Design, synthesis and conformational analysis of hGM-CSF(13-31)-Gly-Pro-Gly-(103-116)

Identification of an Antigenic Domain Near the C Terminus of Human Granulocyte-Macrophage Colony-stimulating Factor and Its Spatial Localization

The goal of this study was to map an epitope on the human granulocyte-macrophage colony-stimulating factor (hGM-CSF) at its C terminus, a region whose integrity is fundamental in maintaining the normal function of this molecule. Residues including the fourth alpha-helix (D, 103-116) were analyzed for their role in the interaction with antibodies (Abs) raised against the protein. Five peptides homologous to different segments of the C terminus of hGM-CSF were synthesized. Peptide-(102-121) included the same residues of the alpha-helix D and the next five amino acids toward the C terminus; peptide-[E108A]-(102-121) introduced the mutation E108A in order to verify the role of acidic residues; peptide-[C96A](93-110) encompassed the beta-sheet 2 and half of the alpha-helix D; peptide-[C121A]-(110-127) included the second half of the alpha-helix D and the C terminus of hGMCSF; peptide-(13-31)-Gly-Pro-Gly-(103-116) included both the alpha-helices A and D connected by the tripeptide Gly-Pro-Gly, which allows the original antiparallel orientation of the two alpha-helices to be maintained. Both anti-protein and anti-peptide-(102-121) antibodies, capable of neutralizing the stimulatory activity of hGMCSF in the bone marrow colony-forming assays, recognized a specific epitope in the C terminus of hGM-CSF. Molecular modeling estimated the surface accessibility of hGM-CSF and the stability of the synthetic peptides in aqueous solution. Altogether, our results showed that the immunogenic region includes part of the alpha-helix D and the residues 116-120, which are external to this helix and particularly exposed on the protein surface, confirming the feasible participation of this region in antibody binding.

Design, synthesis, and conformational studies of the hGM-CSF derived peptide (13-27)-Gly-(75-87)

An analogue of the human granulocyte-macrophage colony-stimulating factor (hGM-CSF), hGM-CSF(13-27)-Gly-(75-87) was synthesized by solid phase methodology. This analogue was designed to comprise helices A and C of the native growth factor, linked by a glycine bridge. Helices A and C form half of a four-helix bundle motif in the crystal structure of the native factor and are involved in the interaction with alpha- and beta-chains of the heterodimeric receptor. A conformational analysis of the synthetic analogue by CD, two-dimensional nmr spectroscopy, and molecular dynamics calculations is reported. The analogue is in a random structure in water and assumes a partially alpha-helical conformation in a 1 : 1 trifluoroethanol/water mixture. The helix content in this medium is approximately 70%. By 2D-nmr spectroscopy, two helical segments were identified in the sequences corresponding to helices A and C. In addition to medium- and short-range NOESY connectivities, a long-range cross peak was found between the Cbeta proton of Val(16) and NH proton of His(87) (using the numbering of the native protein). Experimentally derived interproton distances were used as restraints in molecular dynamics calculations, utilizing the x-ray coordinates as the initial structure. The final structure is characterized by two helical segments in close spatial proximity, connected by a loop region. This structure is similar to that of the corresponding domain in the x-ray structure of the native growth factor in which helices A and C are oriented in an antiparallel fashion. The N-terminal residues Gly-Pro of helix C are involved in an irregular turn connecting the two helical segments. As a consequence, helix C is appreciably shifted and slightly rotated with respect to helix A compared to the x-ray structure of the native growth factor. These small differences in the topology of the two helices could explain the lower biological activity of this analogue with respect to that of the native growth factor.

Human GM-CSF interaction with the alpha-chain of its receptor studied using surface plasmon resonance

A surface plasmon resonance (SPR) based biosensor has been used for studying the interaction of recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF) with genetically engineered alpha-chain subunits of its specific receptor (GM-Ralpha). Western blot analysis of GM-Ralpha confirmed the correct size (80 kDa) and reactivity of these proteins against anti-GM-Ralpha polyclonal or monoclonal antibodies. GM-CSF was immobilized, using standard amine coupling methods, to the dextran-modified gold biosensor surface in order to capture GM-Ralpha subsequently injected over the sensing layer. GM-Ralpha were shown to specifically form complexes with the immobilized ligand. Pre-incubation of constant amounts of GM-Ralpha with dilutions of soluble GM-CSF before injection of the mixture over the GM-CSF matrix, prevented ligand binding in a dose dependent manner. In contrast, unrelated soluble cytokines or serum proteins (e.g. G-CSF, albumin, etc.) were found to exert no inhibition. Complexes formation blockage by pre-incubation of constant amounts of GM-Ralpha with dilutions of neutralizing anti-GM-Ralpha antibodies was concentration dependent, further assessing the specificity of the interaction. To investigate the possibility of relating the effect on binding affinity of critical conformational changes at the contact site, experiments of multisite binding were performed, flowing a set of neutralizing monoclonal antibodies reacting to different epitopes on GM-CSF over the GM-CSF matrix, before injecting GM-Ralpha. The results indicated that antibody interaction with helix D and helix A of GM-CSF markedly inhibited GM-CSF binding to GM-Ralpha. Comparable results were obtained using the biosensor technology and enzyme-linked immunoassays, in representative experiments performed with the same reagents. These experiments demonstrate that SPR can be successfully used for studying complementary interactions between GM-CSF and its receptor alpha-chains in solution without using labels or secondary tracers and, compared with conventional immunoanalysis methods, significantly saving time.