Taking down the FLAG! How insect cell expression challenges an established tag-system.

In 1988 the preceding journal of Nature Biotechnology, Bio/Technology, reported a work by Hopp and co-workers about a new tag system for the identification and purification of recombinant proteins: the FLAG-tag. Beside the extensively used hexa-his tag system the FLAG-tag has gained broad popularity due to its small size, its high solubility, the presence of an internal Enterokinase cleavage site, and the commercial availability of high-affinity anti-FLAG antibodies. Surprisingly, considering the heavy use of FLAG in numerous laboratories world-wide, we identified in insect cells a post-translational modification (PTM) that abolishes the FLAG-anti-FLAG interaction rendering this tag system ineffectual for secreted proteins. The present publication shows that the tyrosine that is part of the crucial FLAG epitope DYK is highly susceptible to sulfation, a PTM catalysed by the enzyme family of Tyrosylprotein-Sulfo-transferases (TPSTs). We showed that this modification can result in less than 20% of secreted FLAG-tagged protein being accessible for purification questioning the universal applicability of this established tag system.

Total synthesis of the potent anticancer Aglaia metabolites (-)-silvestrol and (-)-episilvestrol and the active analogue (-)-4'-desmethoxyepisilvestrol.

Total synthesis of the anticancer 1,4-dioxane containing natural products silvestrol (1) and episilvestrol (2) is described by an approach based on the proposed biosynthesis of these novel compounds. The key steps included an oxidative rearrangement of the protected d-glucose derivative 11 to afford the 1,4-dioxane 12, which could be elaborated to the coupling partner 5 and a photochemical [3 + 2]-cycloadditon between the 3-hydroxyflavone 27 and methyl cinnamate followed by base-induced alpha-ketol rearrangement and reduction to give the cyclopentabenzofuran core 33. The core (-)-6 and 1,4-dioxane fragment 5 were united by a highly stereoselective Mitsunobu coupling with the modified azodicarboxylate DMEAD to afford the axial coupled product 36. Deprotection then gave episilvestrol (2). Silvestrol (1) was synthesized by a coupling between core (-)-6 and the dioxane 44 followed by deprotection. Compound 1 was also synthesized from episilvestrol (2) by a Mitsunobu inversion. In addition, the analogue 4'-desmethoxyepisilvestrol (46) was synthesized via the same route. It was found that 46 and episilvestrol 2 displayed an unexpected concentration-dependent chemical shift variation for the nonexchangeable dioxane protons. Synthetic compounds 1, 2, 38, 46, and 54 were tested against cancer cells lines, and it was found that the stereochemistry of the core was critical for activity. Synthetic analogue 4'-desmethoxyepisilvestrol (46) was also active against lung and colon cancer cell lines.

Recombinant human zona pellucida proteins ZP1, ZP2 and ZP3 co-expressed in a human cell line.

AIM: To produce biologically active recombinant human (rh) ZP proteins in a human cell for use in sperm function tests. METHODS: The human embryonic kidney cell line 293T was employed to produce rhZP1, rhZP2 and rhZP3 proteins individually and together by co-expression. Presence of these proteins in the culture medium and cell lysate was assessed by Western blotting analysis. The effect of the recombinant proteins on the human AR was assessed. RESULTS: RhZP2 and rhZP3 were secreted into the culture medium, whereas rhZP1 was found only in the cell lysate. Interestingly, when all zona pellucida proteins were co-expressed in the same cells, rhZP1 was also secreted into the culture medium. However, despite the presence of all three ZP proteins in sufficient concentration and evidence of heavy glycosylation on gel electrophoresis, biological activity to induce the AR was not observed. CONCLUSION: RhZP1, rhZP2 and rhZP3 were successfully expressed in the human embryonic kidney cell line 293T. It appears that an interaction amongst these proteins may be required for release of rhZP1 from the cell. Although this approach is not satisfactory for producing active human ZP proteins, it makes a significant contribution to the understanding of the structural and functional characteristics of the ZP proteins.