Specific cleavage of the fibroblast receptor for platelet‐derived growth factor by an endogenous Ca2+‐dependent thiol protease

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Abstract
Previous studies have shown that platelet-derived growth factor (PDGF) stimulates the phosphorylation of two components in membranes prepared from human fibroblasts in the presence of Ca2+. One of these represents the 185-kDa PDGF receptor, which undergoes autophosphorylation, and the other has an Mr of 130000. We show in this communication that the 130-kDa component is derived from the 185-kDa receptor via proteolysis by an endogeneous Ca2+-dependent protease, which is dependent on a reduced -SH group for its activity. The 130-kDa fragment contains several of the characteristics of the receptor, such as the PDGF-binding site and the major autophosphorylation sites. Furthermore, the cleaved receptor retains tyrosine kinase activity.