Sorsby's fundus dystrophy tissue inhibitor of metalloproteinases-3 (TIMP-3) mutants have unimpaired matrix metalloproteinase inhibitory activities, but affect cell adhesion to the extracellular matrix.

The TIMP family of matrix metalloproteinase inhibitors consists of four members, of which TIMP-1, -2 and -4 are secreted, freely diffusible proteins, whereas TIMP-3 is ECM-associated. Mutations in the TIMP3 gene have been linked to Sorsby's fundus dystrophy (SFD), an autosomal dominant inherited retinal degenerative disease that leads to blindness. The SFD mutations characterized result in introduction of an unpaired cysteine residue in the C-terminal domain of TIMP-3. We have expressed four SFD mutant TIMP-3 proteins in baby hamster kidney (BHK) cells and evaluated their characteristics alongside wild-type TIMP-3. Analysis of the mutant proteins (Ser156Cys, Gly167Cys, Tyr168Cys and Ser181Cys) by SDS-PAGE and reverse zymography revealed that each of the mutants retained gelatinase A and gelatinase B inhibitory activity, and were localized to the ECM. Association rate constants for Ser156Cys TIMP-3 with gelatinase-A, gelatinase-B, stromelysin-1 and collagenase-3 were only moderately reduced compared to wild-type TIMP-3. However, all of the mutants displayed aberrant protein-protein interactions, resulting in the presence of additional proteins or complexes in ECM preparations. Two of the mutants (Ser156Cys and Ser181Cys) showed a marked propensity to form multiple higher molecular-weight complexes that retained TIMP activity on reverse zymography. Expression of the SFD mutant TIMP-3 (and to a lesser extent, wild-type TIMP-3) proteins in BHK cells conferred increased cell adhesiveness to the ECM. Our findings indicate that the pathogenesis of Sorsby's fundus dystrophy cannot be attributed to a failure to localize SFD TIMP-3 proteins to the ECM or defects in MMP inhibition, but may involve the formation of aberrant TIMP-3-containing protein complexes and altered cell adhesion.     

Tissue inhibitor of metalloproteinase-3 induces a Fas-associated death domain-dependent type II apoptotic pathway

Tissue inhibitors of metalloproteinases (TIMPs) are important regulators of matrix metalloproteinase (MMP) and adamalysin metalloproteinase activity. We previously reported that overexpression of TIMP-3 inhibits MMPs and induces apoptotic cell death in a variety of cell types and demonstrated that apoptosis is mediated through the N terminus of TIMP-3, which harbors the MMP inhibitory domain. However, little is known about the mechanisms underlying TIMP-3-induced apoptosis. Here we demonstrate that overexpression of TIMP-3 induced activation of initiator caspase-8 and -9 and promoted caspase-mediated cleavage of the death substrates poly(ADP-ribose) polymerase and focal adhesion kinase. Furthermore, TIMP-3 induced mitochondrial activation as demonstrated by loss of mitochondrial membrane potential and release of cytochrome c. Intervention studies demonstrated that overexpression of Bcl-2, the anti-apoptotic mitochondrial membrane protein, or CrmA, a viral serpin inhibitor of caspase-8, completely inhibited TIMP-3-induced apoptosis. Furthermore, a dominant-negative Fas-associated death domain mutant inhibited TIMP-3-induced death substrate cleavage and apoptotic death. Taken together, these results indicate that TIMP-3 overexpression induces a type II apoptotic pathway initiated via a Fas-associated death domain-dependent mechanism.     

Expression of Sorsby's fundus dystrophy mutations in human retinal pigment epithelial cells reduces matrix metalloproteinase inhibition and may promote angiogenesis

Sorsby's fundus dystrophy (SFD) is an autosomal dominant degenerative disease of the macula caused by mutations in the tissue inhibitor of metalloproteinase-3 (TIMP-3) gene. Choroidal neovascularization is a hallmark of this disease, which closely resembles the exudative form of age-related macular degeneration. However, the mechanism by which TIMP-3 mutations induce the disease phenotype in SFD remains unknown. To address this question we established human retinal pigment epithelial cell lines expressing wild type or S156C (Ser(156) changed to cysteine) mutant TIMP-3. S156C TIMP-3 had reduced matrix metalloproteinase (MMP) inhibitory activity in retinal pigment epithelial cells and resulted in increased secretion and activation of gelatinase A and B. The conditioned medium from these cells induced angiogenesis in "in vivo" chick chorioallantoic membrane assays that could be reversed with recombinant wild type TIMP-3. Our data indicate that the choroidal neovascularization in SFD may be a result of increased MMP activity, which could lead to the stimulation of angiogenesis. These results also suggest the potential therapeutic use of TIMP-3 or synthetic MMP inhibitors in this disease.     

A novel tissue inhibitor of metalloproteinases-3 mutation reveals a common molecular phenotype in Sorsby's fundus dystrophy

Sorsby's fundus dystrophy (SFD) is a dominantly inherited degenerative disease of the retina that leads to loss of vision in middle age. It has been shown to be caused by mutations in the gene for tissue inhibitor of metalloproteinases-3 (TIMP-3). Five different mutations have previously been identified, all introducing an extra cysteine residue into exon 5 (which forms part of the C-terminal domain) of the TIMP-3 molecule; however, the significance of these mutations to the disease phenotype was unknown. In this report, we describe the expression of several of these mutated genes, together with a previously unreported novel TIMP-3 mutation from a family with SFD that results in truncation of most of the C-terminal domain of the molecule. Despite these differences, all of these molecules are expressed and exhibit characteristics of the normal protein, including inhibition of metalloproteinases and binding to the extracellular matrix. However, unlike wild-type TIMP-3, they all form dimers. These observations, together with the recent finding that expression of TIMP-3 is increased, rather than decreased, in eyes from patients with SFD, provides compelling evidence that dimerized TIMP-3 plays an active role in the disease process by accumulating in the eye. Increased expression of TIMP-3 is also observed in other degenerative retinal diseases, including the more severe forms of age-related macular degeneration, the most common cause of blindness in the elderly in developed countries. We hypothesize that overexpression of TIMP-3 may prove to be a critical step in the progression of a variety of degenerative retinopathies.     

Evidence for transition-state stabilization by serine-148 in the catalytic mechanism of chloramphenicol acetyltransferase

The function of conserved Ser-148 of chloramphenicol acetyltransferase (CAT) has been investigated by site-directed mutagenesis. Modeling studies (P. C. E. Moody and A. G. W. Leslie, unpublished results) suggested that the hydroxyl group of Ser-148 could be involved in transition-state stabilization via a hydrogen bond to the oxyanion of the putative tetrahedral intermediate. Replacement of serine by alanine results in a mutant enzyme (Ala-148 CAT) with kcat reduced 53-fold and only minor changes in Km values for chloramphenicol and acetyl-CoA. The Ser-148----Gly substitution gives rise to a mutant enzyme (Gly-148 CAT) with kcat reduced only 10-fold. A water molecule may partially replace the hydrogen-bonding potential of Ser-148 in Gly-148 CAT. The three-dimensional structure of Ala-148 CAT at 2.34-A resolution is isosteric with that of wild-type CAT with two exceptions: the absence of the Ser-148 hydroxyl group and the loss of one poorly ordered water molecule from the active site region. The results are consistent with a catalytic role for Ser-148 rather than a structural one and support the hypothesis that Ser-148 is involved in transition-state stabilization. Ser-148 has also been replaced with cysteine and asparagine; the Ser-148----Cys mutation results in a 705-fold decrease in kcat and the Ser-148----Asn substitution in a 214-fold reduction in kcat. Removing the hydrogen bond donor (Ser-148----Ala or Gly) is less deleterious than replacing Ser-148 with alternative possible hydrogen bond donors (Ser-148----Cys or Asn).     

Role of mitochondrial hOGG1 and aconitase in oxidant-induced lung epithelial cell apoptosis

8-Oxoguanine DNA glycosylase (Ogg1) repairs 8-oxo-7,8-dihydroxyguanine (8-oxoG), one of the most abundant DNA adducts caused by oxidative stress. In the mitochondria, Ogg1 is thought to prevent activation of the intrinsic apoptotic pathway in response to oxidative stress by augmenting DNA repair. However, the predominance of the β-Ogg1 isoform, which lacks 8-oxoG DNA glycosylase activity, suggests that mitochondrial Ogg1 functions in a role independent of DNA repair. We report here that overexpression of mitochondria-targeted human α-hOgg1 (mt-hOgg1) in human lung adenocarcinoma cells with some alveolar epithelial cell characteristics (A549 cells) prevents oxidant-induced mitochondrial dysfunction and apoptosis by preserving mitochondrial aconitase. Importantly, mitochondrial α-hOgg1 mutants lacking 8-oxoG DNA repair activity were as effective as wild-type mt-hOgg1 in preventing oxidant-induced caspase-9 activation, reductions in mitochondrial aconitase, and apoptosis, suggesting that the protective effects of mt-hOgg1 occur independent of DNA repair. Notably, wild-type and mutant mt-hOgg1 coprecipitate with mitochondrial aconitase. Furthermore, overexpression of mitochondrial aconitase abolishes oxidant-induced apoptosis whereas hOgg1 silencing using shRNA reduces mitochondrial aconitase and augments apoptosis. These findings suggest a novel mechanism that mt-hOgg1 acts as a mitochondrial aconitase chaperone protein to prevent oxidant-mediated mitochondrial dysfunction and apoptosis that might be important in the molecular events underlying oxidant-induced toxicity.     

Promotion of hyperphosphorylation by frontotemporal dementia tau mutations

Mutations in the tau gene are known to cosegregate with the disease in frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). However, the molecular mechanism by which these mutations might lead to the disease is not understood. Here, we show that four of the FTDP-17 tau mutations, R406W, V337M, G272V, and P301L, result in tau proteins that are more favorable substrates for phosphorylation by brain protein kinases than the wild-type, largest four-repeat protein tau4L and tau4L more than tau3L. In general, at all the sites studied, mutant tau proteins were phosphorylated faster and to a higher extent than tau4L and tau4L > tau3L. The most dramatic difference found was in the rate and level of phosphorylation of tau4L(R406W) at positions Ser-396, Ser-400, Thr-403, and Ser-404. Phosphorylation of this mutant tau was 12 times faster and 400% greater at Ser-396 and less than 30% at Ser-400, Thr-403, and Ser-404 than phosphorylation of tau4L. The mutated tau proteins polymerized into filaments when 4-6 mol of phosphate per mol of tau were incorporated, whereas wild-type tau required approximately 10 mol of phosphate per mol of protein to self-assemble. Mutated and wild-type tau proteins were able to sequester normal tau upon incorporation of approximately 4 mol of phosphate per mol of protein, which was achieved at as early as 30 min of phosphorylation in the case of mutant tau proteins. These findings taken together suggest that the mutations in tau might cause neurodegeneration by making the protein a more favorable substrate for hyperphosphorylation.     

Processing of Bacillus licheniformis penicillinases lacking a lipoprotein modification site in Escherichia coli.

We have previously shown that the penP Ser-27 prepenicillinase is processed into two forms, Ser-35-penicillinase and Asn-29 penicillinase. Two new penicillinase mutants, penP Ser-27 Pro-28 and penP Ser-27,23' (Pro-Asp)24', were derived from the penP Ser-27 mutant by oligonucleotide-directed site-specific mutagenesis. The penP Ser-27 Pro-28 mutant prepenicillinase was also processed into two forms, Ser-35-penicillinase and Gly-26-penicillinase. On the contrary, the penP Ser-27,23' (Pro-Asp)24' mutant prepenicillinase is unprocessed.     

Sorsby fundus dystrophy mutation Timp3(S156C) affects the morphological and biochemical phenotype but not metalloproteinase homeostasis

The tissue inhibitor of metalloproteinases-3 (TIMP3) is a multifunctional protein tightly associated with the extracellular matrix (ECM). A specific type of mutation in TIMP3 which results in potentially unpaired cysteine residues at the C-terminus of the protein has been shown to cause Sorsby fundus dystrophy (SFD), an autosomal dominant retinopathy of late onset. An early finding in SFD is a striking accumulation of protein and lipid material in Bruch's membrane, a multilayered ECM structure located between the choroid and the RPE. To study the molecular mechanisms underlying SFD pathology, we recently generated two mouse lines, one deficient in Timp3 (Timp3(-/-)) and one carrying an SFD-related mutation in the orthologous murine Timp3 gene (Timp3(S156C/S156C)). We now established immortalized fibroblast cells from the mutant mouse strains and provide evidence that the various cell lines display distinct morphological and physiological features that are dependent on the mutational status of the Timp3 protein in the secreted ECM. We show that matrix metalloproteinase (MMP) activity and inhibitory properties of Timp3 are not affected by the SFD-associated mutation. We further demonstrate that Timp3(S156C) protein accumulates in the ECM of the mutant fibroblast cells and that this accumulation is not due to a prolonged turnover rate of mutant vs. normal Timp3. We also show that the relative abundance of mutant and normal Timp3 in the ECM has no measurable effects on cellular phenotypes. Together, these findings suggest (i) a functional role of normal Timp3 in pathways determining cellular morphology and (ii) a loss of this particular function as a consequence of the Ser156Cys mutation. We therefore hypothesize that SFD pathogenesis is due to a loss-of-function mutation in TIMP3.     

Overexpression of LIM Kinase 1 Renders Resistance to Apoptosis in PC12 Cells by Inhibition of Caspase Activation

LIM kinases (LIMKs) regulate actin polymerization by phosphorylating cofilin and are predominantly expressed in neural tissue. In this study, the effect of LIMK1 overexpression in PC12 cell apoptosis was investigated. PC12 cells overexpressing the wild-type LIMK1 were more resistant to serum-withdrawal-induced cell death and the level of caspase 3 activation in these cells was lower than in the control PC12 cells or than in the PC12 cells expressing a mutant LIMK1 lacking the kinase domain. The inhibition of JNK activation was observed in the PC12 cells overexpressing the wild-type LIMK1 after serum withdrawal. These results suggest that the LIMK1 might allow resistance to apoptosis in PC12 cells by inhibiting JNK activation.     

Chronic cigarette smoke causes oxidative damage and apoptosis to retinal pigmented epithelial cells in mice

The purpose of this study was to determine whether mice exposed to chronic cigarette smoke develop features of early age-related macular degeneration (AMD). Two month old C57Bl6 mice were exposed to either filtered air or cigarette smoke in a smoking chamber for 5 h/day, 5 days/week for 6 months. Eyes were fixed in 2.5% glutaraldehyde/2% paraformaldehyde and examined for ultrastructural changes by transmission electron microscopy. The contralateral eye was fixed in 2% paraformaldehyde and examined for oxidative injury to the retinal pigmented epithelium (RPE) by 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-OHdG) immunolabeling and apoptosis by TUNEL labeling. Mice exposed to cigarette smoke had immunolabeling for 8-OHdG in 85+/-3.7% of RPE cells counted compared to 9.5+/-3.9% in controls (p<0.00001). Bruch membrane was thicker in mice exposed to smoke (1086+/-332 nm) than those raised in air (543+/-132 nm; p = 0.0069). The two most pronounced ultrastructural changes (severity grading scale from 0-3) seen were a loss of basal infoldings (mean difference in grade = 1.98; p<0.0001), and an increase in intracellular vacuoles (mean difference in grade = 1.7; p<0.0001). Ultrastructural changes to Bruch membrane in cigarette-smoke exposed mice were smaller in magnitude but consistently demonstrated significantly higher grade injury in cigarette-exposed mice, including basal laminar deposits (mean difference in grade = 0.54; p<0.0001), increased outer collagenous layer deposits (mean difference in grade = 0.59; p = 0.002), and increased basal laminar deposit continuity (mean difference in grade = 0.4; p<0.0001). TUNEL assay showed a higher percentage of apoptotic RPE from mice exposed to cigarette smoke (average 8.0+/-1.1%) than room air (average 0+/-0%; p = 0.043). Mice exposed to chronic cigarette smoke develop evidence of oxidative damage with ultrastructural degeneration to the RPE and Bruch membrane, and RPE cell apoptosis. This model could be useful for studying the mechanism of smoke induced changes during early AMD.     

Overexpression of PTEN induces cell growth arrest and apoptosis in human breast cancer ZR-75-1 cells

Phosphatase and tensin homolog （PTEN） is a tumor suppressor gene located at human chromosome 10q23, might play an important role in cell proliferation, cell cycle and apoptosis of cancer cells. In this study, the eukaryotic expression vectors pBP-wt-PTEN （containing a wild-type PTEN gene） and pBP-G129R-PTEN （containing a mutant PTEN gene） were used to transfect breast cancer ZR-75-1 cells. After transfection, ZR-75-1 cells expressing PTEN were obtained and tested. The blue exclusion assay showed the growth rate of the cells transfected with pBP-wt-PTEN was significantly lower than that of the control cells transfected with pBP-G129R-PTEN. Analysis of the cell cycle by flow cytometry showed that the progression from the G1 to the S phase was arrested in cells expressing wild-type PTEN. Some typical morphological changes of apoptosis were also observed in cells transfected with pBP-wt-PTEN, but not in those transfected with pBP-G 129R-PTEN. This study shows that overexpression of PTEN in ZR-75-1 cells leads to cell growth arrest and apoptosis.     

The insulin-like growth factor I receptor as a physiologically relevant target of p53 in apoptosis caused by interleukin-3 withdrawal.

The wild-type p53 protein is known to modulate apoptosis induced in 32D murine hemopoietic cells by interleukin-3 withdrawal. In 32D cells and in 32D cells constitutively expressing a temperature-sensitive mutant of p53 (32Dtsp53), overexpression of a wild-type (but not a mutant) insulin-like growth factor I receptor (IGF-IR) protects these cells from apoptosis. A tsp53 in its wild-type conformation causes a decrease in the levels of IGF-IRs, and this decrease is accompanied by increased sensitivity of these cells to apoptosis. However, when the expression of the IGF-IR cDNA is regulated by a viral promoter, IGF-IR levels are not decreased by a wild-type p53, and apoptosis does not occur. These findings show that, in 32Dtsp53 cells, the IGF-IR is a physiologically relevant target of p53 in the process of apoptosis.     

Catalytic role of histidine 147 in Escherichia coli thymidylate synthase

Nine mutant thymidylate synthases were isolated that only differed in sequence at position 147. The wild-type enzyme (which had a histidine residue at 147) and mutant enzymes were purified to near homogeneity and their kinetic properties were compared. Although the kcat values for the mutant enzymes were 10-10,000-fold lower than for the wild-type enzyme, the Km values for both 2'-deoxyuridylate and 5,10-methylenetetrahydrofolate were nearly identical for all the enzymes indicating that His-147 is not significantly involved in initial substrate binding. By comparing the wild-type (His-147) to the glycine (Gly-147) enzyme, the side chain of His-147 was estimated to lower the activation energy of the catalytic step by 1.6-2.9 kcal mol-1. In contrast to the wild-type enzyme, the activity of the Gly-147 enzyme decreased when the pH was raised above 7.5. The activity loss coincided with the deprotonation of a residue that had a pKa of 9.46 +/- 0.2 and an enthalpy of ionization (delta Hion) of 12.1 +/- 0.9. These values are consistent with the involvement of a lysine or an arginine residue in the catalytic process. An inspection of the rates of ternary complex formation among enzyme, 5-fluoro-2'-deoxyuridylate, and 5,10-methylenetetrahydrofolate for the mutant enzymes indicated that His-147 is not needed for the proton removal from C-5 of 2'-deoxyuridylate but rather participates in an initial catalytic step and alters the pKa value of a catalytically important lysine or arginine residue.     

Single amino acid substitutions in puroindoline-b mutants influence lipid binding properties

External reflectance Fourier transform infrared (ER-FTIR) spectroscopy and surface pressure measurements have been used to characterize the interaction of wild-type puroindoline-b (Pin-b) and two mutant forms featuring single residue substitutions-namely, Gly-46 to Ser-46 (Pin-bH) and Trp-44 to Arg-44 (Pin-bS)-with condensed-phase monolayers of zwitterionic (L-alpha-dipalmitoylphosphatidylcholine, DPPC) and anionic (L-alpha-dipalmitoylphosphatidyl-dl-glycerol, DPPG) phospholipids. The interaction with anionic DPPG monolayers, monitored by surface pressure isotherms, was influenced significantly by mutations in Pin-b (p < 0.05); wild-type Pin-b showed the highest surface pressure change of 10.6 +/- 1.0 mN m-1, followed by Pin-bH (7.9 +/- 1.6 mN m-1) and Pin-bS (6.3 +/- 1.0 mN m-1), and the surface pressure isotherm kinetics were also different in each case. Integrated Amide I peak areas from corresponding ER-FTIR spectra confirmed the differences in adsorption kinetics, but also showed that differences in adsorbed amount were less significant, suggesting that mutations influence the degree of penetration into DPPG films. All Pin-b types showed evidence of interaction with DPPC films, detected as changes in surface pressure (5.6 +/- 1.1 mN m-1); however, no protein peaks were detected in the ER-FTIR spectra, which indicated that the interaction was via penetration with limited adsorption at the lipid/water interface. The expression of Pin-b mutants is linked to wheat endosperm hardness; therefore, the data presented here suggest that the lipid binding properties may be pivotal within the mechanism for this quality trait. In addition, the data suggest antimicrobial activities of Pin-b mutants would be lower than those of the wild-type Pin-b, because of decreased selectivity toward anionic phospholipids.     

Structural studies on transmembrane proteins. 1. Model study using bacteriorhodopsin mutants containing single cysteine residues

In developing new approaches to structural studies of polytopic transmembrane proteins, we have prepared bacteriorhodopsin mutants containing single cysteine residues at selected sites in different topological domains. Four such mutants were prepared: Gly-72----Cys and Ser-169----Cys in the presumed looped-out regions on the opposite sides of the membrane bilayer and Thr-90----Cys and Leu-92----Cys in the membrane-embedded helix C. The four mutants folded and regenerated the characteristic chromophore in detergent/phospholipid micelles and pumped protons like the wild-type bacteriorhodopsin. After reconstitution in asolectin vesicles, the sulfhydryl groups in the mutants Gly-72----Cys and Ser-169----Cys reacted with iodo[2-3H]acetic acid, while the sulfhydryl groups in the membrane-embedded mutants, Thr-90----Cys and Leu-92----Cys, did not. The sulfhydryl groups in all four mutants could be derivatized in the denatured state by reaction with iodoacetic acid or 6-acryloyl-2-(dimethylamino)naphthalene. Of these derivatives, the two from the mutants Gly-72----Cys and Ser-169----Cys folded like the wild-type bacterioopsin, whereas of the two from the helix C mutants, Thr-90----Cys and Leu-92----Cys, only the latter folded normally. However, the folding of Leu-92----Cys was also impaired when treated with the bulky 5-(iodoacetamido)fluorescein. The reactivity and the folding behavior of the cysteine mutants can thus report on the topographic domain as well as on the orientation of the helices within the membrane.     

Tyrosine 706 and 807 phosphorylation site mutants in the murine colony-stimulating factor-1 receptor are unaffected in their ability to bind or phosphorylate phosphatidylinositol-3 kinase but show differential defects in their ability to induce early response gene transcription.

The receptor for colony-stimulating factor-1 (CSF-1) is a receptor protein-tyrosine kinase. To study the possible function of CSF-1 receptor autophosphorylation, two autophosphorylation sites, Tyr-706, located in the kinase insert, and Tyr-807, a residue conserved in all protein-tyrosine kinases, were changed independently to either phenylalanine or glycine. Wild-type and mutant receptors were stably expressed in Rat-2 cells. In response to CSF-1, cells expressing Phe- or Gly-706 mutant receptors showed increased growth rate and altered cell morphology. Both the Phe- and Gly-706 mutant receptors associated with and phosphorylated phosphatidylinositol-3 kinase at levels comparable with those of wild-type receptors. However, these mutant receptors differed subtly from each other and from the wild-type receptor in their ability to induce different aspects of the response to CSF-1. The Phe-706 mutant receptor was most strongly affected in its ability to increase growth rate or elevate the levels of c-fos and NGF1A mRNAs, whereas the Gly-706 mutant receptor was most markedly affected in its ability to induce a change in cell morphology or increase the levels of c-jun and NGF1A mRNAs. These findings indicate that Tyr-706 itself, or this region of the receptor, may be important for interaction of the CSF-1 receptor with different signalling pathways. Gly-807 mutant receptors lacked protein-tyrosine kinase activity, failed to respond to CSF-1, and were defective in biosynthetic processing. Phe-807 mutant receptors had 40 to 60% reduced protein-tyrosine kinase activity in vitro. Although cells expressing Phe-807 receptors were able to respond to CSF-1, the changes in growth rate and cell morphology were significantly less than seen with wild-type receptors, and the induction of early response genes was also slightly lower than for the wild-type receptor. In contrast, Phe-807 receptors were equivalent to wild-type receptors when tested for their ability to interact with phosphatidylinositol-3 kinase. These findings indicate that phosphorylation of Tyr-807 may be important for full activation of the receptor.     

Studies on the mechanism of formation of the pyruvate prosthetic group of phosphatidylserine decarboxylase from Escherichia coli

Phosphatidylserine decarboxylase from Escherichia coli uses a pyruvate group as the enzyme cofactor (Satre, M., and Kennedy, E. P. (1978) J. Biol. Chem. 253, 479-483). Comparison of the DNA sequence of the psd gene with the partial amino acid sequence of the mature gene product suggests that the two nonidentical subunits of the mature enzyme are formed by cleavage of a proenzyme resulting in the conversion of Ser-254 to an amino-terminal pyruvate residue (Li, Q.-X., and Dowhan, W. (1988) J. Biol. Chem. 263, 11516-11522). The cleavage of the wild-type proenzyme occurs rapidly with a half-time on the order of 2 min. When Ser-254 is changed to cysteine (S254C), threonine (S254T), or alanine (S254A) by site-directed mutagenesis, the rate of processing of the proenzyme and the production of the functional enzyme are drastically affected. Proenzymes with S254C or S254T are cleaved with a half-time of around 2-4 h while the S254A proenzyme does not undergo processing. The reduced processing rate for the mutant proenzymes is consistent with less of the functional enzyme being made. Mutants encoding the S254C and S254T protein produce 16 and 2%, respectively, of the activity of the wild-type allele but can still complement a temperature-sensitive mutant in the psd locus. There is no detectable activity or complementation observed with the S254A protein. These results are consistent with the hydroxyl group of Ser-254 playing a critical role in the cleavage of the peptide bond between Gly-253 and Ser-254 of the prophosphatidylserine decarboxylase and support the mechanism proposed by Snell and coworkers (Recsei and Snell (1984) Annul Rev. Biochem. 53, 357-387) for the formation of the prosthetic group of pyruvate-dependent decarboxylases.