A yeast cytosol is shown to contain two distinct activities that stimulate protein translocation across microsomal membranes. One activity was purified. It consists of two constitutively expressed 70K heat shock related proteins that increase the rate of translocation. Possible mechanisms of action of these proteins are discussed.

SUMMARY Hsp70s are important cancer chaperones that act upstream of Hsp90 and exhibit independent anti-apoptotic activities. To develop chemical tools for the study of human Hsp70, we developed a homology model that unveils a previously unknown allosteric site located in the nucleotide binding domain of Hsp70. Combining structure-based design and phenotypic testing, we discovered a previously unknown inhibitor of this site, YK5. In cancer cells, this compound is a potent and selective binder of the cytosolic but not the organellar human Hsp70s and has biological activity partly by interfering with the formation of active oncogenic Hsp70/Hsp90/client protein complexes. YK5 is a small molecule inhibitor rationally designed to interact with an allosteric pocket of Hsp70 and represents a previously unknown chemical tool to investigate cellular mechanisms associated with Hsp70.

70-kDa heat shock protein (Hsp70) molecular chaperones are ATPases that participate in protein folding by regulating protein-protein interactions. ATP binds to the highly conserved amino-terminal domain, whereas polypeptides bind to the less conserved carboxyl-terminal domain. These domains are functionally coupled. Polypeptides were previously shown to dissociate from Hsp70s upon ATP binding and to stimulate ATPase activity. We probed the structure of the yeast cytosolic Hsp70 Ssa1p using limited proteolysis to determine whether the conformations of its nucleotide and polypeptide binding domains are also coupled. Ssa1p adopted three distinct conformations, nucleotide-free, ADP-dependent, and ATP-dependent. Complete conformational changes required K ؉ and Mg 2؉. Using aminoterminal sequencing, ATP-agarose chromatography, and a carboxyl-terminal-specific antibody, we mapped the locations of the major proteolytic fragments. Nucleotides altered the conformations of both the nucleotide and polypeptide binding domains. Similarly, a polypeptide altered the conformations of both domains. These results indicate that the conformations of the nucleotide and polypeptide binding domains are coupled.

Cell surface heparan sulfate (HS) proteoglycans are carbohydrate-rich regulators of cell migratory, mitogenic, secretory, and inflammatory activity that bind and present soluble heparin-binding growth factors (e.g., fibroblast growth factor, Wnt, Hh, transforming growth factor β, amphiregulin, and hepatocyte growth factor) to their respective signaling receptors. We demonstrate that the deglycanated core protein of syndecan-1 (SDC1) and not HS chains nor SDC2 or -4, appears to target the epithelial selective prosecretory mitogen lacritin. An important and novel step in this mechanism is that binding necessitates prior partial or complete removal of HS chains by endogenous heparanase. This limits lacritin activity to sites where heparanase appears to predominate, such as sites of exocrine cell migration, secretion, renewal, and inflammation. Binding is mutually specified by lacritin's C-terminal mitogenic domain and SDC1's N terminus. Heparanase modification of the latter transforms a widely expressed HS proteoglycan into a highly selective surface-binding protein. This novel example of cell specification through extracellular modification of an HS proteoglycan has broad implications in development, homeostasis, and disease.