series of significant regulatory changes. Several times between October 2016 and July 2018, researchers at the UC Agricultural Issues Center gathered cannabis retail prices published on Weedmaps, a leading online cannabis retail platform.