Implications of the vIL6gp130 Tetramer Structure for the Active Gcsfgcsfr Extracellular Signaling Complex

A crystal structure has been reported for GCSF in complex with the CHR (domains 2 and 3; also called the BN and BC domains, respectively) of the GCSFR (Fig. 2A) [2]. The structure revealed both expected and unexpected results. The complex is a 2:2 tetramer, with each GCSF molecule interacting with each receptor through the canonical site 2 epitope (also called major interface) on the face of the A and C helices (Fig. 2A) [2]. However, the unexpected result was that the two GCSF/GCSFR complexes formed a "side-by-side" dimer mediated by an interaction between the D3 (BC) domain and the N terminus of GCSF (or, minor interface) (Fig. 2A). The minor interface was not expected, as this interaction was not detected in previous structure-function studies. However, because the overall complex in the crystal was 2:2, a case was

Figure 2 Inactive and active models of GCSF complexed to GCSFR. (A) Top view of the crystal structure of GCSF complexed to the GCSFR D2 and D3 domains as reported by Aritomi etal. [2]. Underneath the ribbon drawing is a schematic showing the assembly in the same orientation as the top view. At the bottom is a ribbon diagram of this structure shown from the side. (B) Hypothetical model of the active GCSF-GCSFR complex based on the viral IL-6-gp130 tetramer [9,21]. The D1 domain of GCSFR is included in the model and is shown interacting with site 3 of GCSF from epitope mapping studies (residues in space filling) [21]. A schematic in the middle shows the assembly in the same top-view orientation as in part (A). At the bottom is a ribbon drawing of a side view of the GCSF-GCSFR tetramer model.

Figure 2 Inactive and active models of GCSF complexed to GCSFR. (A) Top view of the crystal structure of GCSF complexed to the GCSFR D2 and D3 domains as reported by Aritomi etal. [2]. Underneath the ribbon drawing is a schematic showing the assembly in the same orientation as the top view. At the bottom is a ribbon diagram of this structure shown from the side. (B) Hypothetical model of the active GCSF-GCSFR complex based on the viral IL-6-gp130 tetramer [9,21]. The D1 domain of GCSFR is included in the model and is shown interacting with site 3 of GCSF from epitope mapping studies (residues in space filling) [21]. A schematic in the middle shows the assembly in the same top-view orientation as in part (A). At the bottom is a ribbon drawing of a side view of the GCSF-GCSFR tetramer model.

made for this assembly representing the active signaling complex even though the GCSFR was missing the N-terminal domain known to be required for signaling[2].

Layton et al. [21] undertook an epitope mapping study that placed the elusive GCSF site 3 at the identical location found for gp130-cytokines (Fig. 2B). Further, these authors constructed a molecular model for the active GCSFR signaling complex (D1, D2, D3) based on the structure of the viral IL-6-gp130 tetramer (Fig. 2B). The model satisfies functional data for GCSF and is essentially identical to the vIL-6-gp130 framework, where site 2 on the A/C face and site 3 at the end of the D-helix enables formation of an interlocking tetramer (Fig. 2B). Thus, the utility of spatially distinct functional epi-topes on both cytokine and receptor for assembling an interlocking dimer has been repeated in a distinct cytokine system.

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