Transfection of insect cell in suspension for efficient baculovirus generation

Targeted protein degradation via intramolecular bivalent glues

Targeted protein degradation is a pharmacological modality that is based on the induced proximity of an E3 ubiquitin ligase and a target protein to promote target ubiquitination and proteasomal degradation. This has been achieved either via proteolysis-targeting chimeras (PROTACs)-bifunctional compounds composed of two separate moieties that individually bind the target and E3 ligase, or via molecular glues that monovalently bind either the ligase or the target1-4. Here, using orthogonal genetic screening, biophysical characterization and structural reconstitution, we investigate the mechanism of action of bifunctional degraders of BRD2 and BRD4, termed intramolecular bivalent glues (IBGs), and find that instead of connecting target and ligase in trans as PROTACs do, they simultaneously engage and connect two adjacent domains of the target protein in cis. This conformational change 'glues' BRD4 to the E3 ligases DCAF11 or DCAF16, leveraging intrinsic target-ligase affinities that do not translate to BRD4 degradation in the absence of compound. Structural insights into the ternary BRD4-IBG1-DCAF16 complex guided the rational design of improved degraders of low picomolar potency. We thus introduce a new modality in targeted protein degradation, which works by bridging protein domains in cis to enhance surface complementarity with E3 ligases for productive ubiquitination and degradation.

A fast-track protocol for protein expression using the BEV system

Baculovirus-insect cell expression (BEV) has become one of the most widely used eukaryotic systems for heterologous protein expression. The combination of engineered baculovirus genomes together with a variety of compatible vectors, robust insect cell lines, serum-free media and commercial kits have made it a standard workhorse in many "non-virology-expert" laboratories. Despite these significant improvements, the BEV system still has major drawbacks, primarily the time required to amplify recombinant virus and its inherent instability. Here we present an easy-to-adopt simplified and shortened protocol.

An Improved Strategy for Fluorescent Tagging of Membrane Proteins for Overexpression and Purification in Mammalian Cells

An essential prerequisite for in vitro biochemical or structural studies is a construct that is amenable to high level expression and purification and is biochemically "well-behaved". In the field of membrane protein research, the use of green fluorescent protein (GFP) to monitor and optimize the heterologous expression in different hosts has radically changed the ease of streamlining and multiplexing the testing of a large number of candidate constructs. This is achieved by genetically fusing the fluorescent proteins to the N- or C-terminus of the proteins of interest to act as reporters which can then be followed by methods such as microscopy, spectroscopy, or in-gel fluorescence. Nonetheless, a systematic study on the effect of GFP and its spectral variants on the expression and yields of recombinant membrane proteins is lacking. In this study, we genetically appended four common fluorescent protein tags, namely, mEGFP, mVenus, mCerulean, and mCherry, to the N- or C-terminus of different membrane proteins and assessed their expression in mammalian cells by fluorescence-detection size exclusion chromatography (FSEC) and protein purification. We find that, of the four fluorescent proteins, tagging with mVenus systematically results in higher expression levels that translates to higher yields in preparative purifications, thus making a case for switching to this yellow spectral variant as a better fusion tag.

Insight into partial agonism by observing multiple equilibria for ligand-bound and Gs-mimetic nanobody-bound β1-adrenergic receptor

A complex conformational energy landscape determines G-protein-coupled receptor (GPCR) signalling via intracellular binding partners (IBPs), e.g., G_s and β-arrestin. Using ¹³C methyl methionine NMR for the β₁-adrenergic receptor, we identify ligand efficacy-dependent equilibria between an inactive and pre-active state and, in complex with G_s-mimetic nanobody, between more and less active ternary complexes. Formation of a basal activity complex through ligand-free nanobody–receptor interaction reveals structural differences on the cytoplasmic receptor side compared to the full agonist-bound nanobody-coupled form, suggesting that ligand-induced variations in G-protein interaction underpin partial agonism. Significant differences in receptor dynamics are observed ranging from rigid nanobody-coupled states to extensive μs-to-ms timescale dynamics when bound to a full agonist. We suggest that the mobility of the full agonist-bound form primes the GPCR to couple to IBPs. On formation of the ternary complex, ligand efficacy determines the quality of the interaction between the rigidified receptor and an IBP and consequently the signalling level.

β₁-adrenergic receptors are expressed in cardiac tissue and stimulated by the sympathetic nervous system. Here, the authors use NMR spectroscopy to unravel the conformational diversity upon β₁-adrenergic receptor activation and provide structural insights into partial agonism and basal activity.

A new single-step protocol for rapid baculovirus-driven protein production in insect cells

Background

In the last three decades, the Baculovirus expression vector system (BEV) has evolved to one of the most widely used eukaryotic systems for heterologous protein expression including approved vaccines and therapies. Despite the significant improvements introduced during the past years, the BEV system still has major drawbacks, primarily the time required to generate recombinant virus and virus instability for certain target proteins. In this study we show that the conventional method to generate recombinant Baculovirus using a Tn7 transposition based system can be shortened to a single-step transfection-only procedure without further amplification.

Methods

In a first step we have adapted a recently published protocol that replaces the standard liposome-based transfection procedure of adherent insect cells by transfecting insect cells in suspension with a preformed DNA-PEI complex generating P0 virus. We have then expressed and purified six different target proteins, among them four intracellular and two secreted proteins, by infecting insect cells either with P0 or P1 virus.

Results

We demonstrate that transfection in suspension is as efficient as the standard protocol, but in addition allows generation of high amounts of P0 virus early in the process. To test if this P0 virus generated by bacmid transfection can be used directly for protein expression in either the screening or production process, we compared P0 versus amplified P1 virus-mediated protein expression. We show that protein expression levels, purity and yield of the purified proteins are equally high for P0 and P1.

Conclusion

The standard protocol for generating recombinant baculovirus comprises transfection of the bacmid followed by one or two subsequent virus amplification steps. In this study we show that Baculovirus generated by transfection-only is equally efficient in driving protein expression. This reduces the time from bacmid DNA to protein to eight days and reduces the risk of virus decay. In contrast to transient gene expression protocols, the required amount of DNA is minimal: 100 µg bacmid DNA is sufficient for a production scale of 10 L.