A new biologically active protein has been recently discovered in the soluble fraction of bovine hypothalamus. It appears to be a binding protein for the immunosuppressor FK506 and seems to belong to the family of immunophilins(IPh-s) that are receptors for immunosuppressive drugs, widely used in organ transplantation to prevent transplantation rejection. The aim of this project is to characterize and elucidate the function(s) of immunophilins in brain. Along with the search for multiple forms of IPh-s and their endogenous ligands immunosuppressors in brain, the following aspects will be also investigated: the structure function aspects of the peptidyl-prolyl cistrans isomerase activity (PPIase) of IPh-s; the role of IPh-s in synapse function of brain; and the effect of IPh-s and their ligands complexes on calmodulin-dependent enzymes; interactions of these complexes with immunogenic compounds such as thymosin (4 and thymosin (1 which have been recently isolated from hypothalamus.