The epidermal growth factor receptor as a substrate for a kinase-splitting membranal proteinase.

A brush-border membranal proteinase, which specifically clips the catalytic subunit of cAMP-dependent protein kinase, is shown to cleave the receptor for the epidermal growth factor (EGF) (Mr = 170,000) into two fragments of Mr = 140,000 and 30,000. The 140-kDa fragment retains its EGF-binding site and its EGF-dependent protein tyrosine kinase activity on exogenous substrates, but it loses its capacity to undergo self-phosphorylation. It is shown to be distinct from the 150-kDa fragment of the EGF receptor obtained by the Ca2+-activated neutral proteinase. The membranal proteinase strictly recognizes the native structure of the receptor and fails to cleave either the denatured receptor or its 150-kDa degradation product. Thus the membranal proteinase acts as a conformation-recognizing probe for both the protein-tyrosine kinase domain of the EGF receptor and the catalytic subunit of cAMP-dependent protein-Ser/Thr kinase, suggesting that the known sequence homology between these two kinases is also reflected in their conformation. The well defined 140-kDa fragment described here is useful for structure-function studies of the EGF receptor.