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From Proteopedia
The complex of wild type B-RAF and BAY439006
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Introduction
The B-RAF Protein belongs to a large family of protein kinases that have a very important role on different aspects of cellular biology and biomedical sciences. The human protein kinase family consists of 518 genes, making it one of the largest gene families. Based upon the phosphorylation of the hydroxyl group these proteins are classified on 3 groups: protein serine threonine kinase (385 members), protein tyrosine kinase (90 members) and tyrosine kinase like proteins (43 members). A-RAF, B-RAF and C-RAF proteins are a family of serine threonine kinase proteins that participate in the RAS-RAF-MEK-ERK signal transduction cascade. The general reaction they catalyse is:
This cascade participates in the regulation of several cellular mechanisms like: apoptosis, cell cycle progression, differentiation, proliferation and transformation to the cancerous state in response to growth factors, cytokines and hormones. First discovered in 1938 as a retroviral oncogene B-RAF plays a key role on all studies concerning cancer therapies and other biomedical applications.
B-RAF Structure[1]
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B. Schematic B-RAF Primary Structure
As we distinguished before, there are 3 types of RAF proteins: A-RAF, B-RAF and C-RAF. All of them share 3 much conserved regions (CR): CR1, CR2 and CR3. CR1 is composed of a RAS binding domain (RBD) and a cysteine rich domain (CRD) which can bind 2 zinc ions (zinc finger domain). It is possible for the CR1 to interact with the RAS and with the membrane phospholipids (PH domain) CR2 is a serine threonine rich domain. When a serine is phosphorylated this domain can bind a regulatory protein that can bind the C-terminal region in the same time. Binding of 14-3-3 (C-terminal region) to this phosphorylated serine is inhibitory for the enzyme. CR3 is the protein kinase domain on the C-terminal region. We can find just after this kinase domain a stimulatory 14-3-3 binding site for other regulatory proteins.
As all RAF Proteins, B-RAF has the characteristic small N-terminal lobe and the large C-terminal lobe. The small lobe is constituted of an antiparallel β-shift that anchors and orient the ATP. It contains a glycine rich ATP phosphate binding loop, called P-loop. The large lobe interacts with the substrate that in our case is MEK1/2 who needs to be phosphorylated to be active. The catalytic site is just between the two lobes. These 2 lobes can move relative to each other, opening or closing the cleft. This has 2 major consequences on the functioning of this enzyme: 1. The open form allows access of ATP and release of ADP from the active site. 2. The closed form brings the residues of the substrate into the active site. On the other hand each lobe has her proper polypeptide segment that can change conformation from active to inactive and vice versa. 1. In the small lobe, this segment is a α-helix, which is called αC-helix. The αC-helix rotates and translates with respect to the rest of the lobe, making or breaking part of the active site. 2. In the large lobe, the activation segment can make or break part of the ATP binding site. The activation segment of each protein kinase has a specific domain that begins with a DFG amino acid sequence that can easily change its conformation (active or inactive conformation). In the inactive conformation the phenylalanine side chain occupies the ATP binding pocket and the aspartate side chain faces away from the active site. This is the DFG Aspartate Out Conformation. In the active conformation, the phenylalanine side chain is rotated out of the ATP binding pocket and the Aspartate side chain can now face the ATP binding pocket and form coordinated links with the Mg²⁺. This is called the DFG Aspartate In Conformation. The activation segment can be phosphorylated by members of the same protein kinase family or by other protein kinases.
Regulation
signaling ways
Other Inhibitors
drugs and inhibitor paradox
Applications in Medicine
External Ressources
References
- ↑ Wan PT, Garnett MJ, Roe SM, Lee S, Niculescu-Duvaz D, Good VM, Jones CM, Marshall CJ, Springer CJ, Barford D, Marais R. Mechanism of activation of the RAF-ERK signaling pathway by oncogenic mutations of B-RAF. Cell. 2004 Mar 19;116(6):855-67. PMID:15035987
Protreopedia Page Contributors and Editors
Kostika Sofroni and Gilles Dupouy
