Williams syndrome (WS) is a developmental disorder caused by haploinsufficiency of genes at 7q11.23. Hemizygosity of elastin is responsible for one feature of WS, supravalvular aortic stenosis. LIM-kinase 1 hemizygosity has been implicated as a contributing factor to impaired visual-spatial constructive cognition in WS. A novel gene, FKBP6, has been identified and characterized within the common WS deletion region. FKBP6 shows homology to the FK-506 binding protein (FKBP) class of immunophilins. FKBP6 has a putative N-terminal FK-506 binding and peptidylproyl isomerase (rotamase) domain and, like known high-molecular-weight FKBPs, an imperfect C-terminal tetratricopeptide repeat domain. FKBP6 is expressed in testis, heart, skeletal muscle, liver, and kidney. FKBP6 consists of nine exons and is completely contained within a 35-kb cosmid clone. Fluorescence in situ hybridization experiments show that the FKBP6 gene is deleted in 40/40 WS individuals. Hemizygous deletion of FKBP6 may contribute to certain defects such as hypercalcemia and growth delay in WS (Meng, 1998).

The structurally related immunophilins cyclophilin 40 (CyP-40) and FKBP52 have been identified as components of the unactivated estrogen receptor. Both immunophilins have a similar molecular architecture that includes a C-terminal segment with a tetratricopeptide repeat (TPR) domain predicted to mediate protein interaction. hsp90 is a common cellular target for CyP-40 and FKBP52. Deletion mutants of CyP-40 fused to glutathione S-transferase were immobilized on glutathione-agarose and then used in a rapid hsp90 retention assay to define regions of the CyP-40 C terminus that are important for hsp90 binding. The evidence suggests that the TPR domain is not sufficient for stable association of CyP-40 with hsp90 and requires the participation of flanking acidic and basic residues clustered at the N- and C-terminal ends, respectively. Both microdomains are characterized by alpha-helical structures with segregated hydrophobic and charged residues. Corresponding regions were identified in FKBP52. By preincubating myometrial cytosol with lysates containing bacterially expressed FKBP52, it has been shown that FKBP52 competes with CyP-40 for hsp90 binding. These results raise the possibility of a mutually exclusive association of CyP-40 and FKBP52 with hsp90. This would lead to separate immunophilin-hsp90-receptor complexes and place the estrogen receptor under the control of distinct immunophilin signaling pathways (Ratajczak, 1996).

FKBP52 is a high molecular mass immunophilin possessing peptidylprolyl isomerase (PPIase) activity that is inhibited by the immunosuppressant drug FK506. FKBP52 is a component of steroid receptor-hsp90 heterocomplexes, and it binds to hsp90 via a region containing three tetratricopeptide repeats (TPRs). By cross-linking of the purified proteins it has been demonstrated that there is one binding site for FKBP52/dimer of hsp90. This accounts for the common heterotetrameric structure of native receptor heterocomplexes being 1 molecule of receptor, 2 molecules of hsp90, and 1 molecule of a TPR domain protein. Immunoadsorption of FKBP52 from reticulocyte lysate also yields co-immunoadsorption of cytoplasmic dynein, co-immunoadsorption of dynein is competed by a fragment of FKBP52 containing its PPIase domain, but not by a TPR domain fragment that blocks FKBP52 binding to hsp90. Using purified proteins, it has been shown that FKBP52 binds directly to the hsp90-free glucocorticoid receptor. Because neither the PPIase fragment nor the TPR fragment affects the binding of FKBP52 to the glucocorticoid receptor under conditions in which they block FKBP52 binding to dynein or hsp90, respectively, different regions of FKBP52 must determine its association with these three proteins (Silverstein, 1999).

Interferon regulatory factor-4 (IRF-4) plays an important role in immunoregulatory gene expression in B and T lymphocytes and is also highly expressed in human T cell leukemia virus type 1 infected cells. A novel interaction has been characterized between IRF-4 and the FK506-binding protein 52 (FKBP52), a 59 kDa member of the immunophilin family with peptidyl-prolyl isomerase activity (PPIase). IRF-4-FKBP52 association inhibits IRF4-PU.1 binding to the immunoglobulin light chain enhancer E(lambda2-4) as well as IRF-4-PU.1 transactivation, effects that are dependent on functional PPIase activity. FKBP52 association also results in a structural modification of IRF-4, detectable by immunoblot analysis and by IRF-4 partial proteolysis. These results demonstrate a novel posttranslational mechanism of transcriptional control, mediated through the interaction of an immunophilin with a transcriptional regulator (Mamane, 2000).

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