Department of Pharmacology, University of Nebraska Medical Center, Omaha 68198.
Avian progesterone receptor complexes contain five major co-purifying proteins, including hsp90, hsp70, and a 23-kDa protein. The other receptor-associated proteins, p50 and p54, share amino acid sequence similarities with a 52-59-kDa component (p59, hsp56, or FKBP52) of mammalian steroid receptor complexes that is also a member of the FK506-binding proteins (FKBP) family of immunophilins. We show here that p50, but not p54, cross-reacts with a rabbit antiserum prepared against human FKBP52. Both p50 and p54 bind an FK506 affinity resin at 0.5 M KCl, but only p50 binds efficiently in low salt conditions. Glycerol density gradient analyses show that both p50 and p54 exist predominantly in oligomeric complexes at low ionic strength. The poor retention of p54 on FK506 resin at low ionic strength compared with the high retention of p50 suggests that these proteins may largely exist in separate complexes and may interact with other proteins, such as progesterone receptor, in distinctive manners. In HeLa cell extracts, a 55-kDa FK506-binding protein, distinct from FKBP52, cross-reacts with anti-p54 antibody FF1. We conclude that p50 is avian FKBP52, whereas p54 and the 55-kDa human FF1 antigen are FKBP54, a novel immunophilin.