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A novel assay for the measurement of Raf-1 kinase activity
C Bondzi1, S Grant, G W Krystal
1Department of Microbiology/Immunology, Medical College of Virginia of Virginia Commonwealth University and McGuire VA Medical Center, Richmond, Virginia, VA 23249, USA.
Abstract:
Raf-1 is a serine-threonine protein kinase that functions as a central component of the mitogen-activated protein kinase signal transduction pathway. Raf-1 activity is currently assayed in vitro by either measuring 32P incorporation into MEK, Raf-1's only characterized substrate, or by using the phosphorylated MEK to initiate a coupled assay culminating in the phosphorylation of myelin basic protein by MAP kinase. These assays are plagued by a potential lack of specificity in the case of the former, and the time consuming and error-prone nature of the later indirect assay. In this report, we demonstrate a novel single step assay for Raf-1 kinase activity based on phosphorylation of recombinant MEK-1, detected using an activation-specific MEK antibody that recognizes MEK only when specifically phosphorylated by Raf-1 on Ser 217 and Ser 221. The assay readily detected stem cell factor-mediated Raf-1 activation. MEK phosphorylation by immunoprecipitated Raf-1 plateaued at 10 min following initiation of the kinase reaction and was completely dependent on the inclusion of Raf-1. There was a linear correlation between the degree of MEK phosphorylation and the amount of Raf-1 protein immunoprecipitated. In addition to detecting growth factor-mediated activation, the assay was also able to detect paclitaxel-mediated Raf-1 activation. This assay is rapid, sensitive, and specific and therefore is a marked improvement over currently utilized techniques.
Insights
A new assay simplifies measuring Raf-1 kinase activity by detecting phosphorylated MEK-1. This rapid, sensitive, and specific method improves upon existing techniques for studying the mitogen-activated protein kinase pathway.
Area of Science:
- Molecular Biology
- Biochemistry
- Cell Signaling
Background:
- Raf-1 is a key serine-threonine protein kinase in the mitogen-activated protein kinase (MAPK) pathway.
- Current in vitro assays for Raf-1 activity, such as 32P incorporation into MEK or coupled assays, suffer from potential lack of specificity and are time-consuming.
Purpose of the Study:
- To develop a novel, single-step assay for quantifying Raf-1 kinase activity.
- To improve upon the speed, sensitivity, and specificity of existing Raf-1 activity assays.
Main Methods:
- A new assay was developed utilizing recombinant MEK-1 phosphorylation.
- Detection employed an activation-specific MEK antibody recognizing phosphorylation at Ser 217 and Ser 221.
- The assay was tested for its ability to detect Raf-1 activation mediated by stem cell factor and paclitaxel.
Main Results:
- The novel assay successfully detected stem cell factor-mediated Raf-1 activation.
- MEK phosphorylation by immunoprecipitated Raf-1 reached a plateau within 10 minutes and was Raf-1 dependent.
- A linear correlation was observed between MEK phosphorylation levels and the amount of immunoprecipitated Raf-1.
Conclusions:
- The developed assay is rapid, sensitive, and specific for measuring Raf-1 kinase activity.
- This method represents a significant advancement over current techniques for assessing Raf-1 activation.
- The assay can detect Raf-1 activation induced by both growth factors and chemotherapeutic agents like paclitaxel.