THE CYCLOPHILIN COMPONENT OF THE UNACTIVATED ESTROGEN-RECEPTOR CONTAINS A TETRATRICOPEPTIDE REPEAT DOMAIN AND SHARES IDENTITY WITH P59 (FKBP59)

Using a rapid single-step affinity chromatography procedure we have is olated the unactivated estrogen receptor from bovine uterus. Results o f sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Wester n analyses for protein extracts recovered from affinity chromatography of receptor cytosols, either preincubated or untreated with estradiol , suggest a component structure for the intact oligomeric receptor whi ch includes hsp90, hsp70, p59, a 40-kDa cyclophilin-related protein, a nd an uncharacterized 22-kDa protein species. We have chemically deter mined the amino acid sequences of eight peptides derived from the 40-k Da component and now report the cloning and primary sequence of a cDNA encoding this protein, which is designated estrogen receptor-binding cyclophilin (ERBC). Homology analyses confirm that ERBC is a new membe r of the cyclophilin family and contains a C-terminal domain with sign ificant sequence homology to an internal region of p59, a binding prot ein for the immunosuppressant FK506 (FKBP59). This conserved region in cludes a 3-unit tetratricopeptide repeat domain bounded at the C termi nus by a putative calmodulin binding site. We propose that the tetratr icopeptide repeat domain mediates the protein interaction properties o f ERBC and p59. Both immunophilins may have important roles in recepto r assembly and may represent a new category of ligand- and calcium-dep endent modulators of protein function.