An increasing number of therapeutic antibodies targeting tumors that express the epidermal growth factor receptor (EGFR) are in clinical use or late stages of clinical development. Here we investigate the molecular basis for inhibition of EGFR activation by the therapeutic antibody matuzumab (EMD72000). We describe the X-ray crystal structure of the Fab fragment of matuzumab (Fab72000) in complex with isolated domain III from the extracellular region of EGFR. Fab72000 interacts with an epitope on EGFR that is distinct from the ligand-binding region on domain III and from the cetuximab/Erbitux epitope. Matuzumab blocks ligand-induced receptor activation indirectly by sterically preventing the domain rearrangement and local conformational changes that must occur for high-affinity ligand binding and receptor dimerization.

Anthranilate synthase catalyzes the synthesis of anthranilate from chorismate and glutamine and is feedback-inhibited by tryptophan. The enzyme of the hyperthermophile Sulfolobus solfataricus has been crystallized in the absence of physiological ligands, and its three-dimensional structure has been determined at 2.5-Å resolution with x-ray crystallography. It is a heterotetramer of anthranilate synthase (TrpE) and glutamine amidotransferase (TrpG) subunits, in which two TrpG:TrpE protomers associate mainly via the TrpG subunits. The small TrpG subunit (195 residues) has the known ''triad'' glutamine amidotransferase fold. The large TrpE subunit (421 residues) has a novel fold. It displays a cleft between two domains, the tips of which contact the TrpG subunit across its active site. Clusters of catalytically essential residues are located inside the cleft, spatially separated from clustered residues involved in feedback inhibition. The structure suggests a model in which chorismate binding triggers a relative movement of the two domain tips of the TrpE subunit, activating the TrpG subunit and creating a channel for passage of ammonia toward the active site of the TrpE subunit. Tryptophan presumably blocks this rearrangement, thus stabilizing the inactive states of both subunits. The structure of the TrpE subunit is a likely prototype for the related enzymes 4-amino 4-deoxychorismate synthase and isochorismate synthase.Anthranilate synthase (AnthS) from bacteria and yeast is a multifunctional enzyme composed of small TrpG and large TrpE subunits or domains (1). TrpG belongs to the family of ''triad'' glutamine amidotransferases (2, 3), which hydrolyze glutamine and transfer nascent ammonia through an intramolecular channel to a synthase active site.The TrpE subunit is a bifunctional enzyme (4). It catalyzes the synthesis of anthranilate in two steps (Scheme 1): the reversible reaction of chorismate with ammonia to 2-amino 2-deoxyisochorismate (ADIC synthase reaction) followed by the irreversible elimination of pyruvate from ADIC (ADIC lyase reaction). Both reactions require Mg 2ϩ ions, and ADIC is not released into the solvent. The TrpG 2 :TrpE 2 complex mediates communication between three distinct ligandbinding sites on the two subunits (1): (i) chorismate binding to the TrpE subunit activates the release of ammonia from glutamine bound to the TrpG subunit; (ii) nascent ammonia is transferred intramolecularly from the TrpG to the TrpE subunit, in preference to ammonia from the bulk solvent (1), and (iii) tryptophan binding to a distinct site on the TrpE subunit (5) inhibits all partial reactions of the TrpG 2 :TrpE 2 complex. The strictly ordered addition of chorismate before glutamine (6) and the cooperative binding of both chorismate and tryptophan (7) to the complex suggest that conformational changes mediate the communication between the various ligand-binding sites.Here, we report the crystal structure of the unliganded AnthS complex from the hyperthermophile Sulfolobus solfataricus. The TrpE subun...