Time-dependent inhibition of peptidylprolyl cis-trans-isomerases by FK506 is probably due to cis-trans isomerization of the inhibitor's imide bond

Functional identification of a Streptomyces lividans FKBP-like protein involved in the folding of overproduced secreted proteins

Some bacterial peptidyl-prolyl cis/trans isomerases (PPIases) are involved in secretory protein folding after the translocation step. Streptomyces lividans has been used as a host for engineering extracellular overproduction of homologous and heterologous proteins in industrial applications. Although the mechanisms governing the major secretory pathway (Sec route) and the minor secretory pathway (Tat route) are reasonably well described, the function of proteins responsible for the extracellular secretory protein folding is not characterized as yet. We have characterized a Tat-dependent S. lividans FK506-binding protein-like lipoprotein (FKBP) that has PPIase activity. A mutant in the sli-fkbp gene induces a secretion stress response and affects secretion and activity of the Sec-dependent protein α-amylase. Additionally, propagation in high copy number of the sli-fkbp gene has a positive effect on the activity of both the overproduced α-amylase and the overproduced Tat-dependent agarase, both containing proline cis isomers. Targeted proteomic analyses showed that a relevant group of secreted proteins in S. lividans TK21 are affected by Sli-FKBP, revealing a wide substrate range. The results obtained indicate that, regardless of the secretory route used by proteins in S. lividans, adjusting the expression of sli-fkbp may facilitate folding of dependent proteins when engineering Streptomyces strains for the overproduction of homologous or heterologous secretory proteins.

Pharmacological targeting of catalyzed protein folding: the example of peptide bond cis/trans isomerases

Peptide bond isomerases are involved in important physiological processes that can be targeted in order to treat neurodegenerative disease, cancer, diseases of the immune system, allergies, and many others. The folding helper enzyme class of Peptidyl-Prolyl-cis/trans Isomerases (PPIases) contains the three enzyme families of cyclophilins (Cyps), FK506 binding proteins (FKBPs), and parvulins (Pars). Although they are structurally unrelated, all PPIases catalyze the cis/trans isomerization of the peptide bond preceding the proline in a polypeptide chain. This process not only plays an important role in de novo protein folding, but also in isomerization of native proteins. The native state isomerization plays a role in physiological processes by influencing receptor ligand recognition or isomer-specific enzyme reaction or by regulating protein function by catalyzing the switch between native isomers differing in their activity, e.g., ion channel regulation. Therefore elucidating PPIase involvement in physiological processes and development of specific inhibitors will be a suitable attempt to design therapies for fatal and deadly diseases.

Reversible inhibition of calcineurin by the polyphenolic aldehyde gossypol

The reversible inhibition of calcineurin (CaN), which is the only Ca(2+)/calmodulin-dependent protein Ser/Thr phosphatase, is thought to be a key functional event for most cyclosporin A (CsA)- and tacrolimus (FK506)-mediated biological effects. In addition to CaN inhibition, however, CsA and FK506 have multiple biochemical effects because of their action in a gain-of-function model that requires prior binding to immunophilic proteins. We screened a small molecule library for direct inhibitors of CaN using CaN-mediated dephosphorylation of (33)P-labeled 19-residue phosphopeptide substrate (RII phosphopeptide) as an assay and found the polyphenolic aldehyde gossypol to be a novel CaN inhibitor. Unlike CsA and FK506, gossypol does not require a matchmaker protein for reversible CaN inhibition with an IC(50) value of 15 microm. Gossypolone, a gossypol analog, showed improved inhibition of both RII phosphopeptide and p-nitrophenyl phosphate dephosphorylation with an IC(50) of 9 and 6 microm, respectively. In contrast, apogossypol hexaacetate was inactive. Gossypol acts noncompetitively, interfering with the binding site for the cyclophilin 18.CsA complex in CaN. In contrast to CsA and FK506, gossypol does not inactivate the peptidyl-prolyl-cis/trans-isomerase activity of immunophilins. Similar to CsA and FK506, T cell receptor signaling induced by phorbol 12-myristate 13-acetate/ionomycin is inhibited by gossypol in a dose-dependent manner, demonstrated by the inhibition of nuclear factor of activated T cell (NFAT) c1 translocation from the cytosol into the nucleus and suppression of NFAT-luciferase reporter gene activity.

Augmentation of SR Ca(2+) release by rapamycin and FK506 causes K(+)-channel activation and membrane hyperpolarization in bladder smooth muscle

1. The immunosuppressants rapamycin and FK506 are known to relax smooth muscle despite facilitating Ca(2+) release through ryanodine-receptors of the sarcoplasmic reticulum (SR). The apparent contradiction was studied in isolated guinea-pig urinary bladder myocytes. 2. Modulation of spontaneous SR Ca(2+) release was monitored by means of spontaneous transient outward currents (or STOCs) in isolated smooth muscle cells voltage-clamped to -20 mV. Rapamycin (10 microM, n=18) significantly increased amplitude (50+/-12%, mean+/-s.d.), life time (77+/-19%), and time integral of STOCs (113+/-22%), and it reduced the interval between STOCs (20+/-7%). FK506 (20 microM, n=24) increased amplitude (15+/-7%), life time (50+/-7%), time integral (104+/-26%). Cyclosporin A (20 microM, n=18) had no significant effects on STOCs. 3. The basal cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) measured by Indo1-fluorescence was insensitive to rapamycin or FK506. Pretreatment with rapamycin (20 microM, 2 min) did not impair the SR Ca(2+) load as can be concluded from caffeine-induced Ca(2+)-transients. 4. As it was expected from the enhanced STOC activity, the non-clamped membrane was hyperpolarized by rapamycin (15+/-2 mV) or by FK506 (15+/-3 mV). 5. The data are consistent with the idea that rapamycin and FK506 augment spontaneous SR Ca(2+) release by removal of FK-binding proteins from the RyR-complex. Smooth muscle relaxation is interpreted as negative Ca(2+) feedback: augmented Ca(2+) activation of STOCs induces membrane hyperpolarization that reduces Ca(2+) influx through voltage gated channels.

Modular structure of the trigger factor required for high activity in protein folding

The Escherichia coli trigger factor is a peptidyl-prolyl cis/trans isomerase (PPIase) which catalyzes proline-limited protein folding extremely well. It has been found associated with nascent protein chains as well as with the chaperone GroEL. The trigger factor utilizes protein regions outside the central catalytic domain for catalyzing refolding of unfolded proteins efficiently. Here we produced several fragments which encompass individual domains or combinations of the middle FKBP-like domain (M) with the N-terminal (N) and C-terminal (C) regions, respectively. These fragments appear to be stably folded. They show ordered structure and cooperative urea-induced unfolding transitions, and the far-UV CD spectrum of the intact trigger factor is well represented by the sum of the spectra of the fragments. This suggests that the native trigger factor shows a modular structure, which is composed of three fairly independent folding units. In the intact protein there is a slight mutual stabilization of these units. The high enzymatic activity in protein folding could not be restored by fusing alternatively the N or the C-terminal regions to the catalytic domain (in NM and MC constructs, respectively). Surprisingly, the high folding activity of the intact trigger factor has been regained partially by functional complementation of the overlapping NM and MC constructs.

Comparative mutational analysis of peptidyl prolyl cis/trans isomerases: active sites of Escherichia coli trigger factor and human FKBP12

A low degree of amino acid sequence similarity to FK506-binding proteins (FKBPs) has been obtained for the peptidyl prolyl cis/trans isomerase (PPIase) domain of E. coli trigger factor (TF) that was thought to be significant with regard to the enzymatic properties of the bacterial enzyme. We examined whether the alteration of a negatively charged side-chain at position 37 (FKBP numbering) and a phenylalanine at position 99, both highly conserved through both types of enzymes, leads to parallel effects on the catalytic activity of both FKBP12 and TF-PPIase domain in a series of tetrapeptide substrates with different P1 subsites. For the latter enzyme, substitution of Glu178 by Val or Lys, which aligns to Asp37 in human FKBP12, enhanced the PPIase activity, whereas a strongly decreased enzymatic activity was determined for the Asp37Leu and Asp37Val variants of FKBP12. Regardless of the P1 subsite of the substrate used for the assay, mutation of Phe233Tyr generated a protein variant of the TF-PPIase domain with about 1% of the wild type PPIase activity. Dependent on the substrate nature, a moderate decrease as well as a 4.8-fold increase in k(cat)/K(M) could be determined for the corresponding Phe99Tyr FKBP12 variant. Neither of the mutations of the TF-PPIase domain was able to implant FK506 inhibition found as a major characteristic of the FKBP family of PPIases.

Autocatalytic folding of the folding catalyst FKBP12

Prolyl isomerases are folding enzymes and thus have the potential to catalyze their own folding. We show here that the folding of cytosolic FKBP12 (FK 506 binding protein) is an autocatalytic process both for the mature protein and for a fusion protein with an amino-terminal extension of 16 residues. Native FKBP contains seven trans-prolyl peptide bonds, and the cis-to-trans isomerizations of some or all of them constitute the slow, rate-limiting events in folding. The rate of an autocatalytic reaction increases with reactant concentration, because the product catalyzes its own formation. Accordingly, the folding of the fusion protein was more than 10-fold accelerated when the protein concentration was increased from 0.05 microM to 10 microM. At high concentrations of both forms of FKBP12 autocatalysis was very efficient, and the observed folding rate seemed to approach the rate of the fast direct folding reaction of the protein molecules with the correct (all trans) peptidyl-prolyl bond conformation.