Regulation of the Hsp90-binding immunophilin, cyclophilin 40, is mediated by multiple sites for CA-binding protein (GABP)

Within steroid receptor heterocomplexes the large tetraticopeptide repeat-c ontaining immunophilins, cyclophilin 40 (CyP40), FKBP51, and FKBP52, target a common interaction site in heat shock protein 90 (HspSO) and act coordin ately with HspSO to modulate receptor activity. The reversible nature of th e interaction between the immunophilins and HspSO suggests that relative ce llular abundance might be a key determinant of the immunophilin component w ithin steroid receptor complexes. To investigate CyP40 gene regulation, we have isolated a fi-kilobase (kb) 5 ' -flanking region of the human gene and demonstrated that a similar to 50 base pair (bp) sequence adjacent to the transcription start site is essential for CyP40 basal expression. Three tan demly arranged Ets sites within this critical region were identified as bin ding elements for the multimeric Ets-related transcription factor, GA bindi ng protein (GABP). Functional studies of this proximal promoter sequence, i n combination with mutational analysis, confirmed these sites to be crucial for basal promoter function. Furthermore, overexpression of both GABP alph a and GABP beta subunits in Cos1 cells resulted in increased endogenous CyP 40 mRNA levels. Significantly, a parallel increase in FKBP52 mRNA expressio n was not observed, highlighting an important difference in the mode of reg ulation of the CyP40 and FKBP52 genes. Our results identify GABP as a key r egulator of CyP40 expression. GAFF is a common target of mitogen and stress -activated pathways and may integrate these diverse extracellular signals t o regulate CyP40 gene expression.