FKBP54, A NOVEL FK506-BINDING PROTEIN IN AVIAN PROGESTERONE-RECEPTOR COMPLEXES AND HELA EXTRACTS

Avian progesterone receptor complexes contain five major co-purifying proteins, including hsp90, hsp70, and a 23-kDa protein. The other rece ptor-associated proteins, p50 and p54, share amino acid sequence simil arities with a 52-59-kDa component (p59, hsp56, or FKBP52) of mammalia n steroid receptor complexes that is also a member of the FK506-bindin g proteins (FKBP) family of immunophilins. We show here that p50, but not p54, cross-reacts with a rabbit antiserum prepared against human F KBP52. 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 g radient analyses show that both p50 and p54 exist predominantly in oli gomeric 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 complex es and may interact with other proteins, such as progesterone receptor , in distinctive manners. In HeLa cell extracts, a 55-kDa FK506-bindin g protein, distinct from FKBP52, cross-reacts with anti-p54 antibody F F1. We conclude that p50 is avian FKBP52, whereas p54 and the 55-kDa h uman FF1 antigen are FKBP54, a novel immunophilin.