Hsc70, the constitutive form of the heat shock protein 70 family of proteins, is involved in a number of biological activities which include protein folding and molecular chaperoning. Previously, we had shown that the immunosuppressant 15-deoxyspergualin (DSG) specifically interacted with Hsc70, as well as the Hsp90 family of proteins. Although the exact binding site on Hsc70 for protein substrates is unknown, a recent study shows that the extreme C-terminal four amino acids 647EEVD650 play a role in regulating AT-Pase activity, substrate binding, and interaction with HDJ-1. These four amino acids are also found at the C-terminus of Hsp90 and may be involved in similar functions. In this study, we show that DSG binds specifically to this EEVD regulatory domain. Binding of DSG to Hsc70 did not affect its ability to bind peptides. These results suggest that in addition to the ATP binding domain, there are two additional substrate binding domains on Hsc70. DSG should provide a tool for understanding the role of the EEVD motif in biological processes.