Growth Factor Bound Protein 2 (GRB2)
Created by Christine Bell
Growth Factor Bound Protein 2, or Grb2, is an important protein utilized in signal transduction in mammalian cells. The protein is identified as a Signal Transduction Adaptor, and is functionally relevant in transduction pathways responsible for propagation of signals originating at epidermal growth factor receptors (EGF) and platelet-derived growth factors (PDGF) (1). In the EGF cell signaling pathways, the specific role of Grb2 is to mediate the functional activity of Ras, another protein necessary to the signal transduction pathway. The protein is located in the Golgi apparatus and is generally translated in high quantities in the cell. The molecular weight of Grb2 is 25,206.35 Da, and the theoretical isoelectric point of the protein, as calculated using the molecular database ExPASy, is 5.89 (10).
The protein is comprised of two identical subunits, and when these subunits are linked together, they are broken into three domains. The protein contains one SH2 domain and two SH3 domains.The majority of the secondary structure of Grb2 is comprised mainly of beta sheets; all three domains of the protein contain beta sheets. Alpha helices, while present, are much less prevalent in the structure. Only two alpha helices are present per subunit, accounting for four alpha helices total. Random coils are intermittent throughout the secondary structure. Turns are less prevalent, again only two turns occur per subunit. A graphical representation of the secondary structure is provided in the form of a Ramachandran plot, shown in the Ramachandran Plot tab. The ramachandran plot demonstrates the heavy present of beta sheets within the structure, with a lower concentration of alpha helices.
The SH2 domain spans residues 53-163, thus is located in the center of the protein. The two SH3 domains flank either side of the central SH2 domain; the first SH3 domain spans residues 1-52 and the second SH3 domain spans residues 164-217. Specific association of the Grb2 protein with its affiliated EGF and PDGF receptors occurs through the SH2 domain of Grb2 when these growth factor receptors have been phosphorylated on a tyrosine residue (2). Without the phosphorylation of these residues, Grb2 would be functionally unable to operate, as the signals would not have a pathway through which to propagate in the cell. Examples of tyrosine-phosphorylated proteins that typically interact with the SH2 domain of Grb2 are SIT1 and IRS1 (2, 11 respectively). TheSH3 domains in turn are responsible for binding to a subset of proteins which tend to contain domains rich in proline residues. These proline-rich regions are the particular regions that bind to the N-terminal SH3 domain of Grb2. The chemical basis for the binding specificity of proline-rich regions to the N-terminal SH3 domain is that the SH3 domain has ahydrophobic platform, allowing for hydrophobic proline residues to interact (7). Pictured here is one subunit of Grb2 with the residues color-coded according to their hydrophobicity, to demonstrate the presence of the hydrophobic platform.
The majority of the secondary structure of Grb2 is comprised mainly of beta sheets; all three domains of the protein contain beta sheets. Alpha helices, while present, are much less prevalent in the structure. Only two alpha helices are present per subunit, accounting for four alpha helices total. Random coils are intermittent throughout the secondary structure. Turns are less prevalent, again only two turns occur per subunit. Pictured here is one of the two identical subunits of Grb2, color-coded by secondary structural element for clarity.
Functions of Grb2
As mentioned previously,Grb2 is functionally relevant to signal transduction pathways in the human body. Shown here is a space-filling model of the protein, to more easily view the surface-structure of the protein. The specific location in the pathway is between the event of growth factor receptor binding to a signal and the activation of Ras, the protein responsible for continuing propagation of the cell signal.
When a growth factor receptor binds to its particular cellular signal, the protein will interact with the SH2 domain of Grb2. Research has been conducted to determine whether this preliminary binding of the SH2 domain of Grb2 to the growth factor receptor affects subsequent binding at the SH3 domains. Studies have shown that binding at the SH2 domain does not induce conformational changes in the SH3 domain, however. The chemical event that allows Grb2 to activate Ras is the interaction between Grb2 and Son of sevenless, or Sos, at the SH3 domains of Grb2. Once interaction between these proteins has occurred, then Grb2 is able to activate Ras (3).
Binding of the SH3 domain of Grb2 to Sos (the nucleotide exchange factor of the Ras signaling pathway) is only one of the general functions of this domain. When the SH3 domain of Grb2 interacts with the protein dynamin, then the complex is able to contribute to the process of receptor endocytosis (7).
The basic functions of Grb2 relate to the presence of the SH2 and SH3 domains, and so functional similarities are recognized in some proteins also containing these domains. One specific example is the protein Grb2-related Adaptor Protein 2, or GRAP2. This protein is generally located in the hematopoietic cells of humans. GRAP2 serves an important role in these cells as part of the signal transduction pathway, just as the main function of Grb2 is signal transduction.
While no drug-protein interactions exist at the current time, research is currently being conducted to explore the relationship between Grb3-3 (the isoform of Grb2 with a deletion of amino acid residues 60-100) and individuals infected with HIV-1. Following the initial infection of the individual, levels of T cells from the individual's immune system begin to rapidly decline. A correlation has been found between the infection of T cells with HIV-1 and the upregulation of the Grb2-isoform Grb3-3. Grb3-3 is associated with pathways relating to apoptosis, which could explain the marked decline in T cells following infection with the HIV-1 viral strain (4).
Functionally Important Residues
As mentioned above, binding of the SH2 domain of Grb2 to growth factor receptors does not induce any conformational changes in the SH3 domains to lead to either activation or inhibition of binding at these regions to Sos. Research has recently been conducted to show that modification of the SH3 domain can affect binding of this region to Sos, however. When phosphorylation occurs at
Tyr-160 and Tyr-209, both located in the C-terminal SH3 domain, this leads to efficient dissociation of Sos from the growth factor receptor-Grb2 complex (5).
Structurally Similar Protein
The SH2 domain of Src exhibits a high degree of structural similarity with the SH2 domain of Grb2 (6). Both domains contain two alpha helices surrounded by beta sheets. Functional similarities are also present, as the SH2 domains of both Grb2 and Src are influenced by tyrosine phosphorylation, and both proteins play a role in protein kinase pathways. Superimposition of these two domains demonstrates the high degree of similarity between the secondary structures. While the major secondary substructures show a high degree of similarity, areas of structural variation are evident in the random coils (shown here in grey) throughout the SH2 domains of both Grb2 and Src.