Owing the standard deviation. Each data point represented intensity of different amounts of FAM-SP interacting with NK1R-NLPs subtracted by non-specific adsorption of the same amounts of FAM-SP to the paper. The solid curve represents specific binding. The dashed curve represents nonspecific binding (NK1R-NLPs saturated by excessive amount of nonlabeled SP). The binding curve was fit to an “OneSiteBind” model Y = Bmax 6 X/(Kd + X), where Y represents fluorescence intensity caused by binding and X represents concentration of FAM-SP in the solution after reaction. The fitting results gave (3.560.3) 6106 (fluorescence intensity) for Bmax and 3467.8 nM for Kd (dissociation constant). doi:10.1371/Salmon calcitonin journal.pone.0044911.gspecies in solution, and therefore caused an increase in diffusion times, until the binding of FAM-SP to NK1R became saturated. Each of the individual diffusion curves for FAM-SP were fitted to a 2-species 18055761 blot assays (Kd = 3467.8 nM) in the range of tens of nanomolar but albeit 2,3 fold higher when measured by FCS. FCS measured ligand binding directly in solution and maintained an aqueous environment where background and non-specific binding were minimal. Compared with results from dot blot assays, the fitting results obtained from FCS are considered more accurate when the background/noise and non-specific binding were seen in the dot blot assays but not included for the fitting analysis.DiscussionCompared to other approaches for obtaining membrane-bound receptor proteins, cell-free co-expression provides a one-step viable method to produce functional GPCRs such as NK1R. The NLP serves as an ideal membrane mimetic that renders the proteinGPCRs Supported in Nanolipoprotein DiscsFigure 5. Diffusion curves of FAM-SP after binding with different amounts of NK1R-NLPs. Sample a to e represent 80 mL FAM-SP binding with 0.5, 4, 8, 12, 20 mL NK1R-NLP complexes respectively. Sample f to h represent 20, 10, 4, 2 mL FAM-SP binding with 10 mL NK1R-NLPs complexes respectively. doi:10.1371/journal.pone.0044911.gsoluble and thus easily accessible for ligand binding studies using methods such as EPR spectroscopy and FCS. Compared to other “nanodisc” or NLP based studies, we showed the first functionalFigure 6. Saturation binding curve of NK1R-NLPs to FAM-SP. [NK1R-NLPs] is the concentration of free NK1R-NLPs at equilibrium and was calculated from the subtraction of the amount of NK1R-NLPs bound with FAM-SP from the total amount of NK1R-NLPs added. The ligand bound was calculated by comparing the average diffusion time of the mixture (free FAM-SP and FAM-SP bound with NK1R-NLPs) with the individual controls (diffusion times of free FAM-SP and free NK1R-NLPs). The experimental data (blue dots) were fitted to a.Owing the standard deviation. Each data point represented intensity of different amounts of FAM-SP interacting with NK1R-NLPs subtracted by non-specific adsorption of the same amounts of FAM-SP to the paper. The solid curve represents specific binding. The dashed curve represents nonspecific binding (NK1R-NLPs saturated by excessive amount of nonlabeled SP). The binding curve was fit to an “OneSiteBind” model Y = Bmax 6 X/(Kd + X), where Y represents fluorescence intensity caused by binding and X represents concentration of FAM-SP in the solution after reaction. The fitting results gave (3.560.3) 6106 (fluorescence intensity) for Bmax and 3467.8 nM for 
Kd (dissociation constant). doi:10.1371/journal.pone.0044911.gspecies in solution, and therefore caused an increase in diffusion times, until the binding of FAM-SP to NK1R became saturated. Each of the individual diffusion curves for FAM-SP were fitted to a 2-species 1326631 complex diffusion model. [26] The average diffusion times were determined as a result of the fit parameters for bound FAM-SP and unbound FAM-SP. Since we also had control data of diffusion times for FAM-SP and NK1R-NLPs alone, by comparing the average diffusion times of the mixture with the controls, we were able to infer the percentage of bound FAM-SP (Ligand bound ) versus the total amount of FAM-SP added and the concentration of free NK1R-NLPs at equilibrium ([NK1RNLPs]). By plotting Ligand bound versus [NK1R-NLPs] and fitting it to an “OneSiteBind” model (Figure 6), we calculated the dissociation constants Kd and Bmax. The results are 83633 nM and 3665.6 nM, respectively. This dissociation constant is consistent with the dot 18055761 blot assays (Kd = 3467.8 nM) in the range of tens of nanomolar but albeit 2,3 fold higher when measured by FCS. FCS measured ligand binding directly in solution and maintained an aqueous environment where background and non-specific binding were minimal. Compared with results from dot blot assays, the fitting results obtained from FCS are considered more accurate when the background/noise and non-specific binding were seen in the dot blot assays but not included for the fitting analysis.DiscussionCompared to other approaches for obtaining membrane-bound receptor proteins, cell-free co-expression provides a one-step viable method to produce functional GPCRs such as NK1R. The NLP serves as an ideal membrane mimetic that renders the proteinGPCRs Supported in Nanolipoprotein DiscsFigure 5. Diffusion curves of FAM-SP after binding with different amounts of NK1R-NLPs. Sample a to e represent 80 mL FAM-SP binding with 0.5, 4, 8, 12, 20 mL NK1R-NLP complexes respectively. Sample f to h represent 20, 10, 4, 2 mL FAM-SP binding with 10 mL NK1R-NLPs complexes respectively. doi:10.1371/journal.pone.0044911.gsoluble and thus easily accessible for ligand binding studies using methods such as EPR spectroscopy and FCS. Compared to other “nanodisc” or NLP based studies, we showed the first functionalFigure 6. Saturation binding curve of NK1R-NLPs to FAM-SP. [NK1R-NLPs] is the concentration of free NK1R-NLPs at equilibrium and was calculated from the subtraction of the amount of NK1R-NLPs bound with FAM-SP from the total amount of NK1R-NLPs added. The ligand bound was calculated by comparing the average diffusion time of the mixture (free FAM-SP and FAM-SP bound with NK1R-NLPs) with the individual controls (diffusion times of free FAM-SP and free NK1R-NLPs). The experimental data (blue dots) were fitted to a.
