PDBsum entry 3c9a

Hormone/signaling protein

PDB id

3c9a

Contents

			Protein chains

					208 a.a.


					48 a.a.

			Metals

					_BR ×18

			Waters ×860

References listed in PDB file

Key reference

Title

Structural basis for egfr ligand sequestration by argos.

Authors

D.E.Klein, S.E.Stayrook, F.Shi, K.Narayan, M.A.Lemmon.

Ref.

Nature, 2008, 453, 1271-1275. [DOI no: 10.1038/nature06978]

PubMed id

18500331

Abstract

Members of the epidermal growth factor receptor (EGFR) or ErbB/HER family and their activating ligands are essential regulators of diverse developmental processes. Inappropriate activation of these receptors is a key feature of many human cancers, and its reversal is an important clinical goal. A natural secreted antagonist of EGFR signalling, called Argos, was identified in Drosophila. We showed previously that Argos functions by directly binding (and sequestering) growth factor ligands that activate EGFR. Here we describe the 1.6-A resolution crystal structure of Argos bound to an EGFR ligand. Contrary to expectations, Argos contains no EGF-like domain. Instead, a trio of closely related domains (resembling a three-finger toxin fold) form a clamp-like structure around the bound EGF ligand. Although structurally unrelated to the receptor, Argos mimics EGFR by using a bipartite binding surface to entrap EGF. The individual Argos domains share unexpected structural similarities with the extracellular ligand-binding regions of transforming growth factor-beta family receptors. The three-domain clamp of Argos also resembles the urokinase-type plasminogen activator (uPA) receptor, which uses a similar mechanism to engulf the EGF-like module of uPA. Our results indicate that undiscovered mammalian counterparts of Argos may exist among other poorly characterized structural homologues. In addition, the structures presented here define requirements for the design of artificial EGF-sequestering proteins that would be valuable anti-cancer therapeutics.



	Figure 2. Figure 2: Argos has three similar domains that resemble the three-finger toxin fold of TGF- beta-receptors. a, The overall fold of the three constituent domains in Argos is illustrated with domain 3. The four strands ( 1– 4) form two fingers (fingers 1 and 2) that resemble those on a left hand (as shown). The 'palm' side of the domain faces out of the page. A knuckle-like protrusion projects below the page. At the top of the domain is a disulphide-bonded core from which emanate the two fingers plus the thumb (marked). Cysteine residues C1 to C6, which make C1–C3, C2–C5 and C4–C6 disulphides, are labelled, as are N and C termini. b, Domains 1, 2 and 3 are overlaid (as C ribbons) in the same orientation used in a. Colours are as in Fig. 1. Strand 1', unique to domain 1, is labelled, as is the location of the 120 residue insert (Ins.) removed to generate Argos[217]. Two orthogonal views are shown. c, Domain 3 of Argos (red) overlaid with the 100-residue extracellular ligand-binding domain (ECD) of the type II activin receptor (ActRII)^14 (coloured light grey; from Protein Data Bank code 2GOO).		Figure 4. Figure 4: Argos, EGFR and structural homologues entrap the EGF domain with two binding sites. a, The leftmost and rightmost panels show EGF domains bound to Argos and the human EGFR extracellular region^17 (sEGFR), respectively. Spitz is green and hEGF is cyan. In the central upper panels, Spitz[EGF] and hEGF are shown (in identical orientations) bound to Argos domain 2 (grey) and sEGFR domain I (beige). The side chains of EGF-domain-interacting residues are drawn. Site 1 on sEGFR domain I (defined in ref. 17) and its counterpart on Argos (which includes site d2A) are marked by blue and red ovals, respectively. In the lower central panels, Spitz[EGF] and hEGF (again in identical orientations) are shown bound to Argos domain 3 and sEGFR domain III. Sites 2 and 3 in the sEGFR/hEGF interface are marked with blue ovals. Argos site d3B mimics sEGFR site 2, but Argos does not mimic sEGFR site 3. Instead, Argos makes a unique set of interactions with Spitz[EGF] (site d3A). A key aliphatic side chain critical for the binding of hEGF to site 3 of EGFR (L47 in hEGF, I98 in Spitz) is disordered and exposed in the Spitz[EGF]–Argos complex. b, Domain organization of uPAR^9, ^19. The three domains in uPAR are coloured with the order used for Argos in Fig. 1. Like Argos, uPAR uses three copies of this domain type—although in a different arrangement—to form a C-clamp-like structure for enveloping an EGF domain^9, ^19.

PROCHECK

	Headers