The ubiquitous nucleoside adenosine exerts multiple physiological actions via specific receptors, four of which (A 1 , A 2a , A 2b , and A 3 ) have been cloned (see reference 25 for a review). Activation of A 1 adenosine receptors (A 1 Rs) by adenosine or synthetic analogs, in turn regulates the activity of several membrane and intracellular proteins, such as adenylate cyclase, Ca 2ϩ channels, K ϩ channels and phospholipase C (see reference 25 for a review). Towards the intracellular side of the plasma membrane, A 1 Rs are functionally coupled to members of the pertussis toxin-sensitive family of G proteins, G i1 , G i2 , G i3 , and G o (13,20). For this coupling Nanoff et al. (23) have shown that a proteic factor of 30 to 40 kDa is necessary (23). On the other side of the plasma membrane, A 1 Rs interact with ecto-adenosine deaminase (ecto-ADA) an interaction which is needed for efficient signaling (11,27). Moreover, in response to agonists, A 1 Rs and ADA are internalized together via the same endocytic pathway (28).There is recent evidence indicating that members of the superfamily of G protein-coupled receptors interact with cytosolic proteins. As an example a recent report has described the interaction of the C terminus of metabotropic glutamate receptor type 1␣ with glyceraldehyde-3-phosphate dehydrogenase, tubulin, and an unknown protein (9). The aim of this study was to identify proteins interacting with the third intracellular loop of A 1 R. By means of affinity chromatography, coimmunoprecipitation, and biosensor technology we have found that the heat shock cognate protein hsc73 is able to bind to A 1 R. Interestingly, the interaction between this protein and the A 1 R results in reduced binding of ligands, thus suggesting a role for hsc73 in the regulation of the receptor function. MATERIALS AND METHODSMaterials. ADA (EC 3.5.4.4), electrophoresis reagents, fetal bovine serum and 5Ј-guanylylimidodiphosphate [Gpp(NH)p] were purchased from Boehringer Mannheim (Barcelona, Spain). Activated thiol Sepharose-4B, activated CH-Sepharose 4B, Sephadex G-25, and protein-A Sepharose CL4B were from Pharmacia Biotech (Barcelona, Spain). digitonin,