[Analysis of long-term outcomes of penetrating keratoplasty for congenital corneal opacity]

Objective: To investigate the long-term outcomes of corneal grafts after penetrating keratoplasty(PK) for congenital corneal opacity(CCO) in children aged 0 to 5 years and the related influencing factors. Methods: It was a retrospective series case study. Data of 39 children (55 eyes) who underwent PK surgery due to CCO in the keratology Department of Beijing Tongren Hospital from April 2014 to April 2018 and were followed up for more than 30 months were collected. Among them, there were 17 males (43.6%) and 22 females (56.4%). The age at operation was (16.2±13.3) months, and the follow-up time was (46.4±13.8) months. Clinical data such as basic information, preoperative diagnosis, operation age, operation method and postoperative complications were recorded. The corneal graft transparency was analyzed according to preoperative diagnosis, corneal neovascularization area, age at surgery, monocular or binocular surgery interval, primary surgery type and further surgery, and postoperative complications were observed. Results: At 12 months, 24 months and the last follow-up after PK, 78.2% (43/55), 70.9% (39/55) and 58.2% (32/55) of the affected eyes had clear corneal grafts, respectively.There was no statistical significance between Peters anomaly and sclerocornea (P>0.05), while the extent of neovascularization in the limbus had a significant effect on corneal graft transparency, and graft opacity was more likely to occur in patients with vessel area exceeding 2 quadrants (P<0.05).The highest corneal graft transparency was found in children aged 1 to 3 years 80.8%(21/26) (P<0.05), followed by children younger than 6 months (7/15).The translucency rate of the corneal graft was higher in patients undergoing unilateral surgery than in those undergoing bilateral surgery (P<0.05).Translucency of corneal graft was higher in children with simple surgery than with combined surgery (P<0.05), however, cataract surgery after PK had no significant effect on corneal graft transparency (P>0.05).The postoperative complications mainly included immune rejection in 19 eyes (34.5%), complicated cataract in 13 eyes (23.6%), glaucoma in 7 eyes (13.2%), persistent corneal epithelial defect in 7 eyes (13.2%). Conclusions: After PK in children with CCO, the transparent rate of corneal grafts decreases gradually with time, but the long-term translucency of corneal grafts can still be obtained. The range of corneal neovascularization, age at the time of surgery, whether the surgery was binocular and whether the surgery was combined had an effect on the transparency of corneal graft.

[Corneal melt after intracorneal ring segment implantations: a case report]

A 50-year-old female patient presented to the Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Institute of Ophthalmology, with complaints of right eye pain, tearing, and difficulty opening the eye for over a month after intrastromal corneal ring segment (ICRS) implantation 18 years prior in both eyes. Slit lamp examination revealed corneal stromal melting around the ICRS at the 3 to 4 o'clock position of the right eye, with fluorescein staining. Optical coherence tomography showed epithelial and superficial stromal layer defects in the area of the lesion. The patient was diagnosed with corneal melting after ICRS implantation in the right eye. Under general anesthesia, the corneal stromal ring was removed, and deep lamellar keratoplasty was performed. The patient had no discomfort and the corneal graft remained transparent after the surgery.

[Clinical analysis of penetrating keratoplasty for infants with congenital corneal opacity]

Objective: To investigate the corneal graft survival and related risk factors of primary penetrating keratoplasty in congenital corneal opacity infants. Methods: It was a retrospective cohort study. Data were collected from forty-two infants (51 eyes) who were aged ≤12 months and diagnosed with congenital corneal opacity in Beijing Tongren Hospital and Beijing Anzhen Hospital from January 1, 2017 to January 31, 2018. The mean age at surgery was (5.7±2.2) months (3-12 months). The mean follow-up duration was (28.6±2.6) months (24-33 months). All the patients underwent penetrating keratoplasty. The status of the corneal grafts and complications were observed and recorded during the regular follow-up. The survival probabilities were estimated by using the Kaplan-Meier and Log-rank test. The graft survival between different influence factors was analyzed by using the χ² test. Results: The Kaplan-Meier survival rates for penetrating keratoplasty were 84.3% (43/51) at 6 months, 78.4% (40/51) at 12 months and 60.8% (31/51) at the last follow-up. The presence of corneal neovascularization was significantly correlated with graft failure (χ²=5.264, P=0.022). The graft survival differed between eyes receiving combined surgery and mere penetrating keratoplasty and in eyes with varied surgical indications (P=0.039, <0.01). Increased intraocular pressure (7 eyes, 13.7%) and persistent epithelial defects (7 eyes, 13.7%) were the most common postoperative complications, followed by complicated cataract (4 eyes, 7.8%) and posterior capsule opacification (2 eyes, 3.9%). Conclusions: The graft survival rate was satisfactory following pediatric keratoplasty although it had a tendency to decrease with the follow-up time. Corneal neovascularization was a major risk factor of graft failure. Surgical indications and procedures also had a certain effect on the graft survival.

[Preliminary observation on the treatment of primary pterygium with modified conjunctival transplantation and amniotic membrane transplantation combined with use of interferon eye drops]

Objective: To observe the clinical effect of modified conjunctival transplantation and amniotic membrane transplantation combined with use of interferon (IFN) alpha-2b eye drops in the treatment of primary pterygium. Methods: This was a prospective case-control study. Patients with primary pterygium were treated from June 1, 2018 to December 31, 2018 in the Department of Ophthalmology, Beijing Tongren Hospital, and they were divided into two groups (the experimental group and the control group) by the method of randomized block design. Patients in the experimental group received modified conjunctival transplantation and amniotic membrane transplantation combined with use of IFN alpha-2b eye drops, while patients in the control group received pterygium resection combined with conjunctival autograft transplantation. The pterygium type and size were observed before operation, while visual acuity, intraocular pressure and anterior segment details were recorded either. The follow-up was done at 1 week, 2 weeks, 1 month, 3 months, 6 months and 12 months after operation. The visual acuity, corneal epithelial defect, and pterygium recurrence were observed. All data in this manuscript are enumeration data, the expected frequency of pterygium type distribution in the two groups was more than 5, and the chi square test was used, fisher's exact test was used to compare the other data between the two groups. Results: Seventy patients (77 eyes) with pterygium were in this study, including 30 males and 40 females, aged from 50-70 years old. There were 35 cases (38 eyes) in the experimental group and 35 cases (39 eyes) in the control group. 12 months after operation there were 54 cases (60 eyes) including 28 cases (30 eyes) in the experimental group and 26 cases (30 eyes) in the control group with complete data. The corneal epithelium defects of 1 eye in each group was repaired within 7-14 days after operation, and the rest eyes were completely repaired within 7 days after operation. There was no significant difference in the distribution of corneal epithelial healing between the two groups (P= 1.00). There was no significant difference between the two groups in the number of eyes distribute with decreased visual acuity (2 eyes in each group), stable visual acuity (15 eyes in the experimental group and 23 eyes in the control group), and improved visual acuity (13 eyes in the experimental group and 5 eyes in the control group) (P=0.053). There was no recurrence in the two groups at 12 months after surgery, and there was no significant difference between the two groups in the number of patients with conjunctival hyperplasia of grades 1, 2 and 3 (P=0.405). Conclusions: Modified conjunctival transplantation and amniotic membrane transplantation combined with use of IFN alpha-2b eye drops got low recurrence rate for primary pterygium and less damage to the healthy conjunctival tissue. This combined treatment strategy provides a new choice for the treatment of pterygium. (Chin J Ophthalmol, 2020, 56: 768-773).

[Clinical observation of autologous simple limbal epithelial transplantation for unilateral limbal stem cell deficiency]

Objective: To discuss effect of autologous simple limbal epithelial transplantation (SLET) performed for unilateral limbal stem cell deficiency (LSCD). Methods: Retrospective case study. In this retrospective study, records of 7 patients (7 eyes) who had undergone autologous SLET for unilateral LSCD, with a minimum of 6 months of follow-up, were reviewed. Demographic details, etiology of LSCD, duration between ocular burn and SLET, prior surgery performed, presence or absence of symblepharon, pre-and post-operative visual acuity, and complications were noted. Results: Seven eyes of 7 patients underwent autologous SLET. With a follow-up of 6 months, a completely epithelialised and stable corneal surface was obtained in all recipient eyes. Visual acuity improved in all patients, while none of the eyes developed any complications. Conclusions: Autologous SLET is an effective and safe modality for treatment of unilateral LSCD. Clinical success rates and visual acuity improvement are equal to or better than those reported with earlier techniques. (Chin J Ophthalmol, 2019, 55:923-927).

[The effectiveness and safety of topical 0.1% tacrolimus after high-risk penetrating keratoplasty]

Objective: To observe the effectiveness and safety of topical 0.1% tacrolimus(FK506) as immunosuppressant in high-risk penetrating corneal transplantation to prevent the immune rejection and to compare the outcomes with topical 1% Cyclosporin A (CsA). Methods: The study consists of 49 patients (50 eyes), who were fitted with the high-risk corneal transplantation standard and undergone the penetrating keratoplasty(PKP) or combined operation in Beijing Tongren hospital between March 2015 to September. With the time sequence, the patients were divided into observation group (FK506 group) and the control group (CsA group). The observation group included 9 females and 16 males with an average age of 57.8±14.8. Twenty-four patients were in the control group (25 eyes), including 10 females and 14 males, with an average age of (45.1±16.2). Observation group was treated with topical 0.1% tacrolimus, and the control group treated with topical 1%CsA. Both groups' treatment combined glucocorticoid as well. Two groups had 1 year follow-up observation. The incidence of rejection was compared by statistical methods of Cox regression. The adverse reactions were graded and compared using Mann-Whitney U test. Results: After one year, 22 cases of the observation group and 23 cases of the control group were accomplished all observations. The rejection rate was 4.54% in observation group and 27.23% in control group. The difference between the groups was statistically significant (χ(2)=4.291, P=0.038). Control group had high rejection rate. Besides, there was no severe side effects happened in both groups. After 1 month after surgery, 36.4% of the observation group showed mild corneal edema. The ratio of mild to moderate corneal edema in the control group was 26.1% and 8.7%. Three months after surgery, 4.5% of the observation group showed mild corneal edema, while 13.0% and 13.0% of the control group was found mild to moderate corneal edema. Six months after surgery, 4.5% of the observation group showed moderate corneal edema. The ratio of mild, moderate to severe corneal edema in the control group was 17.4%, 17.4% and 8.7%. The degree of corneal edema in the control group was more serious in three monthes(Z=-2.770, -2.018, -2.941, P<0.05). The differences in both monthes were statistically significant. Mild neovascularization occurred in the 13.6% of observation group. Mild to severe neovascularization occurred in the 13.0%, 4.3%, and 4.3% control groups. The degree of neovascularization in the control group was higher than that in the observation group(Z=-3.221, P=0.001). The differences in both months were statistically significant. Mild to moderate neovascularization occurred in the 18.2% and 9.1% of observation group. Mild to extremely severe neovascularization occurred in the 17.4%, 26.1%, 4.3% and 4.3% control groups. The degree of neovascularization in the control group was higher than that in the observation group(Z=-1.988, P=0.047).The differences in both monthes were statistically significant. Conclusions: Both 0.1% tacrolimus and 1% cyclosporine A are safe and effective in reducing the rejection after high-risk corneal transplantation. (Chin J Ophthalmol, 2019, 55: 419-427).

Demographic Characteristics of Voluntary Donors Registered in Beijing Tongren Hospital Eye Bank of China: A Retrospective Study From 2007 to 2016

BACKGROUND

Lack of willingness to pledge eyes among the general population is the main cause for the shortage of cornea tissue in China. A few studies have implied that general-population adults with specific demographics showed more willingness to donate their eyes.

METHODS

In this study, we analyzed the demographic characteristics of 918 voluntary donors registered in Beijing Tongren Hospital Eye Bank in the past 10 years for possible predictors that might help us to identify potential donors in Beijing and increase the donation rate. All copies of voluntary eye donation application forms filled by the registrants from 2007 to 2016 were collected. Basic demographics listed in the application form were extracted for analysis. Demographics were described as proportions and compared by means of a χ² test. Besides that, donor counts and proportions of combining 2 demographics from the 4 main demographics were described and compared.

RESULTS

Voluntary donors greater than 50 years of age (n = 477, 53.0%) predominated the proportions. Regarding education level and occupation, donors with tertiary education (n = 484, 57.4%) were more numerous than donors with primary and secondary education (n = 355, 42.1%); office clerks, workers, and government officers were more willing to pledge eyes than were donors engaged in other occupations. In addition, donors of the Han race (n = 856, 94.9%) made up the majority and mainly were distributed in 5 central administrative districts of Beijing (n = 629, 77.5%).

CONCLUSIONS

The present study suggests that older women (>50 years of age), living in a central district, with higher education level and engaged in white-collar work, were possible predictors for potential donors.

[Pay attention to the donor material supply for corneal transplantation]

Corneal transplantation is an important method in the treatment of corneal blindness. It is imperative to improve the treatment effectiveness of corneal disease and reduce the possibility of corneal blindness with the progress of corneal transplantation surgery, the construction and development of eye banks and the rational use of donor materials. This article reviews the component corneal transplantation technology promotion, eye bank construction and preparation of donor slices for component corneal transplantation surgery. (Chin J Ophthalmol, 2016, 52: 641-643).

[Antibody-based agents as experimental therapeutic interventions for corneal allograft rejection]

The corneal allograft rejection is the primary reason for graft failure, but the existing agents are of limited efficacy and may be accompanied by unacceptable morbidity. Recently, antibody-based agents have received great attention and have become an important part of therapeutic intervention for organ transplantation, which is also a research focus in the field of corneal transplantation. This review summarizes the history, current situation and mechanism of antibody-based agents in corneal transplantation.

Cullin1 is a novel marker of poor prognosis and a potential therapeutic target in human breast cancer

BACKGROUND

To investigate the role of Cullin1 (Cul1) in the development of breast cancer, we examined the expression of Cul1 in breast cancer tissues and analyzed the correlation between Cul1 expression and clinicopathologic variables and patients survival.

PATIENTS AND METHODS

We evaluated the Cul1 expression by immunohistochemistry using a tissue microarray (TMA) which includes 393 breast cancer tissues. We also studied the role of Cul1 in breast cancer cell proliferation, migration and invasion by carrying out CCK8 cell proliferation assay, cell migration and invasion assay.

RESULTS

The Cul1 expression was significantly correlated with breast cancer histology grade (P = 0.000), estrogen receptor status (P = 0.001), progesterone receptor status (P = 0.001) and human epidermal growth factor receptor 2 status (P = 0.002). Furthermore, we showed a strong correlation between high Cul1 expression and worse 5-year overall and disease-specific survival rates in breast cancer patients (P = 0.026 and P = 0.015, respectively). Finally, we found that Cul1 knockdown inhibits cell proliferation, migration and invasion abilities.

CONCLUSIONS

Cul1 overexpression is significantly correlated with breast cancer progression and predicts worse survival. Cul1 regulates breast cancer cell proliferation, migration and invasion.

Scope of radiological protection control measures

In this report, the Commission recommends approaches to national authorities for their definition of the scope of radiological protection control measures through regulations, by using its principles of justification and optimisation. The report provides advice for deciding the radiation exposure situations that should be covered by the relevant regulations because their regulatory control can be justified, and, conversely, those that may be considered for exclusion from the regulations because their regulatory control is deemed to be unamenable and unjustified. It also provides advice on the situations resulting from regulated circumstances but which may be considered by regulators for exemption from complying with specific requirements because the application of these requirements is unwarranted and exemption is the optimum option. Thus, the report describes exclusion criteria for defining the scope of radiological protection regulations, exemption criteria for planned exposure situations, and the application of these concepts in emergency exposure situations and in existing exposure situations. The report also addresses specific exposure situations such as exposure to low-energy or low-intensity adventitious radiation, cosmic radiation, naturally occurring radioactive materials, radon, commodities, and low-level radioactive waste. The quantitative criteria in the report are intended only as generic suggestions to regulators for defining the regulatory scope, in the understanding that the definitive boundaries for establishing the situations that can be or need to be regulated will depend on national approaches.

Preliminary analysis of biomarkers in plasma by SELDI to predict the response to EGFR tyrosine kinase inhibitors (TKIs) in NSCLC patients

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Background: The identification of NSCLC patients who are most likely to respond to EGFR tyrosine kinase inhibitors (TKIs) have been investigated intensively. Although screenings for EGFR mutation and gene copy number are promising, these tests are not yet widely available. New predictor markers are urgently needed. The objective of this study was to identify proteomic markers in plasma to predict benefits for patients treated with EGFR TKIs. Methods: Proteomic spectra derived from plasma samples from EGFR TKIs-responsive patients and non-responsive patients were generated by surface-enhanced laser desorption/ionization time-of-flight mass spectrometry (SELDI-TOF MS). These proteomic spectra (WCX2 chips, Ciphergen Biosystems, Inc.) were then analyzed by comparing protein profiles in different response groups (PR Vs PD, training set). Another group of patients treaded with EGFR TKIs will be serving as testing set to validate the result of training set. Results: Totally, fifty-four advanced NSCLC patients were included in this study. All patients were treated with single agent of gefitinib or erlotinib. Twenty-nine patients were included in training set of this study. All were suitable for response evaluation. Ten patients (34.5%) were PR, 8 (27.6%) were SD, and 11 (37.9%) were progressive disease (PD). Total six significant protein peaks were significant (m/z 4288, 4595, 9191, 9349, 9397, and 9563) between PR group and PD group (table). Another twenty-five patients were included for testing set. Analyzing of testing set is still going on. Table shows PR and PD patients’ plasma comparison on WCX2 chips. Conclusions: This preliminary “training” set of spectra that uses SELDI-TOF MS technology found that six protein peaks seemed to be very important biomarkers to predict the response to gefitinib. Prospective tests to confirm these proteins and peptides will be present at this meeting. These results are promising for identifying new biomarkers of EGFR TKIs with SELDI.

[Table: see text]

No significant financial relationships to disclose.

Identification of ubiquitin ligases required for skeletal muscle atrophy

Skeletal muscle adapts to decreases in activity and load by undergoing atrophy. To identify candidate molecular mediators of muscle atrophy, we performed transcript profiling. Although many genes were up-regulated in a single rat model of atrophy, only a small subset was universal in all atrophy models. Two of these genes encode ubiquitin ligases: Muscle RING Finger 1 (MuRF1), and a gene we designate Muscle Atrophy F-box (MAFbx), the latter being a member of the SCF family of E3 ubiquitin ligases. Overexpression of MAFbx in myotubes produced atrophy, whereas mice deficient in either MAFbx or MuRF1 were found to be resistant to atrophy. These proteins are potential drug targets for the treatment of muscle atrophy.

The RING domains of the promyelocytic leukemia protein PML and the arenaviral protein Z repress translation by directly inhibiting translation initiation factor eIF4E

The promyelocytic leukemia protein (PML) is a mammalian regulator of cell growth which is characteristically disrupted in acute promyelocytic leukemia and by a variety of viruses. PML contains a RING domain which is required for its growth-suppressive and antiviral properties. Although normally nuclear, in certain pathogenic conditions, including arenaviral infection, PML is relocated to the cytoplasm, where its functions are poorly understood. Here, we observe that PML and arenavirus protein Z use regions around the first zinc-binding site of their respective RING domains to directly interact, with sub-micromolar affinity, with the dorsal surface of translation initiation factor eIF4E, representing a novel mode of eIF4E recognition. PML and Z profoundly reduce the affinity of eIF4E for its substrate, the 5' 7-methyl guanosine cap of mRNA, by over 100-fold. Association with the dorsal surface of eIF4E and direct antagonism of mRNA cap binding by PML and Z lead to direct inhibition of translation. These activities of the RING domains of PML and Z do not involve ubiquitin-mediated protein degradation, in contrast to many RINGs which have been observed to do so. Although PML and Z have well characterized physiological functions in regulation of growth and apoptosis, this work establishes the first discrete biochemical mechanism which underlies the biological activities of their RING domains. Thus, we establish PML and Z as translational repressors, with potential contributions to the pathogenesis of acute promyelocytic leukemia and variety of viral infections.

Conjugation of Nedd8 to CUL1 enhances the ability of the ROC1-CUL1 complex to promote ubiquitin polymerization

The SCF-ROC1 ubiquitin-protein isopeptide ligase (E3) ubiquitin ligase complex targets the ubiquitination and subsequent degradation of protein substrates required for the regulation of cell cycle progression and signal transduction pathways. We have previously shown that ROC1-CUL1 is a core subassembly within the SCF-ROC1 complex, capable of supporting the polymerization of ubiquitin. This report describes that the CUL1 subunit of the bacterially expressed, unmodified ROC1-CUL1 complex is conjugated with Nedd8 at Lys-720 by HeLa cell extracts or by a purified Nedd8 conjugation system (consisting of APP-BP1/Uba3, Ubc12, and Nedd8). This covalent linkage of Nedd8 to CUL1 is both necessary and sufficient to markedly enhance the ability of the ROC1-CUL1 complex to promote ubiquitin polymerization. A mutation of Lys-720 to arginine in CUL1 eliminates the Nedd8 modification, abolishes the activation of the ROC1-CUL1 ubiquitin ligase complex, and significantly reduces the ability of SCF(HOS/beta)(-TRCP)-ROC1 to support the ubiquitination of phosphorylated IkappaBalpha. Thus, although regulation of the SCF-ROC1 action has been previously shown to preside at the level of recognition of a phosphorylated substrate, we demonstrate that Nedd8 is a novel regulator of the efficiency of polyubiquitin chain synthesis and, hence, promotes rapid turnover of protein substrates.

NMR observation of hydrolysis of acetonitrile to acetamide catalyzed by binuclear silver cryptate

NMR analyses (TOCSY, HMQC and DOSY) indicate that, in the presence of water, acetonitrile is exclusively hydrolyzed to acetamide catalyzed by bisilver cryptate complex, which contains coordinating acetonitrile molecule.

Mutations in conserved regions of the predicted RAG2 kelch repeats block initiation of V(D)J recombination and result in primary immunodeficiencies

The V(D)J recombination reaction is composed of multiple nucleolytic processing steps mediated by the recombination-activating proteins RAG1 and RAG2. Sequence analysis has suggested that RAG2 contains six kelch repeat motifs that are predicted to form a six-bladed beta-propeller structure, with the second beta-strand of each repeat demonstrating marked conservation both within and between kelch repeat-containing proteins. Here we demonstrate that mutations G95R and DeltaI273 within the predicted second beta-strand of repeats 2 and 5 of RAG2 lead to immunodeficiency in patients P1 and P2. Green fluorescent protein fusions with the mutant proteins reveal appropriate localization to the nucleus. However, both mutations reduce the capacity of RAG2 to interact with RAG1 and block recombination signal cleavage, therefore implicating a defect in the early steps of the recombination reaction as the basis of the clinical phenotype. The present experiments, performed with an extensive panel of site-directed mutations within each of the six kelch motifs, further support the critical role of both hydrophobic and glycine-rich regions within the second beta-strand for RAG1-RAG2 interaction and recombination signal recognition and cleavage. In contrast, multiple mutations within the variable-loop regions of the kelch repeats had either mild or no effects on RAG1-RAG2 interaction and hence on the ability to mediate recombination. In all, the data demonstrate a critical role of the RAG2 kelch repeats for V(D)J recombination and highlight the importance of the conserved elements of the kelch motif.

SUMO-1 modification of Mdm2 prevents its self-ubiquitination and increases Mdm2 ability to ubiquitinate p53

Mdm2 is an E3 ubiquitin ligase for the p53 tumor suppressor protein. We demonstrate that Mdm2 is conjugated with SUMO-1 (sumoylated) at Lys-446, which is located within the RING finger domain and plays a critical role in Mdm2 self-ubiquitination. Whereas mutant Mdm2(K446R) is stabilized, it elicits increased degradation of p53 and concomitant inhibition of p53-mediated apoptosis. In vitro sumoylation of Mdm2 abrogates its self-ubiquitination and increases its ubiquitin ligase activity toward p53. Radiation caused a dose- and time-dependent decrease in the degree of Mdm2 SUMO-1 modification, which is inversely correlated with the levels of p53. Our results suggest that the maintenance of the intrinsic activity of a RING finger E3 ubiquitin ligase is sumoylation dependent and that reduced Mdm2 sumoylation in response to DNA damage contributes to p53 stability.

The conserved RING-H2 finger of ROC1 is required for ubiquitin ligation

ROC1 is a common component of a large family of ubiquitin E3 ligases that regulate cell cycle progression and signal transduction pathways. Here we present evidence suggesting that a conserved RING-H2 structure within ROC1 is critical for its ubiquitin ligation function. Mercury-containing sulfhydryl modification agents (rho-hydroxymercuribenzoate and mercuric chloride) irreversibly inhibit the ROC1-CUL1 ubiquitin ligase activity without disrupting the complex. Consistent with this, these reagents also eliminate the ability of the Skp1-CUL1-HOS-ROC1 E3 ligase complex to support the ubiquitination of IkappaBalpha. Site-directed mutagenesis analysis identifies RING-H2 finger residues Cys(42), Cys(45), Cys(75), His(77), His(80), Cys(83), Cys(94), and Asp(97) as being essential for the ROC1-dependent ubiquitin ligase activity. Furthermore, C42S/C45S and H80A mutations reduce the ability of ROC1 to interact with CUL1 in transfected cells and diminish the capacity of ROC1-CUL1 to form a stable complex with Cdc34 in vitro. However, C75S, H77A, C94S, and D97A substitutions have no detectable effect on ROC1 binding activities. Thus, the ROC1 RING-H2 finger may possess multiple biochemical properties that include stabilizing an interaction with CUL1 and recruiting Cdc34. A possible role of the RING finger in facilitating the Ub transfer reaction is discussed.

The SCF(HOS/beta-TRCP)-ROC1 E3 ubiquitin ligase utilizes two distinct domains within CUL1 for substrate targeting and ubiquitin ligation

We describe a purified ubiquitination system capable of rapidly catalyzing the covalent linkage of polyubiquitin chains onto a model substrate, phosphorylated IkappaBalpha. The initial ubiquitin transfer and subsequent polymerization steps of this reaction require the coordinated action of Cdc34 and the SCF(HOS/beta-TRCP)-ROC1 E3 ligase complex, comprised of four subunits (Skp1, cullin 1 [CUL1], HOS/beta-TRCP, and ROC1). Deletion analysis reveals that the N terminus of CUL1 is both necessary and sufficient for binding Skp1 but is devoid of ROC1-binding activity and, hence, is inactive in catalyzing ubiquitin ligation. Consistent with this, introduction of the N-terminal CUL1 polypeptide into cells blocks the tumor necrosis factor alpha-induced and SCF-mediated degradation of IkappaB by forming catalytically inactive complexes lacking ROC1. In contrast, the C terminus of CUL1 alone interacts with ROC1 through a region containing the cullin consensus domain, to form a complex fully active in supporting ubiquitin polymerization. These results suggest the mode of action of SCF-ROC1, where CUL1 serves as a dual-function molecule that recruits an F-box protein for substrate targeting through Skp1 at its N terminus, while the C terminus of CUL1 binds ROC1 to assemble a core ubiquitin ligase.

The RAG1/RAG2 complex constitutes a 3' flap endonuclease: implications for junctional diversity in V(D)J and transpositional recombination

During V(D)J recombination, processing of branched coding end intermediates is essential for generating junctional diversity. Here, we report that the RAG1/ RAG2 recombinase is a 3' flap endonuclease. Substrates of this nuclease activity include various coding end intermediates, suggesting a direct role for RAG1/ RAG2 in generating junctional diversity during V(D)J recombination. Evidence is also provided indicating that site-specific RSS nicking involves RAG1/RAG2-mediated processing of a localized flap-like structure, implying 3' flap nicking in multiple DNA processing reactions. We have also demonstrated that the bacterial transposase Tn10 contains a 3' flap endonuclease activity, suggesting a mechanistic parallel between RAG1/RAG2 and other transposases. Based on these data, we propose that numerous transposases may facilitate genomic evolution by removing single-stranded extensions during the processing of excision site junctions.

Inhibition of transcription by the trimeric cyclin-dependent kinase 7 complex

Cyclin-dependent kinase 7 (CDK7) can be isolated as a subunit of a trimeric kinase complex functional in activation of the mitotic promoting factor. In this study, we demonstrate that the trimeric cdk-activating kinase (CAK) acts as a transcriptional repressor of class II promoters and show that repression results from CAK impeding the entry of RNA polymerase II and basal transcription factor IIF into a competent preinitiation complex. This repression is independent of CDK7 kinase activity. We find that the p36/MAT1 subunit of CAK is required for transcriptional repression and the repression is independent of the promoter used. Our results demonstrate a central role for CAK in regulation of messenger RNA synthesis by either inhibition of RNA polymerase II-catalyzed transcription or stimulation of transcription through association with basal transcription repair factor IIH.

Recruitment of a ROC1-CUL1 ubiquitin ligase by Skp1 and HOS to catalyze the ubiquitination of I kappa B alpha

Activation of the transcription factor NF-kappa B in response to proinflammatory stimuli requires the phosphorylation-triggered and ubiquitin-dependent degradation of the NF-kappa B inhibitor, I kappa B alpha. Here, we show the in vitro reconstitution of the phosphorylation-dependent ubiquitination of I kappa B alpha with purified components. ROC1, a novel SCF-associated protein, is recruited by cullin 1 to form a quatemary SCFHOS-ROC1 holenzyme (with Skp1 and the beta-TRCP homolog HOS). SCFHOS-ROC1 binds IKK beta-phosphorylated I kappa B alpha and catalyzes its ubiquitination in the presence of ubiquitin, E1, and Cdc34. ROC1 plays a unique role in the ubiquitination reaction by heterodimerizing with cullin 1 to catalyze ubiquitin polymerization.

HOS, a human homolog of Slimb, forms an SCF complex with Skp1 and Cullin1 and targets the phosphorylation-dependent degradation of IkappaB and beta-catenin

SCF E3 ubiquitin ligases mediate ubiquitination and proteasome-dependent degradation of phosphorylated substrates. We identified a human F-box/WD40 repeats protein (HOS), which is homologous to Slimb/h betaTrCP. Being a part of SCF complex with Skp1 and Cullin1, HOS specifically interacted with the phosphorylated IkappaB and beta-catenin, targeting these proteins for proteasome-dependent degradation in vivo. This targeting required Cullin1 as expression of a mutant Cullin1 abrogated the degradation of IkappaB and of beta-catenin. Mutant HOS which lacks the F-box blocked TNF alpha-induced degradation of IkappaB as well as GSK3beta-mediated degradation of beta-catenin. This mutant also inhibited NF-kappaB transactivation and increased the beta-catenin-dependent transcription activity of Tcf. These results demonstrate that SCF(HOS) E3 ubiquitin ligase regulate both NF-kappaB and beta-catenin signaling pathways.

Suppression of cell transformation by the cyclin-dependent kinase inhibitor p57KIP2 requires binding to proliferating cell nuclear antigen

Proper control of the mammalian cell cycle requires the function of cyclin-dependent kinase (CDK) inhibitors. The p21 family currently includes three distinct genes, p21, p27(Kip1), and p57(Kip2), that share a common N-terminal domain for binding to and inhibiting the kinase activity of CDK-cyclin complexes. The p21 protein also binds to proliferating cell nuclear antigen (PCNA) through a separate C-terminal domain affecting DNA replication and repair. The p27 and p57 proteins also each contain unique C-terminal domains whose functions are unknown. Here we show that the human p57 protein, like p21, contains a PCNA-binding domain within its C terminus that, when separated from its N-terminal CDK-cyclin binding domain, can prevent DNA replication in vitro and S phase entry in vivo. Disruption of either CDK/cyclin or PCNA binding partially reduced p57's ability to suppress myc/RAS-mediated transformation in primary cells, while loss of both inhibitory functions completely eliminated p57's suppressive activity. Thus, control of cell cycle and suppression of cell transformation by p57 require both CDK and PCNA inhibitory activity, and disruption of either or both functions may lead to uncontrolled cell growth.

Mapping of the human genes encoding cyclin H (CCNH) and the CDK-activating kinase (CAK) assembly factor MAT1 (MNAT1) to chromosome bands 5q13.3-q14 and 14q23, respectively

Cyclin-dependent kinases (CDKs), which play a key role in cell cycle control, are activated by the CDK-activating kinase (CAK), which activates cyclin-bound CDKs by phosphorylation at the specific threonine residue. Mammalian CAK contains three components: CDK7, cyclin H, and an assembly factor called MAT1. The CDK7-cyclin H-MAT1 complex is tightly associated with a multiprotein complex TFIIH, which plays a dual role in transcription and DNA repair. Here, we have determined chromosomal localizations of the human genes encoding cyclin H (CCNH) and MAT1 (HGMW-approved symbol MNAT1) to chromosome bands 5q13.3-q14 and 14q23, respectively, by using fluorescence in situ hybridization, somatic cell hybrid analyses, and mapping to the human YAC contigs.

p53 is phosphorylated by CDK7-cyclin H in a p36MAT1-dependent manner

The tumor suppressor protein p53 acts as a transcriptional activator that can mediate cellular responses to DNA damage by inducing apoptosis and cell cycle arrest. p53 is a nuclear phosphoprotein, and phosphorylation has been proposed to be a means by which the activity of p53 is regulated. The cyclin-dependent kinase (CDK)-activating kinase (CAK) was originally identified as a cellular kinase required for the activation of a CDK-cyclin complex, and CAK is comprised of three subunits: CDK7, cyclin H, and p36MAT1. CAK is part of the transcription factor IIH multiprotein complex, which is required for RNA polymerase II transcription and nucleotide excision repair. Because of the similarities between p53 and CAK in their involvement in the cell cycle, transcription, and repair, we investigated whether p53 could act as a substrate for phosphorylation by CAK. While CDK7-cyclin H is sufficient for phosphorylation of CDK2, we show that p36MAT1 is required for efficient phosphorylation of p53 by CDK7-cyclin H, suggesting that p36MAT1 can act as a substrate specificity-determining factor for CDK7-cyclin H. We have mapped a major site of phosphorylation by CAK to Ser-33 of p53 and have demonstrated as well that p53 is phosphorylated at this site in vivo. Both wild-type and tumor-derived mutant p53 proteins are efficiently phosphorylated by CAK. Furthermore, we show that p36 and p53 can interact both in vitro and in vivo. These studies reveal a potential mechanism for coupling the regulation of p53 with DNA repair and the basal transcriptional machinery.

The cyclin-dependent kinase-activating kinase (CAK) assembly factor, MAT1, targets and enhances CAK activity on the POU domains of octamer transcription factors

Octamer binding transcription factors (Oct factors) play important roles in activation of transcription of various genes but, in some cases, require cofactors that interact with the DNA binding (POU) domain. In the present study, a yeast two-hybrid screen with the Oct-1 POU domain as a bait identified MAT1 as a POU domain-binding protein. MAT1 is known to be required for the assembly of cyclin-dependent kinase (CDK)-activating kinase (CAK), which is functionally associated with the general transcription factor IIH (TFIIH). Further analyses showed that MAT1 interacts with POU domains of Oct-1, Oct-2, and Oct-3 in vitro in a DNA-independent manner. MAT1-containing TFIIH was also shown to interact with POU domains of Oct-1 and Oct-2. MAT1 is shown to enhance the ability of a recombinant CDK7-cyclin H complex (bipartite CAK) to phosphorylate isolated POU domains, intact Oct-1, and the C-terminal domain of RNA polymerase II, but not the originally defined substrate, CDK2. Phosphopeptide mapping indicates that the site (Ser385) of a mitosis-specific phosphorylation that inhibits Oct-1 binding to DNA is not phosphorylated by CAK. However, one CAK-phosphorylated phosphopeptide comigrates with a Cdc2-phosphorylated phosphopeptide previously shown to be mitosis-specific, suggesting that, in vitro, CAK is able to phosphorylate at least one site that is also phosphorylated in vivo. These results suggest (i) that interactions between POU domains and MAT1 can target CAK to Oct factors and result in their phosphorylation, (ii) that MAT1 not only functions as a CAK assembly factor but also acts to alter the spectrum of CAK substrates, and (iii) that a POU-MAT1 interaction may play a role in the recruitment of TFIIH to the preinitiation complex or in subsequent initiation and elongation reactions.

Mapping of amino acid residues in the p34 subunit of human single-stranded DNA-binding protein phosphorylated by DNA-dependent protein kinase and Cdc2 kinase in vitro

Human single-stranded DNA-binding protein (HSSB, also called RPA), is a heterotrimeric complex that consists of three subunits, p70, p34, and p11. HSSB is essential for the in vitro replication of SV40 DNA and nucleotide excision repair. It also has important functions in other DNA transactions, including DNA recombination, transcription, and double-stranded DNA break repair. The p34 subunit of HSSB is phosphorylated in a cell cycle-dependent manner. Both Cdc2 kinase and the DNA-dependent protein kinase (DNA-PK) phosphorylate HSSB-p34 in vitro. In this study, we show that efficient phosphorylation of HSSB-p34 by DNA-PK requires Ku as well as DNA. The DNA-PK phosphorylation sites in HSSB-p34 have been mapped at Thr-21 and Ser-33. Kinetic studies demonstrated that a phosphate residue is first incorporated at Thr-21 followed by the incorporation of a second phosphate residue at Ser-33. We also identified Ser-29 as the major Cdc2 kinase phosphorylation site in the p34 subunit.

Studies on the in vitro phosphorylation of HSSB-p34 and -p107 by cyclin-dependent kinases. Cyclin-substrate interactions dictate the efficiency of phosphorylation

Cyclin-dependent kinases (Cdks) are required for cell cycle progression. Two potentially significant Cdk substrates in human cells are the human single-stranded binding protein (HSSB or RPA), which plays an essential role in DNA replication, repair, and recombination, and the tumor suppressor p107 which acts to negatively regulate cell growth. In this report we describe the in vitro phosphorylation of these two proteins by Cdks in an attempt to understand how cyclin-substrate interactions direct phosphorylation efficiencies. We show that cyclin A-Cdk2 efficiently phosphorylates the p34 subunit of HSSB (HSSB-p34) alone or as a part of the heterotrimeric complex. In contrast, cyclin E-Cdk2 that is active in phosphorylating histone H1, does not support the phosphorylation of the p34 subunit of HSSB. We provide evidence that this differential phosphorylation results from a specific interaction between HSSB-p34 and cyclin A, but not cyclin E. Thus the observed cell cycle-dependent phosphorylation of HSSB-p34 at the G1 to S transition is most likely catalyzed by cyclin A-Cdk2 initiated by the direct interaction between cyclin A and the HSSB-p34 subunit. These studies are consistent with our previous observation that p107, which directly binds cyclin A, is efficiently phosphorylated by cyclin A-Cdk2 but not cyclin B-associated kinases. Here we further demonstrate that cyclin A only complexes with p107 in its unphosphorylated form. These data suggest a catalytic mechanism by which Cdk acts: substrate targeting by a cyclin-substrate interaction followed by dissociation of the Cdk upon phosphate incorporation allowing the Cdk to become available for the next cycle of phosphorylation.

Transcription-positive cofactor 4 forms complexes with HSSB (RPA) on single-stranded DNA and influences HSSB-dependent enzymatic synthesis of simian virus 40 DNA

The replication of simian virus 40 (SV40) DNA in vitro requires a trimeric single-stranded DNA (ssDNA)-binding protein called HSSB or RPA. HSSB supports the unwinding of DNA containing the SV40 origin in the presence of the viral-encoded T antigen and is required for the initiation of RNA primer synthesis as well as processive elongation of DNA catalyzed by the DNA polymerase delta holoenzyme. In this report we show that the transcription positive cofactor 4 (PC4), a ssDNA-binding protein, forms complexes with HSSB on ssDNA and markedly affects the replication functions of HSSB. PC4 supports T antigen-catalyzed unwinding of SV40 origins in lieu of HSSB but inhibits both RNA primer synthesis and polymerase delta-catalyzed DNA chain elongation reactions. These inhibitory effects can be reversed by the addition of excess HSSB. Depending on the concentration of HSSB, PC4 is capable of either inhibiting or activating SV40 DNA replication measured in both mono- and dipolymerase systems. The possible role of PC4 in the initiation of DNA replication is discussed.

Isolation and characterization of two human transcription factor IIH (TFIIH)-related complexes: ERCC2/CAK and TFIIH

Transcription factor IIH (TFIIH) is a multisubunit protein complex essential for both the initiation of RNA polymerase class II (pol II)-catalyzed transcription and nucleotide excision repair of DNA. Recent studies have shown that TFIIH copurifies with the cyclin-dependent kinase (cdk)-activating kinase complex (CAK) that includes cdk7, cyclin H, and p36/MAT1. Here we report the isolation of two TFIIH-related complexes: TFIIH* and ERCC2/CAK. TFIIH* consists of a subset of the TFIIH complex proteins including ERCC3 (XPB), p62, p44, p41, and p34 but is devoid of detectable levels of ERCC2 (XPD) and CAK. ERCC2/CAK was isolated as a complex that exhibits CAK activity that cosediments with the three CAK components (cdk7, cyclin H, and p36/MAT1) as well as the ERCC2 (XPD) protein. TFIIH* can support pol II-catalyzed transcription in vitro with lower efficiency compared with TFIIH. This TFIIH*-dependent transcription reaction was stimulated by ERCC2/CAK. The ERCC2/CAK and TFIIH* complexes are each active in DNA repair as shown by their ability to complement extracts prepared from ERCC2 (XPD)- and ERCC3 (XPB)-deficient cells, respectively, in supporting the excision of DNA containing a cholesterol lesion. These data suggest that TFIIH* and ERCC2/CAK interact to form the TFIIH holoenzyme capable of efficiently assembling the pol II transcription initiation complex and directly participating in excision repair reactions.

Expression of HIV-1 envelope glycoprotein alters cellular calmodulin

Removal of parts of a known calmodulin binding site at the C-terminus of HIV-1 envelope glycoprotein, gp160, can result in diminished infectivity. We investigated whether expression of full length gp160 would result in changes in intracellular calmodulin compared to expression of gp160 truncated to remove both known calmodulin binding sites. Both Western and Northern blots demonstrated that expression of gp160 led to increased calmodulin when compared to expression of truncated gp160. The induced calmodulin was associated preferentially with a particulate subcellular fraction. Confocal immunomicroscopy confirmed the increase in calmodulin and also showed that there was enhanced colocalization of calmodulin with gp160. Understanding of the role of calmodulin in the viral life-cycle may lead to new therapeutics.

Clamp loading, unloading and intrinsic stability of the PCNA, beta and gp45 sliding clamps of human, E. coli and T4 replicases

BACKGROUND

The high speed and processivity of replicative DNA polymerases reside in a processivity factor which has been shown to be a ring-shaped protein. This protein ("sliding clamp') encircles DNA and tethers the catalytic unit to the template. Although in eukaryotic, prokaryotic and bacteriophage-T4 systems, the processivity factors are ring-shaped, they assume different oligomeric states. The Escherichia coli clamp (the beta subunit) is active as a dimer while the eukaryotic and T4 phage clamps (PCNA and gp45, respectively) are active as trimers. The clamp can not assemble itself on DNA. Instead, a protein complex known as a clamp loader utilizes ATP to assemble the ring around the primer-template. This study compares properties of the human PCNA clamp with those of E. coli and T4 phage.

RESULTS

The PCNA ring is a stable trimer down to a concentration below 100 nM (Kd approximately 21 nM). On DNA, the PCNA clamp slides freely and dissociates from DNA slowly (t1/2 approximately 24 min). beta is more stable in solution (Kd < 60 PM) and on DNA (t1/2 approximately 1 h) than PCNA which may be explained by its simpler oligomeric state. The T4 gp45 clamp is a much less stable trimer than PCNA (Kd approximately 250 nM) and requires association with the polymerase to stabilize it on DNA as observed previously. The consequence of this cooperation between clamp and polymerase is that upon finishing a template and dissociation of the polymerase from DNA, the gp45 clamp spontaneously dissociates from DNA without assistance. However, the greater stability of the PCNA and beta clamps on DNA necessitates an active process for their removal. The clamp loaders (RFC and gamma complex) were also capable of unloading their respective clamps from DNA in the presence of ATP.

CONCLUSIONS

The stability of the different clamps in solution correlates with their stability on DNA. Thus, the low stability of the T4 clamp explains the inability to isolate gp45 on DNA. The stability of the PCNA and beta clamps predicts they will require an unloading factor to recycle them on and off DNA during replication. The clamp loaders of PCNA and beta double as clamp unloaders presumably for the purpose of clamp recycling.

Inhibition of nucleotide excision repair by the cyclin-dependent kinase inhibitor p21

p21, a p53-induced gene product that blocks cell cycle progression at the G1 phase, interacts with both cyclin-dependent kinases and proliferating cell nuclear antigen (PCNA). PCNA functions as a processivity factor for DNA polymerases delta and epsilon and is required for both DNA replication and nucleotide excision repair. Previous studies have shown that p21 inhibits simian virus 40 (SV40) DNA replication in HeLa cell extracts by interacting with PCNA. In this report we show that p21 blocks nucleotide excision repair of DNA that has been damaged by either ultraviolet radiation or alkylating agents, and that this inhibition can be reversed following addition of PCNA. We have determined that p21 is more effective in blocking DNA resynthesis than in inhibiting the excision step. We further show that a peptide derived from the carboxyl terminus of p21, which specifically interacts with PCNA, inhibits polymerase delta-catalyzed elongation of DNA chains almost stoichiometrically relative to the concentration of PCNA. When added at higher levels, this peptide also blocks both SV40 DNA replication and nucleotide excision repair in HeLa cell extracts. These results indicate that p21 interferes with the function of PCNA in both in vitro DNA replication and nucleotide excision repair.

Phosphorylated and unphosphorylated forms of human single-stranded DNA-binding protein are equally active in simian virus 40 DNA replication and in nucleotide excision repair

The trimeric human single-stranded DNA-binding protein (HSSB; also called RP-A) plays an essential role in DNA replication, nucleotide excision repair, and homologous DNA recombination. The p34 subunit of HSSB is phosphorylated at the G1/S boundary of the cell cycle or upon exposure of cells to DNA damage-inducing agents including ionizing and UV radiation. We have previously shown that the phosphorylation of p34 is catalyzed by both cyclin-dependent kinase-cyclin A complex and DNA-dependent protein kinase. In this study, we investigated the effect of phosphorylation of p34 by these kinases on the replication and repair function of HSSB. We observed no significant difference with the unphosphorylated and phosphorylated forms of HSSB in the simian virus 40 DNA replication or nucleotide excision repair systems reconstituted with purified proteins. The phosphorylation status of the p34 subunit of HSSB was unchanged during the reactions. We suggest that the phosphorylated HSSB has no direct effect on the basic mechanism of DNA replication and nucleotide excision repair reactions in vitro, although we cannot exclude a role of p34 phosphorylation in modulating HSSB function in vivo through a yet poorly understood control pathway in the cellular response to DNA damage and replication.

Assignment of the 36.5-kDa (RFC5), 37-kDa (RFC4), 38-kDa (RFC3), and 40-kDa (RFC2) subunit genes of human replication factor C to chromosome bands 12q24.2-q24.3, 3q27, 13q12.3-q13, and 7q11.23

Replication factor C is a multimeric primer-recognition protein consisting of five subunits (p145, p40, p38, p37, and p36.5) and is essential for the processive elongation of DNA chains catalyzed by DNA polymerase delta or epsilon in human cells. We have mapped the locations on human chromosomes of the genes coding for the four smaller subunits [p36.5 (RFC5), p37 (RFC4), p38 (RFC3), and p40 (RFC2)] using both PCR amplification from DNAs of a panel of somatic hybrids and fluorescence in situ hybridization to bands 12q24.2-q24.3, 3q27, 13q12.3-q13, and 7q11.23, respectively.

Cdk-interacting protein 1 directly binds with proliferating cell nuclear antigen and inhibits DNA replication catalyzed by the DNA polymerase delta holoenzyme

Cdk-interacting protein 1 (Cip1) is a p53-regulated 21-kDa protein that inhibits several members of the cyclin-dependent kinase (CDK) family. It was initially observed in complexes containing CDK4, cyclin D, and proliferating cell nuclear antigen (PCNA). PCNA, in conjunction with activator 1, acts as a processivity factor for eukaryotic DNA polymerase (pol) delta, and these three proteins constitute the pol delta holoenzyme. In this report, we demonstrate that Cip1 can also directly inhibit DNA synthesis in vitro by binding to PCNA. Cip1 efficiently inhibits simian virus 40 replication dependent upon pol alpha, activator 1, PCNA, and pol delta, and this inhibition can be overcome by additional PCNA. Simian virus 40 DNA replication, catalyzed solely by high levels of pol alpha-primase complex, is unaffected by Cip1. Using the surface plasmon resonance technique, a direct physical interaction of PCNA and Cip1 was detected. We have observed that Cip1 efficiently inhibits synthesis of long (7.2 kb) but not short (10 nt) templates, suggesting that its association with PCNA is likely to impair the processive movement of pol delta during DNA chain elongation, as opposed to blocking assembly of the pol delta holoenzyme. The implications of the Cip1-PCNA interaction with respect to regulation of DNA synthesis, cell cycle checkpoint control, and DNA repair are discussed.

Phosphorylation of the p34 subunit of human single-stranded-DNA-binding protein in cyclin A-activated G1 extracts is catalyzed by cdk-cyclin A complex and DNA-dependent protein kinase

The human single-stranded-DNA-binding protein (HSSB, also called RP-A) is a trimeric complex (p70, p34, and p14) required for multiple functions in DNA transactions. We report here that the p34 subunit of HSSB was hyperphosphorylated by kinase activities present in G1 extract (obtained from HeLa cells in G1 phase) preincubated with human cyclin A. This hyperphosphorylated HSSB product included at least four species of p34 that migrated more slowly through denaturing polyacrylamide gels than the hypophosphorylated form. Fractionation of cyclin A-activated G1 extract identified two kinases involved in the hyperphosphorylation of HSSB p34: cdk-cyclin A complex and DNA-dependent p350 protein kinase (DNA-PK). Kinetic analysis revealed that in cyclin A-activated G1 extract, p34 was first phosphorylated by cdk-cyclin A prior to the action of DNA-PK. Addition of p21cip1, a specific inhibitor of cdk-cyclin A but not DNA-PK, nearly abolished the hyperphosphorylation of HSSB p34 in G1 extract preincubated with cyclin A. This suggests a requirement of the cdk-cyclin A activity for the phosphorylation of p34 by DNA-PK in G1 extract.

Cloning of the large subunit of activator 1 (replication factor C) reveals homology with bacterial DNA ligases

We have cloned a gene encoding a DNA-binding protein by Southwestern screening of a murine cDNA library with a double-stranded oligonucleotide containing the sequence from the bidirectional promoter of the alpha 1 and alpha 2 collagen IV genes. The middle portion of this 1131-amino acid protein has a region homologous to bacterial DNA ligases, and the more carboxyl portion contains several domains homologous to p40, p38, p37, and p36.5 subunits of activator 1 (A1, also called replication factor C), a human replication protein complex. Western blotting revealed that antiserum generated against part of the recombinant protein reacted specifically with the 145-kDa component of the purified human A1 complex, indicating that it is the murine counterpart of the A1 p145. Characterization of the DNA-binding activity of the recombinant fusion protein by gel mobility-shift assay revealed that it had a preference for a run of pyrimidines on one strand. Deletion analysis using recombinant proteins revealed that the DNA ligase-like domain was required for DNA-binding activity. The finding that the region required for the binding of murine A1 p145 to DNA has similarity to a domain found in DNA ligases suggests that this region may be utilized by both proteins in recognizing DNA.

Reconstitution of cyclin-dependent cdc2 and cdk2 kinase activities in vitro

The genes that encode human cdc2 and cdk2 proteins are essential for cell cycle progression. In this report, we describe the purification of cyclin-associated cdc2 and cdk2 kinases as well as cyclin-free cdc2 and cdk2 protein preparations from HeLa cells. The cdc2-cyclin B kinase complex that we have isolated, consisting of two polypeptides of p60 (cyclin B) and p34 (cdc2), phosphorylated both the p34 and p70 subunits of the three-subunit human single-stranded DNA-binding protein (also called RP-A), a DNA replication and repair factor. We also partially purified a histone H1 kinase activity that is associated with the cdk2 and cyclin A proteins. Purified human cyclins A and B1, overproduced in bacteria, complemented a cellular fraction enriched in cdc2 and cdk2 proteins to reconstitute histone H1 kinase activity. Using this complementation system, human cdc2 and cdk2 proteins were purified and separated from one another. Glycerol gradient analyses demonstrated that the purified cdk2 (p33) protein co-sedimented with a cyclin A-dependent H1 kinase activity. Thus, cdk2 and cyclin A proteins are components that assemble to yield a kinase complex that catalyzes the phosphorylation of histone H1.

Characterization of the in vitro reconstituted cyclin A or B1-dependent cdk2 and cdc2 kinase activities

Human cyclins A and B1 were assembled with the cdk2 or cdc2 protein to reconstitute their respective kinase activities in vitro. Both cyclins complemented either cdk2 or cdc2, yielding kinase activities that supported the phosphorylation of histone H1. Activation of cdk2-catalyzed H1 kinase activity by cyclin A required a 10-min preincubation of the two components, whereas cdc2 kinase supported phosphate incorporation without a detectable time lag upon the addition of cyclin B1, suggesting a slower association rate of cdk2 with cyclin A compared with cdc2 and cyclin B1. Both cdk2 and cyclin A, as well as cdc2 and cyclin B1, formed stable complexes in the absence of ATP and substrate that could be isolated after glycerol gradient centrifugation. Incubation of the isolated complexes with ATP and histone H1 supported the phosphorylation of the substrate. Cyclin A-activated cdk2 or cdc2 phosphorylated p107, a pRB-related cellular protein, 10 times more effectively than the cyclin B1-complexed kinases. This was most likely due to a direct association of cyclin A with p107 (Ewen, M. E., Faha, B., Harlow, E., and Livingston, D. (1992) Science 255, 85-87; Faha, B., Ewen, M. E., Tsai, L.-H., Livingston, D., and Harlow, E. (1992) Science 255, 87-90). The reconstituted cdc2-cyclin B1 complex incorporated 4-5-fold more phosphate into the p34 subunit of the three-subunit (p70, p34, and p14) human single-stranded DNA-binding protein (also called RP-A), a DNA replication and DNA repair factor, than cdc2-cyclin A. No detectable phosphorylation of the p34 protein was observed with cdk2 complexed with either cyclin B1 or A. These data indicate that both cyclins as well as the catalytic subunits are important factors in controlling the rate of phosphorylation of a given substrate. The cyclin-activated cdc2 family kinases may target their cellular substrates through cyclin-mediated protein-protein interactions.

The subunits of activator 1 (replication factor C) carry out multiple functions essential for proliferating-cell nuclear antigen-dependent DNA synthesis

p37 and p40 are two cloned gene products of the five-subunit human cellular DNA replication factor activator 1 (A1) protein complex (also called replication factor C). Here, we describe the solubilization, purification, and characterization of these two proteins that were overproduced in Escherichia coli. Using a nitrocellulose filter binding assay, we demonstrated that the purified A1 p37 protein associated with DNA preferentially at the primer terminus, a property resembling that of the A1 complex. We also show that in the presence of relatively high levels of salt, the recombinant p37 protein alone activated DNA polymerase epsilon but not polymerase delta in catalyzing the elongation of DNA chains. The p40 protein specifically associated with cellular p37 and proliferating-cell nuclear antigen (PCNA) present in HeLa cell cytosolic extract. The addition of purified p40 protein abolished the in vitro polymerase delta-catalyzed DNA elongation reaction dependent on both PCNA and A1. However, this inhibition was reversed by excess polymerase delta, suggesting a specific interaction between the polymerase and the p40 protein. Thus, while p37 binds DNA at the primer end and has a specific affinity for pol epsilon, p40, which binds ATP, interacts with PCNA and pol delta. These activities are essential for the DNA elongation reactions that lead to the synthesis of leading-strand DNA and the maturation of Okazaki fragments.

Studies of the cloned 37-kDa subunit of activator 1 (replication factor C) of HeLa cells

The elongation of primed DNA templates by DNA polymerase delta and DNA polymerase epsilon requires the action of two accessory proteins, proliferating cell nuclear antigen and activator 1 (A1, also called replication factor C). A1 is an enzyme that contains five different subunits (145, 40, 38, 37, and 36.5 kDa). In this paper, we describe the isolation of the gene encoding the 37-kDa subunit from HeLa cells. This gene was cloned, sequenced, and overexpressed in Escherichia coli. The amino acid sequence shows a high degree of homology to the 40-kDa subunit of A1; they both contain the identical ATP-binding motif, but in contrast to the bacterial expressed 40-kDa protein, the 37-kDa expressed protein did not bind ATP. Both the 37- and 40-kDa proteins share substantial homology with the phage T4 gene 44 protein and to a lesser extent with the tau and gamma subunits of the E. coli DNA polymerase III holoenzyme. Polyclonal antibodies against the bacterially expressed 37- and 40-kDa proteins do not crossreact and are specific in their interaction. Antibodies against the 37-kDa protein maximally inhibited (by 50%) the A1-dependent synthesis of DNA by DNA polymerase delta; antibodies against the 40-kDa protein quantitatively inhibited the same reaction. When A1-dependent synthesis of DNA was partially inhibited by antibodies against the 40-kDa subunit, the addition of antibodies against the 37-kDa subunit inhibited DNA synthesis to a greater extent than the anti-37-kDa antibody alone. These results suggest that both the 37- and 40-kDa subunits of A1 are required for the biological role of A1 and that they may function differently in this process.

Sequence and expression in Escherichia coli of the 40-kDa subunit of activator 1 (replication factor C) of HeLa cells

Activator 1 (A1; also called replication factor C), in conjunction with proliferating-cell nuclear antigen (PCNA), is essential for the elongation of primed DNA templates by DNA polymerases delta and epsilon. A1 contains five distinct subunits of 145, 40, 38, 37, and 36.5 kDa. Here we describe the isolation, sequence, and bacterial expression of a cDNA coding for the 40-kDa subunit. In keeping with the presence of an ATP-binding motif, the bacterially expressed 40-kDa subunit binds ATP. The interaction between the 40-kDa subunit and ATP was reduced by the addition of PCNA. In addition, antibodies raised against the 40-kDa subunit abolished the A1- and PCNA-dependent synthesis of DNA catalyzed by polymerase delta. The putative amino acid sequence of the 40-kDa subunit of A1 revealed significantly homology with the bacteriophage T4 gene 44 protein and, to a lesser degree, with the tau and gamma subunits of Escherichia coli DNA polymerase III holoenzyme.

IL-7 receptor mediates tyrosine phosphorylation but does not activate the phosphatidylinositol-phospholipase C-gamma 1 pathway

IL-7 is a glycoprotein involved in the regulation of lymphocyte precursor growth. In addition, it has a comitogenic effect on mature T cells but not on mature B cells. The exact mechanism whereby IL-7R mediates these cell growth properties remains unknown. Because many growth factor receptor systems on various cell types transduce signals by activating a tyrosine kinase, we have studied here the effect of IL-7R ligation on protein tyrosine phosphorylation. We found that human rIL-7 consistently induced tyrosine phosphorylation of five major proteins, of 175, 155, 135, 110, and 85 kDa, and five minor proteins. The effect of human rIL-7 on tyrosine phosphorylation of these substrates was concentration and time dependent. One of the known substrates that is phosphorylated on tyrosine residues after binding of growth factors to their receptors is the phosphoinositide-specific phospholipase C. Several phospholipase C isozymes have been recently recognized; one isozyme, phospholipase C-gamma 1, was demonstrated to be phosphorylated rapidly after ligand binding to the platelet-derived growth factor receptor and the T cell Ag receptor. We show here that, in contrast to Ag receptor ligation, activation of IL-7R does not induce tyrosine phosphorylation on phospholipase C-gamma 1. Consistent with these results, human rIL-7 failed to increase phosphatidylinositol turnover and did not induce a rise in cytosolic free Ca2+ in the thymocytes, mature T cells, or pre-pre-B cells. The results indicate that the IL-7R mediates the activation of the tyrosine phosphorylation pathway but does not induce the phosphatidylinositol-phospholipase C pathway.

IL-7 receptor mediates tyrosine phosphorylation but does not activate the phosphatidylinositol-phospholipase C-gamma 1 pathway

IL-7 is a glycoprotein involved in the regulation of lymphocyte precursor growth. In addition, it has a comitogenic effect on mature T cells but not on mature B cells. The exact mechanism whereby IL-7R mediates these cell growth properties remains unknown. Because many growth factor receptor systems on various cell types transduce signals by activating a tyrosine kinase, we have studied here the effect of IL-7R ligation on protein tyrosine phosphorylation. We found that human rIL-7 consistently induced tyrosine phosphorylation of five major proteins, of 175, 155, 135, 110, and 85 kDa, and five minor proteins. The effect of human rIL-7 on tyrosine phosphorylation of these substrates was concentration and time dependent. One of the known substrates that is phosphorylated on tyrosine residues after binding of growth factors to their receptors is the phosphoinositide-specific phospholipase C. Several phospholipase C isozymes have been recently recognized; one isozyme, phospholipase C-gamma 1, was demonstrated to be phosphorylated rapidly after ligand binding to the platelet-derived growth factor receptor and the T cell Ag receptor. We show here that, in contrast to Ag receptor ligation, activation of IL-7R does not induce tyrosine phosphorylation on phospholipase C-gamma 1. Consistent with these results, human rIL-7 failed to increase phosphatidylinositol turnover and did not induce a rise in cytosolic free Ca2+ in the thymocytes, mature T cells, or pre-pre-B cells. The results indicate that the IL-7R mediates the activation of the tyrosine phosphorylation pathway but does not induce the phosphatidylinositol-phospholipase C pathway.

Synthesis of DNA by DNA polymerase epsilon in vitro

The isolation of DNA polymerase (Pol) epsilon from extracts of HeLa cells is described. The final fractions contained two major subunits of 210 and 50 kDa which cosedimented with Pol epsilon activity, similar to those described previously (Syvaoja, J., and Linn, S. (1989) J. Biol. Chem. 264, 2489-2497). The properties of the human Pol epsilon and the yeast Pol epsilon were compared. Both enzymes elongated singly primed single-stranded circular DNA templates. Yeast Pol epsilon required the presence of a DNA binding protein (SSB) whereas human Pol epsilon required the addition of SSB, Activator 1 and proliferating cell nuclear antigen (PCNA) for maximal activity. Both enzymes were totally unable to elongate primed DNA templates in the presence of salt; however, activity could be restored by the addition of Activator 1 and PCNA. Like Pol delta, Pol epsilon formed complexes with SSB-coated primed DNA templates in the presence of Activator 1 and PCNA which could be isolated by filtration through Bio-Gel A-5m columns. Unlike Pol delta, Pol epsilon bound to SSB-coated primed DNA in the absence of the auxiliary factors. In the presence of salt, Pol epsilon complexes were less stable than they were in the absence of salt. In the in vitro simian virus 40 (SV40) T antigen-dependent synthesis of DNA containing the SV40 origin of replication, yeast Pol epsilon but not human Pol epsilon could substitute for yeast or human Pol delta in the generation of long DNA products. However, human Pol epsilon did increase slightly the length of DNA chains formed by the DNA polymerase alpha-primase complex in SV40 DNA synthesis. The bearing of this observation on the requirement for a PCNA-dependent DNA polymerase in the synthesis and maturation of Okazaki fragments is discussed. However, no unique role for human Pol epsilon in the in vitro SV40 DNA replication system was detected.