Abstract
The surface topography of IGF I(insulin-like growth factor I) was investigated by chemical modification of amino acid residues in free IGF I and bound to type I IGF receptor or to monoclonal antibody MAB43. Tyrosine residues were modified either by chloramine-T or lactoperoxidase catalyzed iodination. In the free IGF I molecule, all 3 tyrosine residues, A19 (Tyr-60), B25 (Tyr-24), and C2 (Tyr-31), were iodinated. Monoclonal antibody MAB43 protected IGF I against modification at tyrosine residue A19, and in the type I IGF receptor-IGF I complex, all 3 tyrosine residues were shielded against iodine incorporation. These results allow the prediction of the binding domains in the IGF I molecule. The minimal receptor binding site in IGF I would include amino acid residues B25 to C2 and, possibly, the C-terminal part of the A-domain with tyrosine residue A19.
Highlights
The Binding Sites of Insulin-like Growth Factor I (IGFI) to Type I IGF Receptor and to a Monoclonal Antibody
These results show that tyrosine residue A19 of IGF Iis protected against modification in the antibody-insulin-like growth factor I (IGF I) complex
Examination of the model of IGF I which is based upon model-building operations, shown in Fig. 4 (Blundell et al, 1978; Honegger, 1985),reveals that 2 tyrosine side chains are located completely free on the surface of IGF I (B25 and C2) andthat 1 tyrosine (A19) is slightly concealed inthe Cterminal region of the molecule
Summary
From the Bwchemisches Znstitut der Uniuersitat Zurich. Winterthurerstrasse 190, CH-8057Zurich, Switzerland. Monoclonal antibody MAB43 protected IGF I against modification at tyrosine residue A19, and in the type I IGF receptor-IGF I complex, all 3 tyrosine residues were shielded against iodine incorporation These results allow the prediction of the binding domains in the a modification reaction with a radioactive reagent had to be chosen, yielding a well-defined chemical product. 500 units of type I IGF receptor, corresponding to approximately 20 pmol of IGF I binding sites (& = 50 p ~ )o,r 500 units of monoclonal antibody MAB43, receptor-IGF I complex, the receptor may protectcertain amino acid residues of IGF I from modification compared to free IGF I.
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