HDAC4, a human histone deacetylase related to yeast HDA1, is a transcriptional corepressor

Histone acetylation plays an important role in regulating chromatin structure and thus gene expression. Here we describe the functional characterization of HDAC4, a human histone deacetylase whose C-terminal part displays significant sequence similarity to the deacetylase domain of yeast HDA1. HDAC4 is expressed in various adult human tissues, and its gene is located at chromosome band 2q37. HDAC4 possesses histone deacetylase activity intrinsic to its C-terminal domain. When tethered to a promoter, HDAC4 represses transcription through two independent repression domains, with repression domain 1 consisting of the N-terminal 208 residues and repression domain 2 containing the deacetylase domain. Through a small region located at its N-terminal domain, HDAC4 interacts with the MADS-box transcription factor MEF2C. Furthermore, HDAC4 and MEF2C individually upregulate but together downmodulate c-jun promoter activity. These results suggest that HDAC4 interacts with transcription factors such as MEF2C to negatively regulate gene expression.

Identification of a human histone acetyltransferase related to monocytic leukemia zinc finger protein

We describe here the identification and functional characterization of a novel human histone acetyltransferase, termed MORF (monocytic leukemia zinc finger protein-related factor). MORF is a 1781-residue protein displaying significant sequence similarity to MOZ (monocytic leukemia zinc finger protein). MORF is ubiquitously expressed in adult human tissues, and its gene is located at human chromosome band 10q22. MORF has intrinsic histone acetyltransferase activity. In addition to its histone acetyltransferase domain, MORF possesses a strong transcriptional repression domain at its N terminus and a highly potent activation domain at its C terminus. Therefore, MORF is a novel histone acetyltransferase that contains multiple functional domains and may be involved in both positive and negative regulation of transcription.

Hypothalamic glucose sensor: similarities to and differences from pancreatic beta-cell mechanisms

Glucose-responsive neurons in the ventromedial hypothalamus (VMH) are stimulated when glucose increases from 5 to 20 mmol/l and are thought to play an essential role in regulating metabolism. The present studies examined the role of glucose metabolism in the mechanism by which glucose-responsive neurons sense glucose. The pancreatic, but not hepatic, form of glucokinase was expressed in the VMH, but not cerebral cortex, of adult rats. In brain slice preparations, the transition from 5 to 20 mmol/l glucose stimulated approximately 17% of the neurons (as determined by single-cell extracellular recording) from VMH but none in cortex. In contrast, most cells in both VMH and cortex were silent below 1 mmol/l and active at 5 mmol/l glucose. Glucosamine, 2-deoxyglucose, phloridzin, and iodoacetic acid blocked the activation of glucose-responsive neurons by the transition from 5 to 20 mmol/l glucose. Adding 15 mmol/l mannose, galactose, glyceraldehyde, glycerol, and lactate to 5 mmol/l glucose stimulated glucose-responsive neurons. In contrast, adding 15 mmol/l pyruvate to 5 mmol/l glucose failed to activate glucose-responsive neurons, although pyruvate added to 0 mmol/l glucose permitted neurons to maintain activity. Tolbutamide activated glucose-responsive neurons; however, diazoxide only blocked the effect of glucose in a minority of neurons. These data suggest that glucose-responsive neurons sense glucose through glycolysis using a mechanism similar to the mechanism of pancreatic beta-cells, except that glucose-responsive neurons are stimulated by glycerol and lactate, and diazoxide does not generally block the effect of glucose.

Significance of small foci of Gleason score 7 or greater prostate cancer on needle biopsy

OBJECTIVES

With increased screening for prostate cancer, we have noted a greater number of patients with small foci of Gleason score 7 or greater prostate cancer on needle biopsy. The significance of these findings is unknown.

METHODS

We studied 57 men with small foci of Gleason score 7 or greater on needle biopsy. Tumor length was less than 1.5 mm in all but 2 cases. In those 2 cases, there were two minute (less than 0.5 mm) foci of cancer separated by 1.8 mm. The length of cancer ranged from 0.2 to 1.8 mm (mean 0.63 mm). In all cases, only one core was involved.

RESULTS

Thirty-three men underwent radical prostatectomy (RP), 14 received radiation, 8 underwent surveillance, and 2 received hormonal therapy. Men who underwent RP were younger (62 years) than those who had radiotherapy (69.1 years), who were younger than those who underwent surveillance (74.5 years). The mean prostate-specific antigen (PSA) for men undergoing RP was 8.0 ng/mL (range 1.4 to 22). Preoperative serum PSA values did not predict organ-confined status. Needle biopsy grades were as follows: 3 + 4 = 7 (n = 30); 4 + 3 = 7 (n = 17); 4 + 4 = 8 (n = 7); 5 + 4 = 9 (n = 1); and 5 + 5 = 10 (n = 2). We were able to review slides in 27 of the RP specimens, of which 24 were well sampled. Of these 24 cases, 33% had positive margins and 33% were not organ confined; the median tumor volume was 0.5 cc (mean 1.04). No difference in RP tumor volume was found between tumors with needle biopsy Gleason primary grade 3 and those with 4 or greater. The percentage of Gleason pattern 4 on needle biopsy weakly correlated with the percentage of Gleason pattern 4 in the RP specimen (P = 0.04). However, the percentage of Gleason pattern 4 only in the RP specimen, but not in the biopsy, correlated with whether the tumor was organ confined.

CONCLUSIONS

The likelihood of having organ-confined disease with small foci of Gleason score 7 or greater on needle biopsy appears to be equivalent to that calculated from the Partin Tables for greater amounts of Gleason score 6 cancer on needle biopsy. In men who are considering RP, small foci of Gleason score 7 or greater adenocarcinoma on needle biopsy should not necessarily be considered an adverse finding.

Does long-term finasteride therapy affect the histologic features of benign prostatic tissue and prostate cancer on needle biopsy? PLESS Study Group. Proscar Long-Term Efficacy and Safety Study

OBJECTIVES

Finasteride, a common agent used to treat benign prostatic hyperplasia (BPH), inhibits 5-alpha-reductase. Testosterone is converted by 5-alpha-reductase to the more potent dihydrotestosterone, which is the primary androgen in the prostate. Leuprolide is a stronger antiandrogen that is used to downstage prostate cancer before radical prostatectomy. Leuprolide induces marked atrophy of prostate carcinoma cells, which sometimes makes pathologic diagnosis of cancer difficult, although evaluation at radical prostatectomy is easier than at biopsy. It is unknown whether finasteride produces similar changes, which would result in greater diagnostic difficulty because such changes would be seen on biopsy to rule out cancer in men with suspicious clinical findings treated for BPH. The current study investigated the histologic effects of finasteride therapy on human prostate cancer and benign prostatic tissue on needle biopsy.

METHODS

In blinded manner, we reviewed 53 needle biopsy specimens showing prostate carcinoma (35 treated with finasteride, 18 with placebo). Also reviewed in blinded manner were 50 benign needle biopsy specimens (25 treated with finasteride, 25 with placebo). The Gleason score, number of cores involved, percentage cancer involvement in a core, percentage of atrophic changes in cancer cells, presence of mitoses, blue-tinged mucinous secretions, prominent nucleoli, and high-grade prostatic intraepithelial neoplasia were documented for each case in the cancer group. The percentage of atrophy, basal cell hyperplasia, transitional metaplasia, chronic inflammation, and stromal proliferation was documented for each case in the benign group.

RESULTS

No significant histologic differences were present in either the benign or cancer group between cases treated with finasteride and placebo.

CONCLUSIONS

We conclude that finasteride treatment for BPH does not cause difficulty in the diagnosis of cancer in prostate needle specimens. It is possible that there are severely atrophic areas resulting from finasteride treatment that are undersampled. However, the conclusion that cancer seen on needle biopsy in men treated with finasteride is unaltered and readily identified as cancer remains valid.

The histone acetylase PCAF is a phorbol-ester-inducible coactivator of the IRF family that confers enhanced interferon responsiveness

Transcription factors of the interferon regulatory factor (IRF) family bind to the type I interferon (IFN)-responsive element (ISRE) and activate transcription from IFN-inducible genes. To identify cofactors that associate with IRF proteins, DNA affinity binding assays were performed with nuclear extracts prepared from tissue culture cells. The results demonstrated that the endogenous IRFs bound to the ISRE are complexed with the histone acetylases, PCAF, GCN5, and p300/CREB binding protein and that histone acetylase activities are accumulated on the IRF-ISRE complexes. By testing recombinant proteins, we show that PCAF directly binds to some but not all members of the IRF family through distinct domains of the two proteins. This interaction was functionally significant, since transfection of PCAF strongly enhanced IRF-1- and IRF-2-dependent promoter activities. Further studies showed that expression of PCAF and other histone acetylases was markedly induced in U937 cells upon phorbol ester treatment, which led to increased recruitment of PCAF to the IRF-ISRE complexes. Coinciding with the induction of histone acetylases, phorbol ester markedly enhanced IFN-alpha-stimulated gene expression in U937 cells. Supporting the role for PCAF in conferring IFN responsiveness, transfection of PCAF into U937 cells led to a large increase in IFN-alpha-inducible promoter activity. These results demonstrate that PCAF is a phorbol ester-inducible coactivator of the IRF proteins which contributes to the establishment of type I IFN responsiveness.

[The investigation analysis of malocclusion in the juvenile in Shanghai Pudong New Area]

OBJECTIVE:To understanding the rate of malocclusion in the juvenile in Shanghai Pudong new area.Emphasising to know the rate of crowding,overbite or overjet and anterior crossbite.METHODS:1785 person with the age from 7 to 12 are investigated.According to Angle's classification of malocclusion they are classify.RESULTS:The rate of malocclusion is 44.02% in the mix dentition and 48.78% in the permanent dentition.The crowding rates are:mix dentition 22.74%,permanent dentition 32.36%.The overbite or overjet rates are:mix dentition 17.18%,permanent dentition 11.06%.The anterior crossbite rates are:mix dentition 4.44%,permanent dentition 4.07%. CONCLUSION:There are no significant differences between the rate of malocclusion in thr mix and permanent dentitions.It is important that the patients with the overbite/overjet and crossbite should be treated as early as possible.

Rare expression of high-molecular-weight cytokeratin in adenocarcinoma of the prostate gland: a study of 100 cases of metastatic and locally advanced prostate cancer

Immunohistochemistry with antibodies for high-molecular-weight cytokeratin labels basal cells and is used as an ancillary study in diagnosing prostate carcinoma, which reportedly lacks expression of high-molecular-weight cytokeratin. A recent report questioned the specificity of this marker, describing immunopositivity for high-molecular-weight cytokeratin in a small series of metastatic prostate cancer. We have also noted rare cases of prostate lesions on biopsy with typical histological features of adenocarcinoma showing immunopositivity for high-molecular-weight cytokeratin, either in tumor cells or in patchy cells with the morphology of basal cells. In some of these cases, it was difficult to distinguish cancer from out-pouching of high-grade prostatic intraepithelial neoplasia. To investigate whether prostate cancer cells express high-molecular-weight cytokeratin, we studied 100 cases of metastatic prostate carcinoma and 10 cases of prostate cancer invading the seminal vesicles from surgical specimens. Metastatic sites included regional lymph nodes (n = 67), bone (n = 19), and miscellaneous (n = 14). Cases with any positivity for high-molecular-weight cytokeratin antibody (34betaE12) were verified as being of prostatic origin with immunohistochemistry for prostate-specific antigen and prostate-specific acid phosphatase. Only four cases were detected positive for high-molecular-weight cytokeratin. In two cases (one metastasis, one seminal vesicle invasion) there was weakly diffuse positivity above background level. Two metastases in lymph nodes showed scattered strong staining of clusters of tumor cells, which represented <0.2% of tumor cells in the metastatic deposits. These positive cells did not have the morphology of basal cells. We conclude that prostate cancer, even high grade, only rarely expresses high-molecular-weight cytokeratin. This marker remains a very useful adjunct in the diagnosis of prostate cancer.

Distinction of basaloid carcinoma of the prostate from benign basal cell lesions by using immunohistochemistry for bcl-2 and Ki-67

The distinction of rare basaloid carcinomas (BC) of the prostate from more common basal cell hyperplasia may be difficult, because basal cell hyperplasia (BCH) may have prominent nucleoli and may appear infiltrative. Using immunohistochemistry, we studied bcl-2 and p53 expression and Ki-67 proliferation index in eight cases of typical BCH, eight cases of BCH with nucleoli, and six cases of BC. Bcl-2 expression (P < .0001) and Ki-67 index (P=.005) were elevated in BC compared with typical BCH or BCH with nucleoli, whereas there was no significant difference between typical BCH and BCH with nucleoli. P53 was not discriminative in separating benign from malignant basal cell lesions of the prostate. Bcl-2 may play a role in the pathogenesis of basal cell lesions of the prostate. Elevated expression of bcl-2 and higher Ki-67 index may aid in the diagnosis of basal cell proliferative lesions of the prostate.

Long-distance transcriptional enhancement by the histone acetyltransferase PCAF

Enhancers are defined by their ability to stimulate gene activity from remote sites and their requirement for promoter-proximal upstream activators to activate transcription. Here we demonstrate that recruitment of the p300/CBP-associated factor PCAF to a reporter gene is sufficient to stimulate promoter activity. The PCAF-mediated stimulation of transcription from either a distant or promoter-proximal position depends on the presence of an upstream activator (Sp1). These data suggest that acetyltransferase activity may be a primary component of enhancer function, and that recruitment of polymerase and enhancement of transcription are separable. Transcriptional activation by PCAF requires both its acetyltransferase activity and an additional activity within its N terminus. We also show that the simian virus 40 enhancer and PCAF itself are sufficient to counteract Mad-mediated repression. These results are compatible with recent models in which gene activity is regulated by the competition between deacetylase-mediated repression and enhancer-mediated recruitment of acetyltransferases.

Use of the appendix to replace the choledochus

Ten cases of choledochal cyst (CC) were treated by biliary-appendicoduodenostomy. The follow-up comprised a patient interview, ultrasonography (US), and single-proton ejected computerized tomography (SPECT) scanning. In all cases an anti-reflux submucosal tunnel was added to the distal appendico-duodenostomy; all showed an uneventful postoperative course. All the dilated intrahepatic bile ducts had normalized on B-US postoperatively. Four children under went SPECT examination; all of them had patent neo-bile ducts. In the authors' opinion: (1) Anastomosing the cecal end of the appendix to the common hepatic duct seemed more favorable than the other way around, because the cecal end could be easily trimmed to the size of the common hepatic duct, which was more or less dilated in the presence of a CC; (2) It is necessary to add a submucosal tunnel to the distal appendicoduodenostomy to achieve a more reliable anti-reflux effect; and (3) Transposing the vascularized appendix through the retro-transverse colon simplified the procedure and might reduce the risk of retroperitoneal complications if bile leakage should occur.

A human RNA polymerase II complex containing factors that modify chromatin structure

We have isolated a human RNA polymerase II complex that contains chromatin structure remodeling activity and histone acetyltransferase activity. This complex contains the Srb proteins, the Swi-Snf complex, and the histone acetyltransferases CBP and PCAF in addition to RNA polymerase II. Notably, the general transcription factors are absent from this complex. The complex was purified by two different methods: conventional chromatography and affinity chromatography using antibodies directed against CDK8, the human homolog of the yeast Srb10 protein. Protein interaction studies demonstrate a direct interaction between RNA polymerase II and the histone acetyltransferases p300 and PCAF. Importantly, p300 interacts specifically with the nonphosphorylated, initiation-competent form of RNA polymerase II. In contrast, PCAF interacts with the elongation-competent, phosphorylated form of RNA polymerase II.

Histone-like TAFs within the PCAF histone acetylase complex

PCAF histone acetylase plays a role in regulation of transcription, cell cycle progression, and differentiation. Here, we show that PCAF is found in a complex consisting of more than 20 distinct polypeptides. Strikingly, some polypeptides are identical to TBP-associated factors (TAFs), which are subunits of TFIID. Like TFIID, histone fold-containing factors are present within the PCAF complex. The histone H3- and H2B-like subunits within the PCAF complex are identical to those within TFIID, namely, hTAF(II)31 and hTAF(II)20/15, respectively. The PCAF complex has a novel histone H4-like subunit with similarity to hTAF(II)80 that interacts with the histone H3-like domain of hTAF(II)31. Moreover, the PCAF complex has a novel subunit with WD40 repeats having a similarity to hTAF(II)100.

Cloning of Drosophila GCN5: conserved features among metazoan GCN5 family members

PCAF and hGCN5 are distinct human genes that encode proteins related to the yeast histone acetyltransferase and transcriptional adapter GCN5. The PCAF protein shares extensive similarity with the 439 amino acids of yGCN5, but it has an approximately 350 amino acid N-terminal extension that interacts with the transcriptional co-activator p300/CBP. Adenoviral protein E1a can disrupt PCAF-CBP interactions and prevent PCAF-dependent cellular differentiation. In this report, we describe the cloning and initial characterization of a Drosophila homolog of yGCN5. In addition to the homology to yGCN5, the Drosophila protein shares sequencesimilarity with the N-terminal portion of human PCAF that is involved in binding to CBP. In the course of characterizing dGCN5, we have discovered that hGCN5 also contains an N-terminal extension with significant similarity to PCAF. Interestingly, in the case of the h GCN5 gene, alternative splicing may regulate the production of full-length hGCN5. The presence of the N-terminal domain in a Drosophila GCN5 homolog and both human homologs suggests that it was part of the ancestral form of metazoan GCN5.

The histone acetylase PCAF is a nuclear receptor coactivator

Whereas the histone acetylase PCAF has been suggested to be part of a coactivator complex mediating transcriptional activation by the nuclear hormone receptors, the physical and functional interactions between nuclear receptors and PCAF have remained unclear. Our efforts to clarify these relationships have revealed two novel properties of nuclear receptors. First, we demonstrate that the RXR/RAR heterodimer directly recruits PCAF from mammalian cell extracts in a ligand-dependent manner and that increased expression of PCAF leads to enhanced retinoid-responsive transcription. Second, we demonstrate that, in vitro, PCAF directly associates with the DNA-binding domain of nuclear receptors, independently of p300/CBP binding, therefore defining a novel cofactor interaction surface. Furthermore, our results show that dissociation of corepressors enables ligand-dependent PCAF binding to the receptors. This observation illuminates how a ligand-dependent receptor function can be propagated to regions outside the ligand-binding domain itself. On the basis of these observations, we suggest that PCAF may play a more central role in nuclear receptor function than previously anticipated.

Human histone acetyltransferase GCN5 exists in a stable macromolecular complex lacking the adapter ADA2

Acetylation of core histones is an important regulatory step in transcriptional activation from chromatin templates. The yeast transcriptional coactivator protein GCN5 was recently shown to be a nuclear histone acetyltransferase (HAT). Genetic and biochemical studies in yeast suggest that GCN5 functions with the adapter proteins ADA1, ADA2, ADA3, and ADA5 in a heteromeric complex. We have established conditions for chromatographic fractionation of HATs and ADA2 from human K562 erythroleukemia cells. Gel-filtration chromatography revealed two populations of GCN5 with Stokes' radii of 67 and 33 A, consistent with a large macromolecular complex and a monomer, respectively. The GCN5-related HAT, PCAF, was resolved as a stable complex with a Stokes' radius of 74 A. The HAT complexes were resistant to 0.3 M NaCl and DNase I. ADA2 was characterized by a Stokes' radius of 35 A, consistent with a monomer. Thus, in contrast to the stable GCN5-adapter complex in yeast, human GCN5 and ADA2 are not stably associated with each other. The implications of this result are discussed vis-a-vis the mechanism of recruitment of GCN5 to regulatory regions of genes.

Differential roles of p300 and PCAF acetyltransferases in muscle differentiation

PCAF is a histone acetyltransferase that associates with p300/CBP and competes with E1A for access to them. While exogenous expression of PCAF potentiates both MyoD-directed transcription and myogenic differentiation, PCAF inactivation by anti-PCAF antibody microinjection prevents differentiation. MyoD interacts directly with both p300/CBP and PCAF, forming a multimeric protein complex on the promoter elements. Viral transforming factors that interfere with muscle differentiation disrupt this complex without affecting the MyoD-DNA interaction, indicating functional significance of the complex formation. Exogenous expression of PCAF or p300 promotes p21 expression and terminal cell-cycle arrest. Both of these activities are dependent on the histone acetyltransferase activity of PCAF, but not on that of p300. These results indicate that recruitment of histone acetyltransferase activity of PCAF by MyoD, through p300/CBP, is crucial for activation of the myogenic program.

The histone acetyltransferase activity of human GCN5 and PCAF is stabilized by coenzymes

Here we report that PCAF and human GCN5, two related type A histone acetyltransferases, are unstable enzymes that under the commonly used assay conditions are rapidly and irreversibly inactivated. In addition, we report that free histone H1, although not acetylated in vivo, is a preferred and convenient in vitro substrate for the study of PCAF, human GCN5, and possibly other type A histone acetyltransferases. Using either histone H1 or histone H3 as substrates, we find that preincubation with either acetyl-CoA or CoA stabilizes the acetyltransferase activities of PCAF, human GCN5 and an enzymatically active PCAF deletion mutant containing the C-terminal half of the protein. The stabilization requires the continuous presence of coenzyme, suggesting that the acetyltransferase-coenzyme complexes are stable, while the isolated apoenzymes are not. Human GCN5 and the N-terminal deletion mutant of PCAF are stabilized equally well by preincubation with either CoA or acetyl-CoA, while intact PCAF is better stabilized by acetyl-CoA than by CoA. Intact PCAF, but not the N-terminal truncation mutant or human GCN5, is autoacetylated. These findings raise the possibility that the intracellular concentrations of the coenzymes affect the stability and therefore the nuclear activity of these acetyltransferases.

Acetylation of general transcription factors by histone acetyltransferases

The acetylation of histones increases the accessibility of nucleosomal DNA to transcription factors [1,2], relieving transcriptional repression [3] and correlating with the potential for transcriptional activity in vivo [4 - 7]. The characterization of several novel histone acetyltransferases - including the human GCN5 homolog PCAF (p300/CBP-associated factor) [8], the transcription coactivator p300/CBP [9], and TAFII250 [10] - has provided a potential explanation for the relationship between histone acetylation and transcriptional activation. In addition to histones, however, other components of the basal transcription machinery might be acetylated by these enzymes and directly affect transcription. Here, we examine the acetylation of the basal transcriptional machinery for RNA polymerase II by PCAF, p300 and TAFII250. We find that all three acetyltransferases can direct the acetylation of TFIIEbetaand TFIIF, and we identify a preferred site of acetylation in TFIIEbeta. Human TFIIE consists of two subunits, alpha(p56) and beta(p34), which form a heterotetramer (alpha2 beta2) in solution ([11], reviewed in [12]). TFIIE enters the preinitiation complex after RNA polymerase II and TFIIF, suggesting that TFIIE may interact directly with RNA polymerase II and/or TFIIF [13,14]. In addition, TFIIE can facilitate promoter melting either in the presence or absence of TFIIH and can stimulate TFIIH-dependent phosphorylation of the carboxy-terminal domain of RNA polymerase II [15-18]. TFIIF has an essential role in both transcription initiation and elongation ([19,20], for review see [21]). We discuss the implications of the acetylation of TFIIEbetaand TFIIF for transcriptional control by PCAF, p300 and TAFII250.

The TAF(II)250 subunit of TFIID has histone acetyltransferase activity

The transcription initiation factor TFIID is a multimeric protein complex composed of TATA box-binding protein (TBP) and many TBP-associated factors (TAF(II)s). TAF(II)s are important cofactors that mediate activated transcription by providing interaction sites for distinct activators. Here, we present evidence that human TAF(II)250 and its homologs in Drosophila and yeast have histone acetyltransferase (HAT) activity in vitro. HAT activity maps to the central, most conserved portion of dTAF(II)230 and yTAF(II)130. The HAT activity of dTAF(II)230 resembles that of yeast and human GCN5 in that it is specific for histones H3 and H4 in vitro. Our findings suggest that targeted histone acetylation at specific promoters by TAF(II)250 may be involved in mechanisms by which TFIID gains access to transcriptionally repressed chromatin.

A p300/CBP-associated factor that competes with the adenoviral oncoprotein E1A

The adenoviral oncoprotein E1A induces progression through the cell cycle by binding to the products of the p300/CBP and retinoblastoma gene families. A new cellular p300/CBP-associated factor (P/CAF) having intrinsic histone acetylase activity has been identified that competes with E1A. Exogenous expression of P/CAF in HeLa cells inhibits cell-cycle progression and counteracts the mitogenic activity of E1A. E1A disturbs the normal cellular interaction between p300/CBP and its associated histone acetylase.

Kinetic characterization of channel impaired mutants of tryptophan synthase

Tryptophan synthase, an alpha 2 beta 2 tetrameric complex, is a classic example of an enzyme that is thought to "channel" a metabolic intermediate (indole) from the active site of the alpha subunit to the active site of the beta subunit. The solution of the three-dimensional structure of the enzyme from Salmonella typhimurium provided physical evidence for a 25-A hydrophobic tunnel which connects the alpha and beta active sites (Hyde, C. C., Ahmed, S. A., Padlan, E. A., Miles, E. W., and Davies, D. R. (1988) J. Biol. Chem. 263, 17857-17871). Using rapid reaction kinetics, we have previously established that indole is indeed channeled and have identified three essential kinetic features which govern efficient channeling. In the current study we have probed the necessity of these features by using site-directed mutagenesis to alter these requirements. We now report the kinetic characterization of two mutants which contain substitutions to block or restrict the tunnel (beta C170F and beta C170W). Preliminary kinetic and structural evidence of a restricted tunnel in the beta C170W has been provided (Schlichting, I., Yang, X. W., Miles, E. W., Kim, A. Y., and Anderson, K. S. (1994) J. Biol. Chem. 269, 26591-26593). The rapid kinetic analysis of these mutant proteins shows that these mutations interfere with efficient channeling of the indole metabolite such that indole can be observed in single enzyme turnover of the physiologically relevant alpha beta reaction. In addition, the beta C170W mutant appears to be impaired in alpha beta intersubunit communication.

Further studies of the role of Ser-16 in the regulation of the activity of phenylalanine hydroxylase

It was previously proposed that the activation of rat liver phenylalanine hydroxylase (EC 1.14.16.1) by cAMP-dependent protein kinase-mediated phosphorylation of Ser-16 is due to the introduction of the negatively charged phosphate group. To explore the validity of this proposal, we have applied site-directed mutagenesis to specifically replace Ser-16 with negatively charged amino acids, glutamic and aspartic; with polar uncharged amino acids, asparagine and glutamine; with the positively charged amino acid lysine; and with the nonpolar hydrophobic amino acid alanine. The wild-type and mutant enzymes were purified to homogeneity, and the importance of Ser-16 in the activation of phenylalanine hydroxylase was examined by comparing the state of activation of the phosphorylated form of the wild-type hydroxylase with that of the mutants. The kinetic studies carried out on the wild-type phosphorylated hydroxylase showed that all the activation could be accounted for by an increase in Vmax with no change in Km for either phenylalanine or the pterin cofactor. Replacement of Ser-16 with a negatively charged residue, glutamate of aspartate, resulted in the activation of the hydroxylase by 2- to 4-fold, whereas replacement with glutamine, asparagine, lysine, or alanine resulted in a much more modest increase. Further, lysolecithin was found to stimulate the phosphorylated hydroxylase and the mutant enzymes S16E and S16D by a factor of 6-7. In contrast, the mutants S16Q, S16N, and S16A all showed the same magnitude of activation as the wild-type with lysolecithin. Therefore, this study demonstrates that activation of the enzyme by phosphorylation of Ser-16 by cAMP-dependent protein kinase is due to the introduction of negative charge(s) and strongly suggests the involvement of electrostatic interaction between the regulatory and catalytic domains of the hydroxylase.

Ligand-mediated changes in the tryptophan synthase indole tunnel probed by nile red fluorescence with wild type, mutant, and chemically modified enzymes

The bacterial tryptophan synthase alpha 2 beta 2 complex contains an unusual structural feature: an intramolecular tunnel that channels indole from the active site of the alpha subunit to the active site of the beta subunit 25 A away. Here we investigate the role of the tunnel in communication between the alpha and beta subunits using the polarity-sensitive fluorescent probe, Nile Red. Interaction of Nile Red in the nonpolar tunnel near beta subunit residues Cys-170 and Phe-280 is supported by studies with enzymes altered at these positions. Restricting the tunnel by enlarging Cys-170 by chemical modification or mutagenesis decreases the fluorescence of Nile Red by 30-70%. Removal of a partial restriction in the tunnel by replacing Phe-280 by Cys or Ser increases the fluorescence of Nile Red more than 2-fold. A binding site for Nile Red in this region near the pyridoxal phosphate coenzyme of the beta subunit is further supported by iodide quenching and fluorescence energy transfer experiments and by molecular modeling based on the three-dimensional structure of the alpha 2 beta 2 complex. Finally, studies using Nile Red as a sensitive probe of conformational changes in the tunnel reveal that allosteric ligands (alpha subunit) or active site ligands (beta subunit) decrease the fluorescence of Nile Red. We speculate that allosteric and active site ligands induce a tunnel restriction near Phe-280 that serves as a gate to control passage of indole through the tunnel.