Bone Morphogenetic Protein 2
Created by Eric Wherley
Protein Function
Bone morphogenetic proteins (BMPs) play a key role in the development and homeostasis of bone structures within the body (1,2). Members of the transforming growth factor beta family (TGF-beta), BMPs activate serine/threonine kinase receptors in the plasma membrane, stimulating a signaling cascade within the cel (1). This cascade works primarily through the SMAD pathway, but also may involve the MAP-K pathway as wel (1). The protein of interest,
BMP2, has a molecular weight of 12.9kDa and an isoelectric point of 4.26 (2). BMPs initiate chondrogenesis as the first step of bone morphogenesis. Chondrogenesis involves the synthesis of a cartilage matrix, with growth plates that calcify to bone during development (3). BMPs further perform the functions of chemotaxis, mitosis and differentiation in the morphogenetic pathway (3). Chemotaxis, the directed migration of cells to an area of interest, provides mesenchymal cells that proliferate and differentiate to cartilage and eventually bone. Bone morphogenetic proteins perform function most similar to activins, growth and differentiating factors, and TGF-betas, in that each serves a function towards the development and maintenance of tissues and organs. An interesting phenomenon with BMPs is that the 35 members of this group bind to two receptors,
BMP Receptor I and
II. This limited specificity offers insight to the structural similarity of these proteins. BMP2 is found to have the most similar sequence, aside from other BMP proteins, to human activin A(PDBID: 2ARV)(4). With a 40% sequence similarity and Dali server z-score of 11.5, activin A serves to induce mesoderm formation in embryos (5). The nearly analogous function of the two proteins in embryonic development is indicative of their
structural similarity. Both proteins are shown to be antagonized by follistatin, indicating a similar 3D structure. Another protein shown to have similar 3D structure is glial cell-derived neurotrophic factor (GDNF), with 83% structural similarity (PDBID: 1AGQ)(5). GDNF maintains neural pathways through the differentiation of peripheral neurons in a similar manner to the function of BMP proteins (7). Comparison of BMP2 with proteins of similar structure and sequence reveals important structural similarities as they relate to cell growth and differentiation. Demonstration of these similarities indicates that these proteins evolved along similar paths, creating the superfamily TGF-beta. Further analysis of the structural entities of BMP2 will provide information to the molecular mechanism of the signaling pathway involved and offer a more complete view of the protein.
Protein Structure
BMP2 is made up of 103 residues (1). The secondary structure is primarily
beta sheets with 49 residues forming 10 strands (1). These strands form a defining structural feature of two finger-like double-stranded beta-sheets (1,8). The protein contains
14% helical structure, concentrated as a four-turn helix perpendicular to the beta-sheets (1). The
cysteine knot is formed by six cysteine residues at position 43, 47, 111 and 113 (8). This serves to stabilize the structure of the protein without a hydrophobic core. The combination of these major structural elements likens the topology of BMP2 as a hand; the helix mimics the wrist, the cysteine the palm, and beta-sheets the fingers (8). A member of the transforming growth factor beta family (TGF-beta), BMP2 shares 90% similarity with other BMP proteins (8). It has also been shown that, though only 30% sequence identity is conserved between BMP7 and TGF-beta2 , the 3D structures are nearly identical (8).
BMP2 exists primarily in dimer form under physiological conditions. A
single disulfide bond connects the subunits at the Cys78 position (8). This dimerization creates a hydrophobic core between the monomers that stabilizes the molecule. Two areas of BMP2 have been identified as binding sites and are identified as the wrist and knuckle epitope (1). The
wrist epitope is defined by the pre-helix segment of the protein (Pro48-Asn56) (1). This segment is evidenced to differentiate the protein from other members of the family by altering the helical angle (2). For example, BMPs have an angle of 48o while TGF-betas have an angle of 38o (8). The knuckle epitope is located on top of the first finger of BMP2. BMP2 initiates a cell signaling response by binding and oligeramizing two types of receptor chains (3). The protein binds surface receptors in a sequential binding mechanism. This is due to the 100-fold higher binding affinity with type I receptors than type II receptors (1). After binding with BMPR type IA or IB, type II receptors are recruited to bind and activate the SMAD pathway, up regulating BMP-responsive genes. The binding of these receptors is shown to proceed following the induced fit mechanism, indicated by conformational changes upon binding, though the total architecture of the protein is conserved.
The type I receptor, generally BMPR-IA or IB,
binds to the wrist epitope of BMP2. A 0.25nm displacement of the pre-helix loop has been noted upon binding to the type I receptor1 (9). Five beta-sheets on the BMPR-IA form a three-fingered structure that is rigid through binding. The loops between the beta-sheets undergo significant changes upon binding. The beta4-beta5 loop changes from a random coil to a 1.6 turn helix upon binding with BMP2 (9).
After binding is completed with the type I receptor, a variety of type II receptors are able to bind.
The beta6 and beta7 stands of BMP2 interact with three beta-strands of the type II receptor (1). The binding site of BMP2 and the receptor is primarily hydrophobic, forming a horseshoe shape. This hydrophobic interaction is postulated to be the reason for the ability of multiple type II receptors to bind BMP2; experimental data of interchanging hydrophobic residues demonstrates similar binding affinity (1,9). It is interesting to note that upon binding with the type II receptor, another conformational change is noted in the type I receptor (1). Examination of the structure of BMP2 and its binding mechanisms explains the interesting number of similar ligands and receptors often seen to work interchangeably. The large number of receptor-ligand complexes possible in the binding of BMP2 to its receptors supports the findings of large conformational changes upon binding. The flexibility allows for small changes in the structure of the receptor-binding site to be accommodated. Further, the hydrophobic nature of the promiscuous type II receptor bond permits a number of receptors to be used. Further examination of the similarities between other members of the TGF-beta family can offer significant clues to the relationship between protein structure and function.