We have studied the direct interaction of the constitutive isoform of Hsp70 (Hsc70) with the DnaJ homolog, auxilin, a cofactor that binds to clathrin-coated vesicles and is required for their uncoating by Hsc70. Auxilin caused a 5-fold increase in Hsc70 ATPase activity and a corresponding increase in steady-state levels of bound ADP; the dissociation constant for this effect was 0.6 M. Auxilin also induced polymerization of Hsc70 and bound to the resulting polymer at a 1:1 molar ratio; here too

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... the dissociation constant was 0.6 M. Both this binding and polymerization required ATP; the Hsc70 depolymerized with a 4-min half-life when ATP was completely hydrolyzed to ADP. Although auxilin induces polymerization stoichiometrically and other DnaJ homologs induce polymerization catalytically, these data show that auxilin is similar to other DnaJ homologs in its ability to activate the Hsc70 ATPase activity, to polymerize Hsc70, and in the nucleotide dependence of this polymerization. Furthermore, the 70-amino acid J-domain of auxilin polymerized Hsc70 with the same nucleotide dependence as intact auxilin. Therefore, although only auxilin and not other DnaJ homologs support uncoating, our data suggest that various DnaJ homologs share a common mechanism of interaction with Hsc70, perhaps because their J-domains interact similarly with Hsc70.