Scf American Crystallographic Association Poster - Model

THE M-CSF STRUCTURE21 The three-dimensional x-ray crystallographic structure of M-CSF has been recently determined. As expected, it has a 4 -helix bundle folding motif characteristic of other such growth factors. It shares about 16% sequence identity with SCF and as with SCF it is a dimeric structure. However, unlike SCF which is a non-covalently bound dimer, the M-CSF dimer is covalently bound via a single intramolecular disulfide bond.

FEATURES OF THE SCF MODEL Based on sequence comparisons and helical hydrophobicity moments a model structure of SCF superimposed on a schematic rendition of M-CSF structure has been proposed that corroborates some of the biochemical data. The conserved cysteines, for example, easily fit into positions that allow the formation of the two intramolecular disulfides. The first disulfide bond attaches the Cterminus to the top of helix B while the second disulfide bond attaches the N-terminus to the top of helix C. It has also been observed that not all of the consensus sites for N-linked glycosylation are, in fact, glycosylated in the native protein. In particular, Asn120 is always glycosylated, Asn69 sometimes glycosylated and Asn72 is never glycosylated. According the model structure Asn120 is on the outer face of helix D while Asn72 is situated towards the core of the molecule at the confluence of helices A and B. Obviously glycosylation at this site would seriously disrupt the SCF structure. Asn69 is situated on the outer edge of helix C. Other important features of this model include the acidic surfaces of the helices, and the conserved dimer interface.

ACIDIC RIDGES The helices in SCF have rather marked hydrophobic moments: the interior facing portions of the helices are quite hydrophobic while the exterior surfaces are often charged. Helices B, C and D, in particular have a large preponderance of negatively charged acidic aspartates and glutamates situated on their surfaces. Helix B for example has an uninterrupted, continuous ridge of five acidic residues on its surface. It is possible that the role of divalent cations in both the precipitation and and crystallization of SCF may in part be derived from the neutralization of these "acidic ridges" by these cations.

CONSERVED DIMER INTERFACE The region of the molecule distal to the C-terminus turns out to be completely and strictly conserved among the SCFs from different species. This region is also analogous to the dimer interface for M-CSF so it is very likely that it too is the dimer interface for SCF. Knowledge of the dimer site may help in designing SCF antagonists that may prove useful in cancer chemotherapy, etc.

PROPOSED RECEPTOR BINDING SITE Because the C-terminus leads towards the cell surface and the dimer interface interacts with the other SCF monomer, it seems plausible to propose that the receptor binding site is composed of the surfaces of helices A & C. A more detailed knowledge of these surfaces in conjunction with mutant studies would assist in the development of SCF agonists --potentially useful agents in treating various clinical anemic states.

REFERENCES 21 Pandit J, et al., (1992), Science, 258:1358.