Search PubMed⌕ Search

Biomedical subjects

S L Tan

Publications and source records attributed to S L Tan.

At least 73 records · Page 4Linked to original sources
PubMed1999

Ovarian stromal echogenicity in women with normal and polycystic ovaries.

Since the widespread use of transvaginal ultrasound to diagnose polycystic ovary syndrome (PCOS), a cardinal feature has been shown to be the presence of a bright, highly echogenic stroma. This is usually assessed subjectively. The objective of this study was to determine whether ovarian stromal echogenicity when measured objectively actually differed between women with polycystic ovaries and those with normal ovaries. A total of 67 women underwent a detailed ultrasound assessment before considering assisted conception treatment. Ovarian morphology was assessed and total ovarian volume, stromal volume, peak stromal blood flow velocity and mean stromal echogenicity were measured. The stromal index (ratio of mean stromal echogenicity to mean echogenicity of the entire ovary) and total stromal echogenicity were also calculated. Ovarian volume, stromal volume, and stromal peak blood flow velocity were all significantly higher in ovaries from women with PCOS. There was no difference in the mean stromal echogenicity, although the stromal index was significantly greater in women with polycystic ovaries. The apparent subjective increase in stromal echogenicity in women with polycystic ovaries, as exemplified by the greater stromal index, is due to a combination of the increased volume of ovarian stroma and the significantly lower mean echogenicity of the entire ovary in these women.

Dehydroepiandrosterone Sulfate↗
PubMed1999

Assessment of multiplex fluorescent PCR for screening single cells for trisomy 21 and single gene defects.

A great majority of patients seeking preimplantation genetic diagnosis (PGD) are women >35 years of age. In addition to being carriers for single gene defects, these women also have a higher risk of having children with Down's syndrome (trisomy 21). For these patients, it would be advantageous if a diagnostic test for trisomy 21 was developed, which could be used in conjunction with tests for single gene defects. Here, we assessed the feasibility of developing an accurate genetic test for diagnosing trisomy 21 and the mutation causing spinal muscular atrophy (SMA) in single cells using multiplex fluorescence polymerase chain reaction (PCR). Single- and two-round PCR were developed using a combination of primers for the survival motor neuron (SMN) gene exons 7 and 8 and two chromosome 21 short tandem repeats (STRs), D21S226 and D21S11. After only 36 cycles, 88 and 68% of normal single cells were screened for SMA mutations and trisomy 21 respectively. In multiplex PCR using only two primers (SMN exon 7 and D21S11) instead of four, the efficiency of SMA diagnosis was increased to 93%. In the same reactions, the D21S11 alleles were detected in 83% of the normal single cells. Clinical applications of this assay should enable detection of those embryos that have inherited three heterozygous alleles and, therefore, benefit many PGD patients who are at an increased risk of Down's syndrome.

Blastomeres↗
PubMed1999

Cumulative probability of clinical pregnancy and live birth after a multiple cycle IVF package: a more realistic assessment of overall and age-specific success rates?

OBJECTIVE: To provide an assessment of pregnancy and live birth probabilities for women presenting for in vitro fertilisation treatment for the first time, when committed in advance to have up to three cycles of treatment in one year. DESIGN: Up to three cycles of in vitro fertilisation within one year, committed in advance. SETTING: A tertiary referral centre for assisted reproduction. PARTICIPANTS: Two hundred and thirty-two women, undergoing a total of 536 cycles of in vitro fertilisation or intracytoplasmic sperm injection between August 1993 and December 1995. METHODS: Analysis of cumulative clinical pregnancy and live birth rates for women having IVF treatment for the first time and undertaking a three-cycle package, using the life-table approach. MAIN OUTCOME MEASURES: Cumulative clinical pregnancy and live birth rates. RESULTS: The cumulative probabilities of clinical pregnancy and live birth after two cycles of treatment were 38.2% and 33.2%, respectively, compared with 54.2% and 48.2%, respectively, after three cycles of treatment. Cumulative clinical pregnancy and live birth rates after three cycles of treatment for women up to the age of 40 years were 57.8% and 51.3%, respectively. Cumulative clinical pregnancy and live birth rates declined with increasing age (P = 0.02 and P= 0.01, respectively). CONCLUSION: The three-cycle package encourages couples to have multiple treatment cycles, thereby improving their ultimate chances of a live birth. The cumulative clinical pregnancy and live birth rates after such a package provide a more realistic assessment of overall and age-specific success rates after multiple treatment cycles.

Adult↗
PubMed1999

Differential sensitivity to 5-fluoro-orotic acid as a screen for bait RNA-independent false positives in a yeast three-hybrid system.

The yeast three-hybrid system presents a valuable tool for detecting and analyzing RNA-protein interactions in vivo. A major drawback of the use of such a transcriptional reporter-based assay in a library screen is the frequent occurrence of false-positive results due to bait RNA-independent activation of the reporter gene. To minimize the isolation of false positives in three-hybrid library screens, we incorporated a rapid and simple procedure based on differential sensitivity to 5-fluoro-orotic acid. The technique effectively eliminates bait RNA-independent false positives and thus greatly enhances the efficiency of the yeast three-hybrid system.

Cloning, Molecular↗
PubMed1998

Using genetic means to dissect homologous and heterologous protein-protein interactions of PKR, the interferon-induced protein kinase.

The interferon-induced protein kinase, PKR, is a pivotal component of interferon (IFN)-induced cellular antiviral and antiproliferative response. The identification and characterization of proteins, of both viral and cellular origins, that interact with PKR have proven to be a valuable probe for unraveling the cellular regulation and function of PKR. Several studies have demonstrated that PKR forms dimers and that dimerization is likely to be required for activation and/or catalytic function. It is therefore important to elucidate the mechanism of PKR dimer formation and the role of PKR effectors in modulating kinase dimerization. Herein we describe the use of the two genetic approaches, the lambda repressor fusion and the yeast two-hybrid systems, to detect and analyze homo- and heterotypic interactions with PKR. We also describe several biochemical methodologies commonly used in our laboratory to validate the genetic results. Although the examples in this article focus on PKR, the techniques can easily be adapted to investigate protein-protein associations in a variety of experimental systems. Finally, given the important role of PKR as a mediator of IFN-induced antiviral and antiproliferative effects, these studies may provide clues to the development of reagents that target PKR to enhance the therapeutic use of IFN in the treatment of disease.

Bacteriophage lambda↗
PubMed1998

Local side effects of subcutaneous and intramuscular urinary gonadotropins for ovarian stimulation in in vitro fertilization: a prospective, randomized study.

OBJECTIVE: To compare the incidence and severity of local side effects of urinary-derived gonadotropins administered s.c. and i.m.. DESIGN: Prospective randomized study of women undergoing IVF treatment. SETTING: Tertiary referral center for assisted reproduction. PATIENT(S): A total of 71 patients were randomized to receive gonadotropins by the s.c. (n = 41) or i.m. (n = 30) route. INTERVENTION(S): One cycle of IVF with gonadotropins administered either s.c. or i.m. for ovarian stimulation. MAIN OUTCOME MEASURE(S): Incidence and severity of local side effects, such as redness, itching, swelling, pain, and bruising. RESULT(S): Pain was the most common side effect, with 55.3% and 70.1% of i.m. and s.c. injections, respectively, resulting in pain. There were no statistically significant differences in the incidence of itching or bruising after i.m. and s.c. injections. Although there was a higher incidence of redness and swelling in the s.c. group compared with the i.m. group, most cases were classified as mild. CONCLUSION(S): There was a significantly higher incidence of some local side effects after s.c. gonadotropin administration but most of these were mild and well tolerated by patients.

Adolescent↗
PubMed1998

Serum vascular endothelial growth factor and Doppler blood flow velocities in in vitro fertilization: relevance to ovarian hyperstimulation syndrome and polycystic ovaries.

OBJECTIVE: To relate changes in serum and follicular fluid vascular endothelial growth factor (VEGF) concentrations to Doppler blood flow velocities within ovarian and uterine blood vessels during IVF cycles and to assess their relation to ovarian hyperstimulation syndrome (OHSS) and polycystic ovaries (PCO)/polycystic ovary syndrome (PCOS). DESIGN: Prospective descriptive study. SETTING: The London Women's Clinic. PATIENT(S): One hundred seven patients receiving a "long" stimulation protocol of IVF. INTERVENTION(S): Doppler blood flows were recorded in ovarian stromal and uterine arteries; serum VEGF was measured in the early follicular phase, after pituitary desensitization, on the day of hCG administration, on the day of oocyte retrieval, and on the day of ET. MAIN OUTCOME MEASURE(S): Serum and follicular fluid VEGF concentrations and Doppler blood flow measurements. RESULT(S): Serum VEGF concentrations rose after hCG administration. The rise was higher in women in whom OHSS developed. Women with PCO/PCOS had higher serum VEGF concentrations throughout the IVF cycle irrespective of whether OHSS developed. Follicular fluid VEGF concentrations were higher in women in whom OHSS developed and in women with PCO/PCOS. Doppler blood flow velocities in the ovarian blood vessels were higher in women in whom OHSS developed. Within ovarian and uterine blood vessels, blood flow velocities were higher in the early follicular phase and on the day of hCG administration in women with PCO/PCOS. A positive correlation was observed between the serum VEGF and E2 concentrations on the days of hCG administration and oocyte retrieval and between the serum VEGF concentration and Doppler blood flow velocities throughout the IVF cycle. CONCLUSION(S): Our results support the role of VEGF as a mediator of OHSS and establish a possible link between VEGF, OHSS, and PCO.

Adult↗
PubMed1998

Effects of pretreatment with an oral contraceptive on the time required to achieve pituitary suppression with gonadotropin-releasing hormone analogues and on subsequent implantation and pregnancy rates.

OBJECTIVE: To assess the effect of pretreatment with an oral contraceptive (OC) on ovarian cyst formation during pituitary suppression with buserelin acetate. DESIGN: Prospective randomized trial. SETTING: Academic medical center. PATIENT(S): Eighty-three patients who were undergoing IVF-ET treatment. INTERVENTION(S): Patients in the study group were pretreated with an OC for 14 days starting on the first day of menstruation. The administration of SC buserelin acetate was initiated on the last day of OC administration. Patients in the control group began to receive buserelin acetate on day 2 of menstruation. Hormonal assays and ultrasound scans were performed on the first day of menstruation, and 7, 11, and 14 days after the commencement of buserelin acetate administration. Thereafter, these tests were performed weekly until pituitary suppression was achieved. MAIN OUTCOME MEASURE(S): Incidence of cyst formation. RESULT(S): A cyst developed in 27 patients in the control group (52.9%) and no patients in the study group (odds ratio [OR]=115; 95% confidence interval [CI]=10-617). Patients in the study group achieved pituitary suppression faster (median difference [MD]=7 days; 95% CI=4-14) and required fewer ampules of gonadotropin (MD=10; 95% CI=6-14). They recruited more follicles (MD=3; 95% CI=0-5) and had higher pregnancy rates (37.2% versus 33.3%). CONCLUSION(S): Pretreatment with an OC abolishes ovarian cyst formation, shortens the time required to achieve pituitary suppression, and decreases gonadotropin requirements without having a negative effect on pregnancy rates.

Adult↗
PubMed1998

Use of luteinizing hormone releasing hormone agonists in polycystic ovary syndrome.

Luteinizing hormone releasing hormone (LHRH) agonists have been used in conjunction with gonadotrophins, and occasionally with pulsatile LHRH, for ovulation induction in women with clomiphene-citrate-resistant polycystic ovary syndrome (PCOS) and also for superovulation for in vitro fertilization (IVF) and gamete intrafallopian transfer in women with PCOS. In IVF, LHRH agonists given by the 'long protocol' before gonadotrophins are commenced have consistently shown higher pregnancy rates and higher live birth rates. Although the optimal time to commence LHRH agonist is not clearly determined, commencement in the early follicular phase possibly with pre-treatment with the combined oral contraceptive pill would avoid the risk of inadvertent administration during early pregnancy. The role of LHRH agonists in ovulation induction is less clear cut, although there may be some advantages in patients with refractory PCOS. The role of LHRH agonists in ovarian hyperstimulation syndrome and recurrent miscarriage is also discussed.

Abortion, Habitual↗
PubMed1998

Endoscopic femoral-popliteal/distal bypass grafting: a preliminary report.

BACKGROUND: Patients requiring limb bypass or coronary artery bypass procedures frequently develop vein harvest-associated complications. Minimally invasive surgical techniques that can be employed during dissection of the greater saphenous vein could potentially reduce the significant incisional morbidity concomitant with this procedure. STUDY DESIGN: An endoscopic saphenous vein balloon dissector was developed and previously tested in the animal model. It was applied to a series of 16 leg bypass patients to dissect totally endoscopically the greater saphenous vein for either translocated or in situ procedures. The clinical course of these 16 patients was compared with the most recent 16 consecutive standard open saphenous vein bypass patients of similar age, disease, and risk factors. RESULTS: No venous conduit injuries occurred with the endoscopic dissection technique, and only one minor wound seroma resulted in the harvest tunnel, requiring simple aspiration. The length of stay averaged 3.8 days, but the trend was downward to 1.8 days for the last 8 consecutive endoscopically dissected patients. In comparison, there were five major wound complications in the 16 open saphenous vein bypass patients with an average length of stay of 6.2 days. CONCLUSIONS: Total endoscopic saphenous vein dissection for either translocated or in situ leg bypass patients can be performed atraumatically for both the patient and the venous conduit. Wound/incisional complications are decreased, and length of stay appears to be reduced. Minimally invasive, endoscopic saphenous vein harvest may be beneficial for both leg bypass patients and coronary artery bypass patients.

Aged↗
PubMed1998

Pretreatment with an oral contraceptive is effective in reducing the incidence of functional ovarian cyst formation during pituitary suppression by gonadotropin-releasing hormone analogues.

PURPOSE: Our purpose was to assess the effect of pretreatment with oral contraceptives (OCs) on the formation of functional ovarian cysts during pituitary suppression with gonadotropin-releasing hormone (GnRH) agonists, subsequent follicular development, and pregnancy rates. METHODS: A retrospective case-controlled study of 31 in vitro fertilization (IVF) patients, all of whom in a previous cycle had commenced the long protocol of GnRH-agonist (Buserelin) in the early follicular phase and were pretreated in a subsequent cycle with 2 weeks of an OC containing 30 micrograms of ethinyl estradiol and 150 micrograms of desogestrel prior to GnRH-agonist administration, was undertaken. Follow-up visits were arranged after a minimum of 11 days of GnRH-agonist administration and weekly thereafter until pituitary suppression was achieved. RESULTS: Cysts were detected in 16 (51.6%) of the 31 patients not pretreated with OCs, and in 0 (0%) of the 31 patients pretreated with OCs (odds ratio = 67.1; 95% confidence interval = 5.6-350.7). Patients pretreated with OCs achieved pituitary suppression more rapidly (median difference = 4 days; 95% confidence interval = 2-7) and had comparable gonadotropin requirements and pregnancy rates. CONCLUSIONS: Pretreatment with OCs prior to pituitary suppression in the early follicular phase decreases ovarian cyst formation, without an apparent effect on subsequent follicular recruitment or pregnancy rates.

Adult↗
PubMed1998

Blood flow changes in the ovarian and uterine arteries in women with polycystic ovary syndrome who respond to clomiphene citrate: correlation with serum hormone concentrations.

OBJECTIVE: Our purpose was to study the hemodynamic changes in the uterine and intraovarian vessels in women with polycystic ovary syndrome (PCOS) and to relate the vascular changes to hormonal indices. METHODS: Six women with a history of previous anovulatory cycles and/or oligomenorrhea and/or elevated serum luteinizing hormone concentrations in addition to polycystic ovaries underwent induction of ovulation with clomiphene citrate. Serial transvaginal ultrasonography was performed on menstrual cycle day 2, daily from the estimated day of ovulation minus 6, 6-hourly from the estimated day of ovulation minus 2 or when the mean follicular diameter was greater than 16 mm (whichever was earlier) until 6 h after follicular rupture, and once 7 days after follicular rupture. At each scan, uterine and intraovarian blood flow was assessed by transvaginal color Doppler ultrasound, and serum concentrations of follicle stimulating hormone (FSH), luteinizing hormone (LH), estradiol and progesterone were assessed by fluoroimmunoassay. RESULTS: All six women demonstrated ultrasonographic features of ovulation. Follicular peak systolic blood flow velocity (Vmax) and ovarian stromal Vmax in the dominant ovary increased during the follicular phase of the menstrual cycle and reached a peak at 36 and 42 h after the onset of the LH surge of 30.7 +/- 4.6 cm/s and 37.0 +/- 3.9 cm/s, respectively. There was no significant change in the ovarian stromal Vmax of the non-dominant ovary. There was no significant change in the pulsatility index (PI) in either the follicular or the ovarian stromal vessels during the menstrual cycle. The changes in follicular Vmax correlated significantly with changes in serum concentrations of FSH, LH and progesterone. Uterine artery time-averaged maximum velocity (TAMX) declined to its lowest levels at LH surge minus 6 days, and thereafter increased. In contrast, the PI increased during the follicular phase of the cycle, reaching a peak at LH surge minus 4 days, and thereafter declined. The changes in uterine artery PI correlated inversely with serum estradiol and serum progesterone concentrations. CONCLUSIONS: Women with PCOS have significant differences in intraovarian and uterine artery hemodynamics, both at the baseline ultrasound scan and during the clomiphene-induced menstrual cycle, compared with women with normal ovaries. The intraovarian and uterine artery vascular differences are likely to be due to a primary disorder within the polycystic ovary and their different hormonal status. Clomiphene citrate may have an effect on uterine artery blood flow.

Adult↗
PubMed1998

Biochemical and genetic evidence for complex formation between the influenza A virus NS1 protein and the interferon-induced PKR protein kinase.

The interferon (IFN)-induced protein kinase (PKR) functions as a gatekeeper of mRNA translation initiation and is, therefore, a key mediator of the host IFN-induced antiviral defense system. Many viruses have invested countermeasures against PKR. Some apparently use more than one mechanism. The influenza virus can repress PKR activity through the use of at least two factors, the cellular P58IPK protein and the viral NS1 protein. The exact mode of action of the latter has not been established. Here, using a coprecipitation assay, we found that PKR could form a complex with NS1 in crude cell extracts prepared from influenza virus-infected HeLa cells. The NS1-PKR interaction was verified by using the yeast two-hybrid system and an in vitro binding assay. Deletion analysis mapped the NS1 binding site to the N-terminal 98 residues of PKR regulatory region. Furthermore, an NS1 mutant, which lacks PKR inhibitory activity, did not bind PKR. Finally, the functional role of NS1 in PKR inhibition was substantiated using an in vivo assay for PKR activity. These results support the role of NS1 in PKR modulation during viral infection that is mediated through a complex formation between the two proteins.

Antiviral Agents↗
PubMed1998

Serum vascular endothelial growth factor concentrations and ovarian stromal blood flow are increased in women with polycystic ovaries.

The aim of this study was to determine basal serum vascular endothelial growth factor (VEGF) concentrations and Doppler blood flow changes within the ovarian stroma of women with polycystic ovaries (PCO) and women with normal ovaries. Pulsed and colour Doppler blood flows within the ovarian stroma were recorded, and serum VEGF concentrations measured, in the early follicular phase (days 2-3 of a menstrual cycle) in 60 women undergoing ovarian stimulation for in-vitro fertilization. 36 women had normal ovaries, 14 women had PCO as seen on pelvic ultrasound examination and 10 had polycystic ovarian syndrome (PCOS). Mean+/-SD serum VEGF concentrations were significantly higher (P < 0.001) in women with PCO and PCOS (3.4+/-0.7 and 3.2+/-0.66 ng/ml respectively) compared with women with normal ovaries (2.3+/-0.5 ng/ml). Mean peak systolic blood flow velocity (PSV) and time-averaged maximum flow velocity (TAMXV) were significantly higher (P < 0.001) in women with PCO and PCOS compared with women with normal ovaries. The mean PSV were 15+/-4 and 16+/-4 cm/s in women with PCO and PCOS respectively, compared with 9+/-2 cm/s in women with normal ovaries. The TAMXV were 9+/-3 and 11+/-3 cm/s in women with PCO and PCOS respectively compared with women with normal ovaries (5.8+/-1.5 cm/s). Serum VEGF concentrations were positively correlated with PSV (r=0.44, P=0.001) and TAMXV (r=0.45, P < 0.000) in all three groups of women. Higher serum concentrations of VEGF in women with PCO and PCOS may relate to the increased vascularity that underlies the increased blood flow demonstrated by Doppler blood flow velocity measurements in these women. The results may explain the higher risk of ovarian hyperstimulation syndrome in programmes of ovarian stimulation in patients with PCO compared with those with normal ovaries.

Adult↗
PubMed1998

A prospective randomized study to assess the clinical efficacy of gonadotrophins administered subcutaneously and intramuscularly.

The purpose of this study was to compare the clinical efficacy of gonadotrophins administered s.c. or i.m., in a prospective randomized study of women undergoing in-vitro fertilization (IVF) treatment at a tertiary referral centre. In all, 71 patients undergoing a total of 162 IVF treatment cycles were randomized to receive either s.c. (n = 41) or i.m. (n = 30) administration of gonadotrophins. Up to three cycles of IVF were assessed for each patient. The main outcome measures were the number of oocytes retrieved, the total amount of gonadotrophins used, the number of follicles recruited and the cumulative pregnancy and live birth rates. The mean number of oocytes retrieved was 10.5 for each group. The number of days of stimulation was significantly shorter for the s.c. group (11.7 +/- 1.9 days, mean +/- SD) than the i.m. group (12.6 +/- 2.3 days). The cumulative conception and live birth rates after three cycles of treatment were similar between the two groups. Our results suggest that the clinical efficacy of s.c. and i.m. administration of gonadotrophins is comparable. Both routes are well tolerated by patients.

Adult↗
PubMed1998

Inhibition of double-stranded RNA-dependent protein kinase PKR by vaccinia virus E3: role of complex formation and the E3 N-terminal domain.

The human double-stranded RNA (dsRNA)-dependent protein kinase PKR inhibits protein synthesis by phosphorylating translation initiation factor 2alpha (eIF2alpha). Vaccinia virus E3L encodes a dsRNA binding protein that inhibits PKR in virus-infected cells, presumably by sequestering dsRNA activators. Expression of PKR in Saccharomyces cerevisiae inhibits protein synthesis by phosphorylation of eIF2alpha, dependent on its two dsRNA binding motifs (DRBMs). We found that expression of E3 in yeast overcomes the lethal effect of PKR in a manner requiring key residues (Lys-167 and Arg-168) needed for dsRNA binding by E3 in vitro. Unexpectedly, the N-terminal half of E3, and residue Trp-66 in particular, also is required for anti-PKR function. Because the E3 N-terminal region does not contribute to dsRNA binding in vitro, it appears that sequestering dsRNA is not the sole function of E3 needed for inhibition of PKR. This conclusion was supported by the fact that E3 activity was antagonized, not augmented, by overexpressing the catalytically defective PKR-K296R protein containing functional DRBMs. Coimmunoprecipitation experiments showed that a majority of PKR in yeast extracts was in a complex with E3, whose formation was completely dependent on the dsRNA binding activity of E3 and enhanced by the N-terminal half of E3. In yeast two-hybrid assays and in vitro protein binding experiments, segments of E3 and PKR containing their respective DRBMs interacted in a manner requiring E3 residues Lys-167 and Arg-168. We also detected interactions between PKR and the N-terminal half of E3 in the yeast two-hybrid and lambda repressor dimerization assays. In the latter case, the N-terminal half of E3 interacted with the kinase domain of PKR, dependent on E3 residue Trp-66. We propose that effective inhibition of PKR in yeast requires formation of an E3-PKR-dsRNA complex, in which the N-terminal half of E3 physically interacts with the protein kinase domain of PKR.

Cell Division↗
PubMed1998

Double-stranded RNA-independent dimerization of interferon-induced protein kinase PKR and inhibition of dimerization by the cellular P58IPK inhibitor.

The interferon (IFN)-induced, double-stranded RNA-activated protein kinase (PKR) mediates the antiviral and antiproliferative actions of IFN, in part, via its translational inhibitory properties. Previous studies have demonstrated that PKR forms dimers and that dimerization is likely to be required for activation and/or function. In the present study we used multiple approaches to examine the modulation of PKR dimerization. Deletion analysis with the lambda repressor fusion system identified a previously unrecognized site involved in PKR dimerization. This site comprised amino acids (aa) 244 to 296, which span part of the third basic region of PKR and the catalytic subdomains I and II. Using the yeast two-hybrid system and far-Western analysis, we verified the importance of this region for dimerization. Furthermore, coexpression of the 52-aa region alone inhibited the formation of full-length PKR dimers in the lambda repressor fusion and two-hybrid systems. Importantly, coexpression of aa 244 to 296 exerted a dominant-negative effect on wild-type kinase activity in a functional assay. Due to its role as a mediator of IFN-induced antiviral resistance, PKR is a target of viral and cellular inhibitors. Curiously, PKR aa 244 to 296 contain the binding site for a select group of specific inhibitors, including the cellular protein P58IPK. We demonstrated, utilizing both the yeast and lambda systems, that P58IPK, a member of the tetratricopeptide repeat protein family, can block kinase activity by preventing PKR dimerization. In contrast, a nonfunctional form of P58IPK lacking a TPR motif did not inhibit kinase activity or perturb PKR dimers. These results highlight a potential mechanism of PKR inhibition and define a novel class of PKR inhibitors. Finally, the data document the first known example of inhibition of protein kinase dimerization by a cellular protein inhibitor. On the basis of these results we propose a model for the regulation of PKR dimerization.

Binding Sites↗