Use of tandem affinity chromatography for purification of cannabinoid receptor CB₂

Recombinant Expression and Purification of Cannabinoid Receptor CB2, a G Protein-Coupled Receptor

G protein-coupled receptors (GPCR) are integral membrane proteins that regulate multiple cellular processes. To obtain insights into structural properties of GPCR and mechanism of activity, these proteins should be isolated in significant (milligram) quantities, in a pure, homogenous, and stable form. Here we describe the expression and purification of type II human cannabinoid receptor CB₂, a class A GPCR, in two different types of expression hosts: in Escherichia coli and in mammalian suspension cell culture Expi293. Our method allows preparation of milligram quantities of the purified receptors suitable for a wide array of downstream applications including high-resolution structural studies and functional assays.

Overproduction of Human Zip (SLC39) Zinc Transporters in Saccharomyces cerevisiae for Biophysical Characterization

Zinc constitutes the second most abundant transition metal in the human body, and it is implicated in numerous cellular processes, including cell division, DNA and protein synthesis as well as for the catalytic activity of many enzymes. Two major membrane protein families facilitate zinc homeostasis in the animal kingdom, i.e., Zrt/Irt-like proteins (ZIPs aka solute carrier 39, SLC39, family) and Zn transporters (ZnTs), essentially conducting zinc flux in the opposite directions. Human ZIPs (hZIPs) regulate import of extracellular zinc to the cytosol, being critical in preventing overaccumulation of this potentially toxic metal, and crucial for diverse physiological and pathological processes, including development of neurodegenerative disorders and several cancers. To date, our understanding of structure-function relationships governing hZIP-mediated zinc transport mechanism is scarce, mainly due to the notorious difficulty in overproduction of these proteins for biophysical characterization. Here we describe employment of a Saccharomyces cerevisiae-based platform for heterologous expression of hZIPs. We demonstrate that yeast is able to produce four full-length hZIP members belonging to three different subfamilies. One target (hZIP1) is purified in the high quantity and homogeneity required for the downstream biochemical analysis. Our work demonstrates the potential of the described production system for future structural and functional studies of hZIP transporters.

Application of Strep-Tactin XT for affinity purification of Twin-Strep-tagged CB2, a G protein-coupled cannabinoid receptor

Human cannabinoid receptor CB2 belongs to the class A of G protein-coupled receptor (GPCR). CB2 is predominantly expressed in membranes of cells of immune origin and is implicated in regulation of metabolic pathways of inflammation, neurodegenerative disorders and pain sensing. High resolution structural studies of CB2 require milligram quantities of purified, structurally intact protein. While we previously reported on the methodology for expression of the recombinant CB2 and its stabilization in a functional state, here we describe an efficient protocol for purification of this protein using the Twin-Strep-tag/Strep-Tactin XT system. To improve the affinity of interaction of the recombinant CB2 with the resin, the double repeat of the Strep-tag (a sequence of eight amino acids WSHPQFEK), named the Twin-Strep-tag was attached either to the N- or C-terminus of CB2 via a short linker, and the recombinant protein was expressed in cytoplasmic membranes of E. coli as a fusion with the N-terminal maltose binding protein (MBP). The CB2 was isolated at high purity from dilute solutions containing high concentrations of detergents, glycerol and salts, by capturing onto the Strep-Tactin XT resin, and was eluted from the resin under mild conditions upon addition of biotin. Surface plasmon resonance studies performed on the purified protein demonstrate the high affinity of interaction between the Twin-Strep-tag fused to the CB2 and Strep-Tactin XT with an estimated Kd in the low nanomolar range. The affinity of binding did not vary significantly in response to the position of the tag at either N- or C-termini of the fusion. The binding capacity of the resin was several-fold higher for the tag located at the N-terminus of the protein as opposed to the C-terminus- or middle of the fusion. The variation in the length of the linker between the double repeats of the Strep-tag from 6 to 12 amino acid residues did not significantly affect the binding. The novel purification protocol reported here enables efficient isolation of a recombinant GPCR expressed at low titers in host cells. This procedure is suitable for preparation of milligram quantities of stable isotope-labelled receptor for high-resolution NMR studies.