Hsc70 was previously isolated by its ability to catalyse the uncoating of clathrin-coated vesicles from bovine brain. We have recently shown that Hsc70 is more active towards coated vesicles from brain than those from other tissues. In order to gain information on the mechanistic reason for this difference we have examined the ability of brain and placental coated vesicles to stimulate partial reactions during a single round of ATP turnover. The Hsc70-ATP complex is turned over to Hsc70-ADP

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... er dot Pi, from which phosphate is slowly released. The resulting Hsc70-ADP complex exchanges ATP for ADP. Dissociation of ATP or ADP from Hsc70 does not seem to occur under physiological conditions. The hydrolysis of ATP is accelerated by the presence of clathrin-coated vesicles, with vesicles from brain being about twice as effective as vesicles from placenta. Additionally, it appears that brain, but not placental, coated vesicles can also stimulate the exchange of ADP for ATP.