Uncoating of clathrin-coated vesicles is mediated by the heat shock cognate protein, hsc70, and requires clathrin light chains (LCa and LCb) and ATP hydrolysis. We demonstrate that purified light chains and synthetic peptides derived from their sequences bind hsc70 to stimulate ATP hydrolysis. LCa is more effective than LCb in stimulating hsc70 ATPase and in inhibiting clathrin uncoating by hsc70. These differences correlate with high sequence divergence in the proline- and glycine-rich region (residues 47-71) that forms the hsc70 binding site. For LCa, but not LCb, this region undergoes reversible conformational changes upon perturbation of the ionic strength or the calcium ion concentration. Our results show that LCa is more important for interactions with hsc70 than is LCb and suggest a model in which the LCa conformation regulates coated vesicle uncoating.