Asp-960/Glu-961 controls the movement of the C-terminal tail of the epidermal growth factor receptor to regulate asymmetric dimer formation

Molecular Docking Studies of a Cyclic Octapeptide-Cyclosaplin from Sandalwood

Natural products from plants, such as chemopreventive agents, attract huge attention because of their low toxicity and high specificity. The rational drug design in combination with structure-based modeling and rapid screening methods offer significant potential for identifying and developing lead anticancer molecules. Thus, the molecular docking method plays an important role in screening a large set of molecules based on their free binding energies and proposes structural hypotheses of how the molecules can inhibit the target. Several peptide-based therapeutics have been developed to combat several health disorders, including cancers, metabolic disorders, heart-related diseases, and infectious diseases. Despite the discovery of hundreds of such therapeutic peptides however, only few peptide-based drugs have made it to the market. Moreover, the in silico activities of cyclic peptides towards molecular targets, such as protein kinases, proteases, and apoptosis related proteins have not been extensively investigated. In this study, we explored the in silico kinase and protease inhibitor potentials of cyclosaplin, and studied the interactions of cyclosaplin with other apoptosis-related proteins. Previously, the structure of cyclosaplin was elucidated by molecular modeling associated with dynamics that were used in the current study as well. Docking studies showed strong affinity of cyclosaplin towards cancer-related proteins. The binding affinity closer to 10 kcal/mol indicated efficient binding. Cyclosaplin showed strong binding affinities towards protein kinases such as EGFR, VEGFR2, PKB, and p38, indicating its potential role in protein kinase inhibition. Moreover, it displayed strong binding affinity to apoptosis-related proteins and revealed the possible role of cyclosaplin in apoptotic cell death. The protein–ligand interactions using LigPlot displayed some similar interactions between cyclosaplin and peptide-based ligands, especially in case of protein kinases and a few apoptosis related proteins. Thus, the in silico analyses gave the insights of cyclosaplin being a potential apoptosis inducer and protein kinase inhibitor.

Dynamic analysis of the epidermal growth factor (EGF) receptor-ErbB2-ErbB3 protein network by luciferase fragment complementation imaging

ErbB3 is a member of the ErbB family of receptor tyrosine kinases. It is unique because it is the only member of the family whose kinase domain is defective. As a result, it is obliged to form heterodimers with other ErbB receptors to signal. In this study, we characterized the interaction of ErbB3 with the EGF receptor and ErbB2 and assessed the effects of Food and Drug Administration-approved therapeutic agents on these interactions. Our findings support the concept that ErbB3 exists in preformed clusters that can be dissociated by NRG-1β and that it interacts with other ErbB receptors in a distinctly hierarchical fashion. Our study also shows that all pairings of the EGF receptor, ErbB2, and ErbB3 form ligand-independent dimers/oligomers. The small-molecule tyrosine kinase inhibitors erlotinib and lapatinib differentially enhance the dimerization of the various ErbB receptor pairings, with the EGFR/ErbB3 heterodimer being particularly sensitive to the effects of erlotinib. The data suggest that the physiological effects of these drugs may involve not only inhibition of tyrosine kinase activity but also a dynamic restructuring of the entire network of receptors.

Quantitation of the effect of ErbB2 on epidermal growth factor receptor binding and dimerization

The epidermal growth factor (EGF) receptor is a member of the ErbB family of receptors that also includes ErbB2, ErbB3, and ErbB4. These receptors form homo- and heterodimers in response to ligand with ErbB2 being the preferred dimerization partner. Here we use (125)I-EGF binding to quantitate the interaction of the EGF receptor with ErbB2. We show that the EGFR/ErbB2 heterodimer binds EGF with a 7-fold higher affinity than the EGFR homodimer. Because it cannot bind a second ligand, the EGFR/ErbB2 heterodimer is not subject to ligand-induced dissociation caused by the negatively cooperative binding of EGF to the second site on the EGFR homodimer. This increases the stability of the heterodimer relative to the homodimer and is associated with enhanced and prolonged EGF receptor autophosphorylation. These effects are independent of the kinase activity of ErbB2 but require back-to-back dimerization of the EGF receptor with ErbB2. Back-to-back dimerization is also required for phosphorylation of ErbB2. These findings provide a molecular explanation for the apparent preference of the EGF receptor for dimerizing with ErbB2 and suggest that the phosphorylation of ErbB2 occurs largely in the context of the EGFR/ErbB2 heterodimer, rather than through lateral phosphorylation of isolated ErbB2 subunits.

Conformational epitopes at cadherin calcium-binding sites and p120-catenin phosphorylation regulate cell adhesion

The activity state of E-cadherin is controlled by conformational epitopes at interfaces between different EC domains, which are coupled to p120-catenin phosphorylation. Dephosphorylation activates adhesion, whereas phosphorylation inhibits activation. p120-dependent changes in the physical state of E-cadherin regulate epithelial cell morphogenesis.

We investigated changes in cadherin structure at the cell surface that regulate its adhesive activity. Colo 205 cells are nonadhesive cells with a full but inactive complement of E-cadherin–catenin complexes at the cell surface, but they can be triggered to adhere and form monolayers. We were able to distinguish the inactive and active states of E-cadherin at the cell surface by using a special set of monoclonal antibodies (mAbs). Another set of mAbs binds E-cadherin and strongly activates adhesion. In other epithelial cell types these activating mAbs inhibit growth factor–induced down-regulation of adhesion and epithelial morphogenesis, indicating that these phenomena are also controlled by E-cadherin activity at the cell surface. Both types of mAbs recognize conformational epitopes at different interfaces between extracellular cadherin repeat domains (ECs), especially near calcium-binding sites. Activation also induces p120-catenin dephosphorylation, as well as changes in the cadherin cytoplasmic domain. Moreover, phospho-site mutations indicate that dephosphorylation of specific Ser/Thr residues in the N-terminal domain of p120-catenin mediate adhesion activation. Thus physiological regulation of the adhesive state of E-cadherin involves physical and/or conformational changes in the EC interface regions of the ectodomain at the cell surface that are mediated by catenin-associated changes across the membrane.

Mechanics of EGF receptor/ErbB2 kinase activation revealed by luciferase fragment complementation imaging

Binding of EGF to its receptor induces dimerization of the normally monomeric receptor. Activation of its intracellular tyrosine kinase then occurs through the formation of an asymmetric kinase dimer in which one subunit, termed the "receiver" kinase, is activated by interaction with the other subunit, termed the "activator" kinase [Zhang, et al. (2006) Cell 125: 1137-1149]. Although there is significant experimental support for this model, the relationship between ligand binding and the mechanics of kinase activation are not known. Here we use luciferase fragment complementation in EGF receptor (EGFR)/ErbB2 heterodimers to probe the mechanics of ErbB kinase activation. Our data support a model in which ligand binding causes the cis-kinase (the EGFR) to adopt the receiver position in the asymmetric dimer and to be activated first. If the EGF receptor is kinase active, this results in the phosphorylation of the trans-kinase (ErbB2). However, if the EGF receptor kinase is kinase dead, the ErbB2 kinase is never activated. Thus, activation of the kinases in the EGFR/ErbB2 asymmetric dimer occurs in a specific sequence and depends on the kinase activity of the EGF receptor.

Co-conserved features associated with cis regulation of ErbB tyrosine kinases

Background

The epidermal growth factor receptor kinases, or ErbB kinases, belong to a large sub-group of receptor tyrosine kinases (RTKs), which share a conserved catalytic core. The catalytic core of ErbB kinases have functionally diverged from other RTKs in that they are activated by a unique allosteric mechanism that involves specific interactions between the kinase core and the flanking Juxtamembrane (JM) and COOH-terminal tail (C-terminal tail). Although extensive studies on ErbB and related tyrosine kinases have provided important insights into the structural basis for ErbB kinase functional divergence, the sequence features that contribute to the unique regulation of ErbB kinases have not been systematically explored.

Methodology/Principal Findings

In this study, we use a Bayesian approach to identify the selective sequence constraints that most distinguish ErbB kinases from other receptor tyrosine kinases. We find that strong ErbB kinase-specific constraints are imposed on residues that tether the JM and C-terminal tail to key functional regions of the kinase core. A conserved RIxKExE motif in the JM-kinase linker region and a glutamine in the inter-lobe linker are identified as two of the most distinguishing features of the ErbB family. While the RIxKExE motif tethers the C-terminal tail to the N-lobe of the kinase domain, the glutamine tethers the C-terminal tail to hinge regions critical for inter-lobe movement. Comparison of the active and inactive crystal structures of ErbB kinases indicates that the identified residues are conformationally malleable and can potentially contribute to the cis regulation of the kinase core by the JM and C-terminal tail. ErbB3, and EGFR orthologs in sponges and parasitic worms, diverge from some of the canonical ErbB features, providing insights into sub-family and lineage-specific functional specialization.

Conclusion/Significance

Our analysis pinpoints key residues for mutational analysis, and provides new clues to cancer mutations that alter the canonical modes of ErbB kinase regulation.