A TRIM21-based bioPROTAC highlights the therapeutic benefit of HuR degradation

Human antigen R (HuR) is a ubiquitously expressed RNA-binding protein, which functions as an RNA regulator. Overexpression of HuR correlates with high grade tumours and poor patient prognosis, implicating it as an attractive therapeutic target. However, an effective small molecule antagonist to HuR for clinical use remains elusive. Here, a single domain antibody (VHH) that binds HuR with low nanomolar affinity was identified and shown to inhibit HuR binding to RNA. This VHH was used to engineer a TRIM21-based biological PROTAC (bioPROTAC) that could degrade endogenous HuR. Significantly, HuR degradation reverses the tumour-promoting properties of cancer cells in vivo by altering the HuR-regulated proteome, highlighting the benefit of HuR degradation and paving the way for the development of HuR-degrading therapeutics. These observations have broader implications for degrading intractable therapeutic targets, with bioPROTACs presenting a unique opportunity to explore targeted-protein degradation through a modular approach.

A211 SPONTANEOUS HBV REMISSION AFTER HBV REACTIVATION FOLLOWING TREATMENT WITH SOFOSBUVIR AND VELPATASVIR IN A PATIENT WITH HCV AND HBV CO-INFECTION: CASE REPORT

Background

In co-infected patients with hepatitis B (HBV) and hepatitis C (HCV), the treatment of HCV with direct-acting antiviral agents (DAA) can cause HBV reactivation. However, there are no clear guidelines on the timing of treatment initiation, especially in the absence of clinical signs of flare.

Aims

Here we discuss the case of a 34-year-old female with HBV and HCV genotype 3 who had HBV reactivation following HCV treatment, but did not require nucleos(t)ide therapy.

Methods

She initially presented with chronic inactive hepatitis B and chronic hepatitis C with HBV DNA level of 67.5 IU/mL and HCV RNA level of 3.33 x 106 IU/mL. She completed a 12 week course of sofosbuvir and velpatasvir for HCV and achieved sustained virologic remission, but subsequently developed reactivation of her HBV with HBV DNA peaking at 3.41 x 104 IU/mL twelve weeks post-treatment. She did not develop any signs of hepatitis and a decision was made to monitor her clinically.

Results

Two years later, she spontaneously went into remission with her HBV DNA levels being <10 IU/mL.

Conclusions

The significance of this case is to illustrate HBV reactivation following treatment of HCV with DAAs may not necessitate immediate treatment, especially if there are no signs of flare. There have been similar reported cases, but larger prospective studies are required to determine the appropriate clinical context where monitoring may be acceptable instead of immediate treatment.

Funding Agencies

None

1202 Validation Of A Novel Wearable Home Sleep Testing Device For Assessment Of Obstructive Sleep Apnea

Introduction

There is a substantial need for an accurate and easy-to-use tool for obstructive sleep apnea (OSA) assessment. Belun Ring Platform (BRP), a novel photoplethysmography (PPG)-based home sleep apnea testing system with a proprietary deep learning algorithm, has been shown to have good sensitivity and specificity in predicting OSA in subjects without significant comorbidities and medications known to affect heart rate (HR). In this study, we further tested its performance in subjects referred for in-lab polysomnography (PSG) assessment of sleep disorders without excluding those with non-arrhythmia comorbidities or the subjects on HR-affecting medications.

Methods

PSG was recorded simultaneously with the Ring in the sleep lab and the studies were manually scored by certified sleep technicians according to the AASM Scoring manual version 2.4. Exclusion criteria include age <18, unstable cardiopulmonary status, recent hospitalization within 30 days, significant arrhythmias, baseline HR <50 or >100, home oxygen use, pacemaker/defibrillator, post-cardiac transplantation or Left ventricular assist device.

Results

A cohort of 78 individuals (26 males and 52 females, age 50.5) were studied with 26 taking HR-affecting medications. Of these, 35 (45%) had AHI < 5; 14 (18%) had AHI 5-15; 15 (19%) had AHI 15-30; 14 (18%) had AHI > 30. The Ring-REI correlated well with the PSG-AHI (r =0.83, P <0.001). The accuracy, sensitivity, specificity in categorizing AHI >15 were 0.808, 0.931, and 0.735 respectively. The positive predictive value, negative predictive value, positive likelihood ratio, and negative likelihood ratio were 0.675, 0.947, 3.509, and 0.094 respectively. The use of HR-affecting medications did not significantly affect the sensitivity and specificity of BRP in predicting OSA (P =0.16 and 0.44 respectively).

Conclusion

BRP is promising as a reasonable tool for OSA assessment and can potentially be incorporated into a broad spectrum of clinical practices for identification of patients with OSA.

Support

This study is supported by a Grant from Belun Technology Company Limited.

Lipid Metabolism in Late-Onset Alzheimer's Disease Differs from Patients Presenting with Other Dementia Phenotypes

Abnormal cerebrospinal fluid (CSF) levels of β-amyloid peptides (Aβ₄₂) and Tau and cognitive decline are typical characteristics of Alzheimer’s disease (AD). Since dysregulation in lipid metabolism accompanies abnormal amyloid formation, we quantified glycerophospholipids (GP) and sphingolipids (SP) in CSF fractions from participants with late-onset AD (LOAD, n = 29) or with Other Dementia (OD, n = 10) to determine if alterations in lipid metabolism account for pathological differences. Aβ₄₂ and total Tau levels were determined using a sandwich ELISA. Liposomal-based fluorescent assays were used to measure phospholipase A₂ (PLA₂) and acid or neutral sphingomyelinase (aSMase, nSMase) activities. Supernatant fluid (SF) and nanoparticle (NP) lipids were quantified using LC-MS/MS. Although CSF Aβ₄₂ and Tau levels are similar, phosphatidylserine (PS) in SF and ceramide (CM) levels in NP are significantly higher in OD compared with LOAD. The aSMase but not the nSMase activity is higher in OD. PLA₂ activity in CSF from OD subjects positively correlates with several GP classes in SF and NP fractions but not in LOAD fractions. Our data indicate differences in CSF lipid metabolism between dementia variants. Higher levels of inflammatory and apoptotic lipids may induce faster neuronal death, resulting in the earlier cognitive decline in patients with OD phenotypes.

Non-anaplastic peripheral T cell lymphoma in children and adolescents-an international review of 143 cases

Peripheral T cell lymphomas (PTCL) are rare in children and adolescents, and data about outcome and treatment results are scarce. The present study is a joint, international, retrospective analysis of 143 reported cases of non-anaplastic PTCL in patients <19 years of age, with a focus on treatment and outcome features. One hundred forty-three patients, between 0.3 and 18.7 years old, diagnosed between 2000 and 2015 were included in the study. PTCL not otherwise specified was the largest subgroup, followed by extranodal NK/T cell lymphoma, hepatosplenic T cell lymphoma (HS TCL), and subcutaneous panniculitis-like T cell lymphoma (SP TCL). Probability of overall survival (pOS) at 5 years for the whole group was 0.56 ± 0.05, and probability of event-free survival was (pEFS) 0.45 ± 0.05. Patients with SP TCL had a good outcome with 5-year pOS of 0.78 ± 0.1 while patients with HS TCL were reported with 5-year pOS of only 0.13 ± 0.12. Twenty-five percent of the patients were reported to have a pre-existing condition, and this group had a dismal outcome with 5-year pOS of 0.29 ± 0.09. The distribution of non-anaplastic PTCL subtypes in pediatric and adolescent patients differs from what is reported in adult patients. Overall outcome depends on the subtype with some doing better than others. Pre-existing conditions are frequent and associated with poor outcomes. There is a clear need for subtype-based treatment recommendations for children and adolescents with PTCL.

Xin-deficient mice display myopathy, impaired contractility, attenuated muscle repair and altered satellite cell functionality

AIM

Xin is an F-actin-binding protein expressed during development of cardiac and skeletal muscle. We used Xin-/- mice to determine the impact of Xin deficiency on different aspects of skeletal muscle health, including functionality and regeneration.

METHODS

Xin-/- skeletal muscles and their satellite cell (SC) population were investigated for the presence of myopathic changes by a series of histological and immunofluorescent stains on resting uninjured muscles. To further understand the effect of Xin loss on muscle health and its SCs, we studied SCs responses following cardiotoxin-induced muscle injury. Functional data were determined using in situ muscle stimulation protocol.

RESULTS

Compared to age-matched wild-type (WT), Xin-/- muscles exhibited generalized myopathy and increased fatigability with a significantly decreased force recovery post-fatiguing contractions. Muscle regeneration was attenuated in Xin-/- mice. This impaired regeneration prompted an investigation into SC content and functionality. Although SC content was not different, significantly more activated SCs were present in Xin-/- vs. WT muscles. Primary Xin-/- myoblasts displayed significant reductions (approx. 50%) in proliferative capacity vs. WT; a finding corroborated by significantly decreased MyoD-positive nuclei in 3 days post-injury Xin-/- muscle vs. WT. As more activated SCs did not translate to more proliferating myoblasts, we investigated whether Xin-/- SCs displayed an exaggerated loss by apoptosis. More apoptotic SCs (TUNEL+/Pax7+) were present in Xin-/- muscle vs. WT. Furthermore, more Xin-/- myoblasts were expressing nuclear caspase-3 compared to WT at 3 days post-injury.

CONCLUSION

Xin deficiency leads to a myopathic condition characterized by increased muscle fatigability, impaired regeneration and SC dysfunction.

An in Situ Hrem Study of Crystal Nucleation in Amorphous Silicon thin Films

In Situ high resolution electron microscopy has proved to be a valuable tool in investigations involving interface reactions in a number of thin film systems. We have applied this technique to dynamically record nucleation and growth sequences during the amorphous (a-) to crystalline (c-) phase transformation in silicon thin films. Interpretation of the recordings has yielded a wealth of information on the mechanisms and to some extent the kinetics of solid phase crystallization. In our recordings, we have been able to capture the critical nucleus at the a-Si-SiO2 interface. Incorporating this into classical nucleation theory enables us to make an estimate of the a-Si-c-Si interfacial energy.

In Situ Hrem Observations of Crystallization in LPCVD Amorphous Silicon

This article describes the application of in situ, high-resolution electron microscopy to the study of crystal nucleation and growth in amorphous silicon. It is shown that dynamic events at elevated temperatures (e.g. 600-750°C) can be recorded at the atomic level by such an approach. It is anticipated that fundamental information, important for the technological development of polysilicon thin films, can be generated by work of this type.

Crystallization of Silicon Ion Implanted LPCVD Amorphous Silicon Films for High Performance Poly-TFT

The mechanism of silicon ion implantation on the crystallization kinetics and the resulting grain sizes of LPCVD α-Si films have been studied by x-ray diffraction and transmission electron microscopy. The solid-phase crystallization was proceeded by random nucleation and growth from the Si/SiO2 interface. The most effective grain size enhancement was found by targeting the peak concentration of implanted siliconbeyond the Si/SiO2 interface, such that the maximum kinetic energy transfer occurred at that interface. The average grain size increases from ∼0.16 μm to ∼2.0 μm by a Si + implantation at 92KeV and a dose of 2X1015 cm-2 for 0.1 μm silicon film. X-ray diffraction intensities were analyzed to optimize implanting dose, beam current and energy for different film thickness. Grain size enhancement was achieved by retarding the random nucleation and increasing the nucleation activation barrier from ∼3.9eV to ∼4.9 eV for the implanted sample. The amorphous to crystalline growth activation barrier of ∼3.2 eV was not altered by Si+ implantation. The observed nucleation and growth kinetics change may be due to the chemical effect of the recoiled oxygen atoms from the substrate. The field-effect mobilities for both n- and p-channel TFTs increase by a factor of two with deep silicon implant.

The possible role of ammonia in phototaxis of migrating slugs of Dictyostelium discoideum

Previously we showed that the rising cell masses of cellular slime molds orient away from high concentrations of ammonia gas, presumably by speeding up the cells on one side. Here we show that in the same way NH(3) could also be involved in the highly sensitive phototaxis found in the migrating slugs of Dictyostelium discoideum. We have evidence that light increases their speed of migration and their production of NH(3). Since unilateral light is concentrated on the distal side of a cell mass by the "lens effect," this leads to the obvious hypothesis that the light stimulates the local production of NH(3), which, in turn, stimulates the cells in the illuminated region to move faster.

A phase II study of intravenous (IV) and intraperitoneal (IP) paclitaxel (Tax), IP cisplatin (Cis), and IV bevacizumab (Bev) as first-line chemotherapy for optimal stage II or III ovarian, primary peritoneal, and fallopian tube cancer

5523

Background: IP Cis plus IV/IP Tax is a standard therapy for optimally debulked ovarian cancer. Bev is a recombinant humanized IgG1 monoclonal antibody directed against vascular endothelial growth factor (VEGF). Activity of Bev against recurrent ovarian cancer has been reported in phase II trials. In this study IP Cis and IV/IP Tax are combined with IV Bev as front-line therapy to assess safety and tolerability. Methods: Patients with optimal (<1 cm residual), FIGO stage II or III, epithelial ovarian, fallopian tube, or peritoneal cancer, acceptable organ function, and KPS = 70% are eligible. Patients receive 6 cycles of chemotherapy plus Bev (starting cycle 2): Tax 135 mg/m2 IV over 3 hours on Day 1, Cis 75 mg/m2 IP on Day 2, Tax 60 mg/m2 IP on Day 8, Bev 15 mg/kg IV on Day 1. Extended treatment with Bev is continued every 3 weeks for 17 treatments after chemotherapy is complete. The study will enroll 41 patients. The primary endpoint is safety and tolerability, determined by the proportion of patients who complete the prescribed 6 cycles of cytotoxic chemotherapy without discontinuation and without dose-limiting non-hematologic and non-electrolyte toxicity. A stopping rule will be applied if excessive toxicity is encountered. Results: To date, 8 women have been treated on the study. Median age: 53 (48–59). All 31 planned doses of chemotherapy have been administered in full. One dose of IP Tax was delayed for 2 days due to abdominal pain. One patient had her first dose of Bev delayed for 1 cycle due to surgical wound infection. There have been no toxicities > grade 3. Grade 1/2 toxicities include: fatigue (87.5%); nausea (50%); and hypomagnesemia (37.5%). Grade 3 toxicities per patient: fatigue (12.5%); hyponatremia (25%); hypokalemia (25%); hypertension (12.5%); abdominal pain (12.5%); and neutropenia (12.5%). Of the 5 patients with pretreatment CA125 >35 Units/mL, 4 normalized their value after 1 cycle of chemotherapy and 1 patient normalized after 2 cycles. Conclusions: Preliminary experience suggests that the combination of IV Bev with IP Cis plus IV/IP Tax may be well tolerated. Enrollment continues and updated results will be presented.

No significant financial relationships to disclose.

Klebsiella infection in patients with thalassemia

Klebsiella infection has previously been reported in a few patients with transfusion-dependent thalassemia. The incidence and clinical spectrum of this infection in our cohort of patients were reviewed retrospectively. Among 160 patients observed for 12 years, there were 15 episodes of Klebsiella infection that occurred in 12 patients (7.5%), resulting in an incidence of 0.78 infections per 100 patient-years. The clinical spectrum included sinusitis (4 cases), intracranial infection (5 cases), septicemia (4 cases), and abscesses of the liver, lung, kidney, and parotid gland (1 case each). Three patients had recurrent infections involving different sites, 2 (16%) died of fulminant septicemia, and 3 (25%) had significant permanent neurological deficits. The antibiotic susceptibility pattern for the isolates was similar to the pattern for isolates recovered in the community. With regard to predisposing factors, iron overload and liver function derangement were found to be significant on univariate analysis (P=.046 and P=.049, respectively) but insignificant on multivariate analysis. Klebsiella infection was a serious and frequently encountered complication in our patients with transfusion-dependent thalassemia, resulting in high mortality and morbidity rates.

Effects of chilling stress on allatal growth and juvenile hormone synthesis in the cockroach, Diploptera punctata

During the ovarian cycle of the cockroach, Diploptera punctata, a mitotic wave occurs in the corpora allata before an increase in gland volume and juvenile hormone (JH) synthesis. Previous studies have demonstrated that the brain inhibits mitosis and JH synthesis in corpus allatum (CA) cells until adult females have mated. Herein, we report that chilling stress effectively suppresses mating induced proliferation of CA cells. In mated females, chilling on melting ice for 0.5-3 hours caused a strong, dose-dependent decrease in mitotic activity. In insects chilled for 3 hours, although the mitotic wave in the CA was practically abolished, CA volume and JH synthesis finally reached peak levels typical of unchilled insects, despite a 2-day delay. Consequently, oocyte maturation and oviposition were also delayed by 2 days, yet in both chilled and unchilled insects, peak values of basal oocyte length were the same. By allowing virgin females to mate on different days after chilling, we found that the chilling effect could be retained in the insect body for at least 2 days. During this period, signals from mating could not effectively remove inhibition of CA cell proliferation. Unilaterally disconnecting the CA from the brain revealed that chilling stress mediated CA cell proliferation via the brain, and did not directly affect the CA.

Purification and characterization of 6xHis-tagged Bm3R1 repressor of Bacillus megaterium

The bm3R1 gene-encoded repressor controls the expression of cytochrome P450BM-3 gene as well as its own expression in Bacillus megaterium. We have developed an efficient system for overexpression and purification of Bm3R1 protein by nickel ion affinity chromatography. Adding six histidine residues at either N-terminus or C-terminus of Bm3R1 repressor caused no loss of its operator DNA binding ability. We have investigated the interaction between Bm3R1(His)6 and its operator DNA by equilibrium and kinetic methods. The results showed that the apparent dissociation constant is around 1.8 x 10(-9) M and the half-life of Bm3R1(His)6-operator complex is about 50 minutes. We have also found by gel filtration chromatography that Bm3R1(His)6 may exist in low oligomeric state(s) in solution, which is competent for binding to its operator DNA.

Discrete Polycomb-binding sites in each parasegmental domain of the bithorax complex

The Polycomb protein of Drosophila melanogaster maintains the segmental expression limits of the homeotic genes in the bithorax complex. Polycomb-binding sites within the bithorax complex were mapped by immunostaining of salivary gland polytene chromosomes. Polycomb bound to four DNA fragments, one in each of four successive parasegmental regulatory regions. These fragments correspond exactly to the ones that can maintain segmentally limited expression of a lacZ reporter gene. Thus, Polycomb acts directly on discrete multiple sites in bithorax regulatory DNA. Constructs combining fragments from different regulatory regions demonstrate that Polycomb-dependent maintenance elements can act on multiple pattern initiation elements, and that maintenance elements can work together. The cooperative action of maintenance elements may motivate the linear order of the bithorax complex.

Elements of the Drosophila bithorax complex that mediate repression by Polycomb group products

The Polycomb (Pc) group genes encode repressors that restrict expression of homeotic genes to precise domains along the anterior-posterior (A-P) axis. We describe germ-line transformation constructs, containing portions of the bxd, iab-2, or iab-3 regulatory regions of the bithorax complex (BX-C), that are controlled by Pc group products in embryos. There are multiple BX-C elements that mediate Pc group control, which we call Pc group response elements (PREs), and they can work when removed from the normal BX-C context. These constructs are each regulated by the genes Polycomb (Pc), extra sex combs (esc), Enhancer of zeste (E[z]), polyhomeotic (ph), Polycomb-like (Pcl), Sex comb on midleg (Scm), Posterior sex combs (Psc), and Additional sex combs (Asx), consistent with multiple Pc group products acting together. Depending upon context, a PRE from the iab-3 region can restrict expression in different A-P positions. Thus, PREs are not specialized for particular parasegments, suggesting that Pc group products do not directly specify A-P boundaries of homeotic expression. Instead, the results support the idea that Pc group products provide stable memory or imprinting of boundaries which are initially specified by gap and pair-rule regulators.

Energy requirements in response to high protein feeding in young male adults

Two experiments were conducted to investigate the effects of moderate (MPI) and high (HPI) levels of protein intake on energy utilization in 12 healthy young male adults who were confined to a metabolic unit for 56 days. The amount of energy supplied was adjusted so that the subjects could maintain a relatively constant body weight, which was used as the criterion to see whether the body was under an energy balance. All foods were supplied in conventional style. In the first experiment, six young men were given a diet of 1.18 g (MPI) and 1.74 g (HPI) protein/kg of body weight/day in consecutive periods. In the second experiment, another six young men were studied at a protein intake of 1.08 g (MPI) and 2.00 g (HPI)/kg/day. The results indicate that both energy intake and energy expenditure are independent of protein intake. The average gross energy intake of the subjects was between 2,300 to 3,240 kcal/day, corresponding to a light-to-moderate grade of energy expenditure. The efficiency of energy utilization did not differ significantly between the moderate and high levels of protein intake in adults.

Effects of excess protein intake on nitrogen utilization in young men

The efficiency of nitrogen (N) utilization was studied in 12 young male subjects. Protein intake levels were adjusted from moderate (1.08 and 1.18 g protein/kg/day) to high (1.74 and 2.00 g protein/kg/day). All of the food was supplied in the form of a normal mixed Chinese diet. Six subjects were admitted to a metabolic unit at a time for 56 days, in two consecutive periods. The results indicate that a higher protein intake causes more N excretion in urine and feces. Biologic value (BV) and net protein utilization (NPU) were markedly decreased during the high protein intake (HPI) period. However, a significant increase in the N balance was found in the presence of excessive protein intake. Digestibility of protein seemed to increase during the HPI period, with the apparent digestibility of the dietary protein being about 83% to 90%. We conclude from this study that excessive N intake reduces the efficiency of N utilization, but still results in a positive N balance in adult human subjects.

Random mutagenesis of Schizosaccharomyces pombe SRP RNA: lethal and conditional lesions cluster in presumptive protein binding sites

Signal recognition particle (SRP), a ribonucleoprotein composed of six polypeptides and one RNA subunit, serves as an adaptor between the cytoplasmic protein synthetic machinery and the translocation apparatus of the endoplasmic reticulum. To begin constructing a functional map of the 7SL RNA component of SRP, we extensively mutagenized the Schizosaccharomyces pombe SRP7 gene. Phenotypes are reported for fifty-two mutant alleles derived from random point mutagenesis, seven alleles created by site-directed mutagenesis to introduce restriction sites into the SRP7 gene, nine alleles designed to pinpoint conditional lesions, and three alleles with extra nucleotides inserted at position 84. Our data indicate that virtually all single nucleotide changes as well as many multiple substitutions in this highly structured RNA are phenotypically silent. Six lethal alleles and eleven which result in sensitivity to the combination of high temperature and elevated osmotic strength were identified. These mutations cluster in conserved regions which, in the mammalian RNA, are protected from nucleolytic agents by SRP proteins. The effects of mutations in the presumptive binding site for a fission yeast SRP 9/14 homolog indicate that both the identity of a conserved residue and the secondary structure within which it is embedded are functionally important. The phenotypes of mutations in Domain IV suggest particular residues as base-specific contacts for the fission yeast SRP54 protein. A single allele which confers temperature-sensitivity in the absence of osmotic perturbants was identified in this study; the growth properties of the mutant strain suggest that the encoded RNA is somewhat defective even at the permissive temperature, and is most likely unable to correctly assemble with SRP proteins at the nonpermissive temperature.

Ten different Polycomb group genes are required for spatial control of the abdA and AbdB homeotic products

Mutations in genes of the Polycomb (Pc) group cause abnormal segmental development due to ectopic expression of the homeotic products of the Antennapedia and bithorax complexes. Here the requirements for Pc group genes in controlling the abdA and AbdB products of the bithorax complex are described. Embryos containing mutations in the genes Polycomb (Pc), extra sex combs (esc), Enhancer of zeste [E(z)], polyhomeotic (ph), Sex comb on midleg (Scm), Polycomb-like (Pcl), Sex comb extra (Sce), Additional sex combs (Asx), Posterior sex combs (Psc) and pleiohomeotic (pho) were examined. In every case, both abdA and AbdB are expressed outside of their normal domains along the anterior-posterior (A-P) axis, consistent with these Pc group products acting in a single pathway or molecular complex. The earliest detectable ectopic expression is highest in the parasegments immediately adjacent to the normal expression boundary. Surprisingly, in the most severe Pc group mutants, the earliest ectopic AbdB is distributed in a pair-rule pattern. At all stages, ectopic abdA in the epidermis is highest along the anterior edges of the parasegments, in a pattern that mimics the normal abdA cell-specific pattern. These examples of highly patterned mis-expression show that Pc group mutations do not cause indiscriminate activation of homeotic products. We suggest that the ectopic expression patterns result from factors that normally activate abdA and AbdB only in certain parasegments, but that in Pc group mutants these factors gain access to regulatory DNA in all parasegments.

Mutations in SPT16/CDC68 suppress cis- and trans-acting mutations that affect promoter function in Saccharomyces cerevisiae

SPT16 was previously identified as a high-copy-number suppressor of delta insertion mutations in the 5' regions of the HIS4 and LYS2 genes of Saccharomyces cerevisiae. We have constructed null mutations in the SPT16 gene and have demonstrated that it is essential for growth. Temperature-sensitive-lethality spt16 alleles have been isolated and shown to be pleiotropic; at a temperature permissive for growth, spt16 mutations suppress delta insertion mutations, a deletion of the SUC2 upstream activating sequence, and mutations in trans-acting genes required for both SUC2 and Ty expression. In addition, SPT16 is identical to CDC68, a gene previously shown to be required for passage through the cell cycle control point START. However, at least some transcriptional effects caused by spt16 mutations are independent of arrest at START. These results and those in the accompanying paper (A. Rowley, R. A. Singer, and G. C. Johnston, Mol. Cell. Biol. 11:5718-5726, 1991) indicate that SPT16/CDC68 is required for normal transcription of many loci in S. cerevisiae.

A yeast gene important for protein assembly into the endoplasmic reticulum and the nucleus has homology to DnaJ, an Escherichia coli heat shock protein

When nuclear localization sequences (termed NLS) are placed at the N terminus of cytochrome c1, a mitochondrial inner membrane protein, the resulting hybrid proteins do not assemble into mitochondria when synthesized in the yeast Saccharomyces cerevisiae. Cells lacking mitochondrial cytochrome c1, but expressing the hybrid NLS-cytochrome c1 proteins, are unable to grow on glycerol since the hybrid proteins are associated primarily with the nucleus. A similar hybrid protein with a mutant NLS is transported to and assembled into the mitochondria. To identify proteins that might be involved in recognition of nuclear localization signals, we isolated conditional-lethal mutants (npl, for nuclear protein localization) that missorted NLS-cytochrome c1 to the mitochondria, allowing growth on glycerol. The gene corresponding to one complementation group (NPL1) encodes a protein with homology to DnaJ, an Escherichia coli heat shock protein. npl1-1 is allelic to sec63, a gene that affects transit of nascent secretory proteins across the endoplasmic reticulum. Rothblatt, J. A., R. J. Deshaies, S. L. Sanders, G. Daum, and R. Schekman. 1989. J. Cell Biol. 109:2641-2652. The npl1 mutants reported here also weakly affect translocation of preprocarboxypeptidaseY across the ER membrane. A normally nuclear hybrid protein containing a NLS fused to invertase and a nucleolar protein are not localized to the nucleus in npl1/sec63 cells at the nonpermissive temperature. Thus, NPL1/SEC63 may act at a very early common step in localization of proteins to the nucleus and the ER. Alternatively, by affecting ER and nuclear envelope assembly, npl1 may indirectly alter assembly of proteins into the nucleus.

Mutations that alter both localization and production of a yeast nuclear protein

The first 74 amino acids of the yeast GAL4 gene product are sufficient to localize a GAL4-beta-galactosidase chimeric protein to the yeast nucleus. Chimeric proteins missing the first 74 GAL4 amino acids, but containing almost all of the rest of GAL4, are not localized to the nucleus and are expressed at higher levels than their nuclear counterparts. On this basis, point mutations within GAL4, which reduce nuclear localization and increase production of a normally nuclear GAL4-beta-galactosidase fusion protein, were isolated and sequenced. The effect of these mutations on the localization and expression of the intact GAL4 protein was examined. The degree to which the mutant proteins are excluded from the nucleus varies, but all mutations cause overproduction of the protein. Point mutations altering two of the six cysteine residues of the GAL4 putative 'zinc finger' abolish gene activation by intact GAL4; however, mutations in nearby residues have no effect on GAL4-dependent gene activation.