- Dual wavelength detection of terbium and ﬂuorescein using dedicated optic settings
- Strong and long-lasting donor signal minimizes background
- Rapid and reliable detection of PKC´s and tyrosine kinase activity
Table of contents
Protein Kinase C (PKC) enzymes are a diverse family of enzymes that under speciﬁc signaling conditions phosphorylate proteins on serine and threonine amino acids. PKCs are involved in many cellular functions including neuronal activity, cell growth, cell proliferation, and cell movement, as well as in several diseases including cardiovascular, cancer, diabetes, and Alzheimer’s.Therefore a TR-FRET screening assay, such as the one described here, can aid in further elucidating PKC’s function.
Tyrosine kinases (TKs) are a diverse family of enzymes that under speciﬁc signaling conditions phosphorylate proteins on the amino acid tyrosine. TKs are involved in many cellular functions such as cell division and cell proliferation, as well as in several diseases including cancer and diabetes. Tyrosine kinases play important roles in cell growth, thereby making them important drug targets in cancer therapy. For that reason a TR-FRET screening assay, such as the one described here, can aid in elucidating speciﬁc inhibitors for tyrosine kinases.
A ﬂuorescein labeled poly-GT or poly-GAT (Glu, Ala, Tyr) substrate is incubated with a tyrosine kinase and ATP. For protein kinase C assays a ﬂuorescein labeled PKC substrate is incubated with PKC and ATP. Subsequently, a terbium labeled antibody that binds to the phosphorylated form of the substrate is added. When the antibody interacts with the phosphorylated substrate, FRET will occur between the terbium label (emits at 490 nm) and the ﬂuorescein moiety (emits at 520 nm) (Figure 1).
Measurements are taken at 490 and 520 nm and the 520/490 ratio is plotted versus the concentration of enzyme to determine an EC50 for the enzyme and its substrate.
Materials & Methods
- Invitrogen’s Fluorescein PKC Substrate and PKC Kinase Buffer (5x), PKCα, Kinase Quench Buffer
- Invitrogen’s LanthaScreen™ Tb-PKC Substrate Antibody
- Invitrogen’s Fluorescein-poly-GT and Fluorescein-poly-GAT, Tyrosine Kinase Buffer, ZAP70, Screen Dilution Buffer
- Invitrogen’s LanthaScreen® Tb-PY20 Antibody
- Black Corning® low volume 384-well microplate
Multiple PKC isoforms were titrated to determine optimal kinase concentrations for screening. The following protocol is an example of the conditions used to determine the EC50 of PKCα and its substrate. A dilution series of PKCα, starting at a ﬁnal concentration of 2.0 µg/ml, was incubated in the presence of 250 nM ﬂuorescein-labeled PKC substrate and 20 µM ATP in a total volume of 10 µl in a microplate. After a 90-minute incubation at room temperature, 10 µl of TR-FRET dilution buffer containing 2X EDTA (20 mM) and 2X Tb-PKC antibody (1.0 nM) was added and mixed to create a ﬁnal volume of 20 µl per well, a ﬁnal 1X EDTA concentration of 10 mM, and a final 1X antibody concentration of 0.5 nM. After incubating for 60 minutes at room temperature, the plate was read on the BMG LABTECH microplate reader. Each data point represents the average of three wells.
Tyrosine Kinase Titration
To demonstrate the functionality of a universal tyrosine kinase assay using LanthaScreen™, both ﬂuorescein-poly-GAT (Glu, Ala, Tyr) and ﬂuorescein-poly-GT (Glu, Tyr) substrates were used with terbium labeled PY20 antibody.
A dilution series of kinase was incubated with 400 nM ﬂuorescein labeled substrate and 200 µM ATP in a total volume of 10 µl in a microplate. After 60 mins of incubation at room temperature, 5 µl of TR-FRET dilution buffer containing 4X EDTA (60 mM) was added. Next, 5 µl of TR-FRET dilution buffer with 4X antibody (8 nM) was added and mixed to create a ﬁnal volume of 20 µl per well, a ﬁnal 1X EDTA concentration of 15 mM, and a ﬁnal 1X antibody concentration of 2 nM. After 60 mins of incubation at room temperature, the plate was read in the microplate reader.
|FLUOstar/ POLARstar Omega||CLARIOstar||PHERAstar FS|
|Detection mode||Time-resolved Fluorescence|
|Method||Endpoint, Top optic|
|Optic settings||Advanced TRF |
|Ex-Filter: Ex TR |
Optic module: 337 520 490
|Integration start||100 µs|
|Integration time||200 µs|
Results & Discussion
Figure 2 shows representative kinase titration curves for PKCα and PKCζ.
Table 1 shows statistical data for all kinases tested.
Table 1: LanthaScreen™ on the PHERAstar: statistics for PKC isoform titrations.
|Fold Diff (max/min)||6.1||6.2||3.3||4.8||3.6||2.6||3.1|
All PKC isoenzymes were titrated in a similar manner as PKCα. Regardless of “fold difference” or signal-to-noise ratios, all assays provided Z’ values far greater than 0.5. This is due to the use of FRET-based, ratiometric data analysis, which leads to low standard deviations of the replicates. Both hill slope and R2 measurements fall within acceptable limits. R2 values approaching 1.0 indicate a perfect curve ﬁt.
In ﬁgure 3 kinase titration curves for ZAP-70 are presented (Fig. 3A+B).
The results show that use of a ﬂuorescein-labeled substrate common for many TKs (poly-GT or poly-GAT) allows for the development of a “universal” TK assay using the LanthaScreen™ format.
Invitrogen’s terbium-based LanthaScreen™ can easily be measured on BMG LABTECH instrumentation. The assay shows several advantages:
- The ratiometric nature of LanthaScreen™ assay eliminates well to well variation.
- The time-resolved nature allows for the use of ﬂuorescein without the associated drawbacks of compound interference.
- The ability to use ﬂuorescein simpliﬁes assay development and costs.