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B-Raf protein isoforms interact with and phosphorylate Mek-1 on serine residues 218 and 222
Abstract:
The B-raf/c-Rmil proto-oncogene belongs to the raf/mil family of serine/threonine protein kinases. It encodes multiple protein isoforms resulting from alternative splicing of two exons located upstream of the kinase domain. Recent studies suggested that B-Raf could be the intermediate molecule between Ras and Mek-1 (MAP Kinase Kinase) in signalling pathways specific of neural cells. However, there has been no evidence for a direct interaction between B-Raf and Mek-1. We report here that different B-Raf isoforms can be co-immunoprecipitated with anti-Mek-1 antisera in COS-1 cells and that the kinase activity of B-Raf is not required for its interaction with Mek-1. We also show that all B-Raf isoforms tested phosphorylate Mek-1 in a time-dependent manner, whereas kinase defective mutants fail to do so. Finally, we demonstrate that the constitutively activated S218D, S222D and S218D/S222D mutants of Mek-1 interact similarly with B-Raf. However, only the S218D and S222D mutants, and not the S218D/S222D double mutant, can be phosphorylated by B-Raf isoforms. Therefore, serine residues 218 and 222, previously shown to regulate Mek-1 activity, appear to be the major phosphorylation sites by B-Raf in vitro.
Insights
This study demonstrates that B-Raf isoforms directly interact with and phosphorylate Mek-1 (MAP Kinase Kinase), identifying key serine residues involved in this crucial signaling pathway.
Area of Science:
- Molecular Biology
- Cell Signaling
- Oncogene Research
Background:
- B-Raf is a serine/threonine protein kinase in the raf/mil family.
- B-Raf isoforms are generated by alternative splicing.
- B-Raf is implicated as an intermediate between Ras and Mek-1 in neural cell signaling.
Purpose of the Study:
- To investigate the direct interaction between B-Raf and Mek-1 (MAP Kinase Kinase).
- To determine if B-Raf kinase activity is necessary for Mek-1 interaction.
- To identify the specific phosphorylation sites on Mek-1 by B-Raf.
Main Methods:
- Co-immunoprecipitation assays in COS-1 cells.
- Kinase activity assays using B-Raf and Mek-1 mutants.
- In vitro phosphorylation studies.
Main Results:
- B-Raf isoforms co-immunoprecipitated with Mek-1, independent of B-Raf kinase activity.
- B-Raf isoforms phosphorylated Mek-1 in a time-dependent manner.
- Serine residues 218 and 222 on Mek-1 were identified as major phosphorylation sites by B-Raf.
Conclusions:
- B-Raf directly interacts with and phosphorylates Mek-1.
- Kinase activity of B-Raf is essential for Mek-1 phosphorylation.
- Serine residues 218 and 222 are key regulatory phosphorylation sites targeted by B-Raf.