We report here the use of a stepwise affinity cross-linking technique in the specific covalent attachment of epidermal growth factor (EGF) to its receptor. A heterobifunctional cross-linking reagent, sulfo-N-succinimidyl 4-(fluorosulfonyl)benzoate (SSFSB), which contains a rapidly reacting sulfo-N-succinimidyl active ester and a much more slowly reacting aromatic fluorosulfonyl moiety, was synthesized and characterized. Murine EGF (mEGF) was modified by the cross-linker to yield as the major product a derivative of mEGF having the (fluorosulfonyl)benzoyl moiety attached covalently at the amino terminus. SSFSB-modified, 125I-labeled mEGF was separated from unreacted SSFSB by size-exclusion chromatography and applied to shed membrane vesicles from A431 human carcinoma cells. Binding of derivatized 125I-mEGF to vesicles led to high yields (greater than 60%) of covalent linkage of 125I-mEGF to the EGF receptor, as determined by measurement of the fraction of specifically bound radiolabel which comigrated with the EGF receptor in NaDodSO4-polyacrylamide gels. The specificity of affinity cross-linking was evident in the negligible degree of labeling of species other than the EGF receptor and in the retention of EGF-stimulated receptor kinase activity after cross-linking.

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