Ideally use cell membranes or soluble recombinant receptors but whole cells can be used as a receptor source. For membrane preparations, PerkinElmer offers the Membrane Target Systems® which are quality assured frozen membranes from cells that express recombinant or endogenous receptors exclusively from PerkinElmer.
Go to: www.perkinelmer.com/gpcrmembranes for a full list of membranes and cell lines.
SELECT-A-BEAD KIT (RPNQ0250)
WGA SPA beads (RPNQ0001, RPNQ0011, RPNQ0003, RPNQ0004) bind glycoproteins and glycolipids in cell membranes. 1 mg of beads will bind 0–30 μg membrane protein.
Polylysine SPA beads (RPNQ0010) bind negatively charged membranes. 1 mg of beads will bind up to 10 μg of membrane protein.
Antibody capture SPA beads (Protein A-coated beads RPNQ0019 or RPN143) can be used in conjunction with a specific antibody to capture soluble and solubilized receptors. They can also be used to capture Ig-fusion proteins.
Streptavidin-coated SPA beads (RPNQ007 or RPNQ0012) can be used to capture biotinylated receptor protein. 1 mg of beads will capture at least 100 pmol of biotin.
Affinity of ligand binding determines the radiolabel choice.
For studies where the receptor density is high, either 3H or 125I-labeled radioligands may be used if the affinity for the receptor is also high (< 10 mM). But, if the affinity of the ligand for the receptor is low (in the range of 10 nM – 10 μM), 125I is probably the radioisotope of choice for labeling.
RECEPTOR EXPRESSION LEVELS
If using 3H-labeled ligands with low specific activity (20 – 80 Ci/mmol) then higher expression levels are required (500,000 receptors per cell) corresponding to densities greater than 2 pmoles receptor protein/mg of membrane protein.
If using 125I-labled ligands with high specific activity (~ 2,000 Ci/mmol) then lower expression levels can be tolerated (50,000 receptors per cell) corresponding to densities of approximately 200 fmol receptor protein/mg of membrane protein.
ESTABLISH ASSAY FORMAT
There are three possible formats for the assay that can be selected, pre-coupled bead addition, T0 addition, or delayed addition. Pre-coupling affords the measure of “on” and “off” rates for receptor binding. The T0 addition format involves sequential addition of test samples, radio-ligand, membrane and bead as separate additions. The coupling of membrane to beads occurs simultaneously with the ligand-binding. This is the most widely used format for screening assys that are automated. For the delayed addition format test samples, radio-ligand and membrane are allowed to equilibrate prior to the addition of beads.
Optimize receptor: SPA bead ratios to ensure complete capture of all the receptor present. Set up a matrix of bead and receptor to identify the combination that produces the optimal signal:background ratio.
OPTIMIZE ASSAY CONDITIONS
Incubation time must be optimized because receptor-binding SPAassays need to be fully equilibrated to achieve stable counting conditions. Perform a time course experiment.
Examine effect of shaking/agitation and temperature on the signal and equilibration time. Receptor-binding SPAassays using yttrium silicate (YSi) beads (RPNQ 0011, RPNQ0010, RPN143, RPNQ0012) will need to be shaken to prevent clumping and poor ligand binding results.
OPTIMIZE ASSAY BUFFER
Examine the effect of NSB reducing agents such as BSA, salts (NaCl or MgCl2), detergents (e.g, Triton-X100) and polyethylenimine coating (RPNQ0003 or RPNQ0004 vs. RPNQ0001) on the level of background signal when added to the reaction buffer.
Examine the effect of pH and co-factors such as divalent cations, on the signal.
If a loss of signal is observed with time, evaluate the requirement for the addition of protease inhibitors in the reaction.
Establish the specificity of the reaction (e.g. by the use of negative control cell membrane). Perform inhibition experiments with known inhibitors to establish IC50 values. Perform saturation-binding studies to establish KD and Bmax values.
PVT SPA beads
WGAPEI Type A RPNQ0003
WGAPEI Type B RPNQ0004
Protein A RPNQ0019
Yttrium silicate SPA beads
Protein A RPN143
Select-a-Bead Kit RPNQ0250