Environmental surveillance for COVID-19 using SARS-CoV-2 RNA concentration in wastewater - a study in District East, Karachi, Pakistan

Background

Wastewater-based surveillance is used to track the temporal patterns of the SARS-CoV-2 virus in communities. Viral RNA particle detection in wastewater samples can indicate an outbreak within a catchment area. We describe the feasibility of using a sewage network to monitor SARS-CoV-2 trend and use of genomic sequencing to describe the viral variant abundance in an urban district in Karachi, Pakistan. This was among the first studies from Pakistan to demonstrate the surveillance for SARS-CoV-2 from a semi-formal sewage system.

Methods

Four sites draining into the Lyari River in District East, Karachi, were identified and included in the current study. Raw sewage samples were collected early morning twice weekly from each site between June 10, 2021 and January 17, 2022, using Bag Mediated Filtration System (BMFS). Secondary concentration of filtered samples was achieved by ultracentrifugation and skim milk flocculation. SARS-CoV-2 RNA concentrations in the samples were estimated using PCR (Qiagen ProMega kits for N1 & N2 genes). A distributed-lag negative binomial regression model within a hierarchical Bayesian framework was used to describe the relationship between wastewater RNA concentration and COVID-19 cases from the catchment area. Genomic sequencing was performed using Illumina iSeq100.

Findings

Among the 151 raw sewage samples included in the study, 123 samples (81.5%) tested positive for N1 or N2 genes. The average SARS-CoV-2 RNA concentrations in the sewage samples at each lag (1-14 days prior) were associated with the cases reported for the respective days, with a peak association observed on lag day 10 (RR: 1.15; 95% Credible Interval: 1.10-1.21). Genomic sequencing showed that the delta variant dominated till September 2022, while the omicron variant was identified in November 2022.

Interpretation

Wastewater-based surveillance, together with genomic sequencing provides valuable information for monitoring the community temporal trend of SARS-CoV-2.

Funding

PATH, Bill & Melinda Gates Foundation, and Global Innovation Fund.

Carbapenem-resistant Enterobacteriaceae in sink drains of 40 healthcare facilities in Sindh, Pakistan: A cross-sectional study

In Pakistan, antimicrobial resistance (AMR) is expected to greatly increase the already high mortality and morbidity rates attributed to infections, making AMR surveillance and prevention a priority in the country. The aims of the project were to characterize the prevalence of carbapenem-resistant Enterobacteriaceae (CRE) in healthcare facility sink drains in Pakistan and to characterize how physical characteristics of sinks and healthcare facility rooms were associated with CRE in those sinks. The study took place in 40 healthcare facilities in Jamshoro Pakistan. Swabs were collected from sink drains in each facility that had a sink, and structured observations of sinks and facilities were performed at each facility. Swabs were plated on CHROMagar KPC to screen for carbapenem-resistant Enterobacteriaceae, which were then isolated on Mueller-Hinton agar plates. Antibiotic susceptibility was determined using the disk diffusion method to assess resistance to carbapenems, cephalosporins, and fluoroquinolones. Thirty-seven of the healthcare facilities had at least one sink, and thirty-nine total sinks were present and sampled from those healthcare facilities. Sinks in these facilities varied in quality; at the time of sampling 68% had water available, 51% had soap/alcohol cleanser at the sink, 28% appeared clean, and 64% drained completely. Twenty-five (64%) of the sink samples grew Enterobacteriaceae on CHROMagar KPC, sixteen (41%) of which were clinically non-susceptible to ertapenem. Seven of the 39 sampled sinks (18%) produced Enterobacteriaceae that were resistant to all three antibiotic classes tested. Several facilities and sink characteristics were associated with CRE. Sinks and drains can serve as undetected reservoirs for carbapenem-resistant Enterobacteriaceae. Control and remediation of such environments will require both systemic strategies and physical improvements to clinical environments.

Differences in chlorine and peracetic acid disinfection kinetics of Enterococcus faecalis and Escherichia fergusonii and their susceptible strains based on gene expressions and genomics

This study was conducted to investigate mechanisms of cross-resistance to chlorine and peracetic acid (PAA) disinfectants by antibiotic-resistant bacteria. Our study evaluated chlorine and PAA based disinfection kinetics of erythromycin-resistant Enterococcus faecalis, meropenem-resistant Escherichia fergusonii, and susceptible strains of these species. Using the integrated second-order disinfectant decay model and first-order Chick-Watson's Law, it was found that the meropenem-resistant Escherichia fergusonii strain showed significantly less log inactivation compared to the susceptible E. fergusonii strain in response to both chlorine and PAA disinfection (p-value = 0.059, 3.5 × 10^-6). On the other hand, the susceptible Enterococcus faecalis strain showed similar log inactivation compared to the erythromycin-resistant strain in response to either treatment (p-value = 0.075, 0.28). Meropenem-resistant E. fergusonii showed an increase in gene expression of New Delhi metallo-β-lactamase (bla_NDM-1) gene to chlorine, but there was no increase in expression to PAA. Whole genome sequencing (WGS) was then conducted to elucidate the differences in genes among both resistant and susceptible table E. fergusonii strains. The average nucleotide identity (ANI) analysis of the draft genomes (>97% similarity) suggests that meropenem-resistant E. fergusonii (S1) and meropenem-susceptible E. fergusonii (S2) are the same species but different strains. Both strains have the same genes for oxidative stress pathways, oxidative scavenger genes, and nearly 40 different antibiotic efflux pump genes. The chromosomal and plasmid draft genomes of meropenem-resistant and susceptible E. fergusonii strains each have 65 and 52 antibiotic resistance genes, respectively. Of these, the resistant E. fergusonii strain harbored the extended-spectrum beta-lactamases bla_CTX-M-15 and bla_TEM-1 genes located on the chromosome, and a bla_TEM-1 gene on the plasmid. The overall findings of this study are significant, as they reveal that antibiotic-resistant and susceptible strains of E. fergusonii exhibit different responses towards chlorine and PAA disinfection.

Microbiome and Antimicrobial Resistance Gene Dynamics in International Travelers

We used metagenomic next-generation sequencing to longitudinally assess the gut microbiota and antimicrobial resistomes of international travelers to clarify global exchange of resistant organisms. Travel resulted in an increase in antimicrobial resistance genes and a greater proportion of Escherichia species within gut microbial communities without impacting diversity.

511. MDRO Carriage in Patients in Two ICUs and Prevalence of Environmental Surface and Healthcare Worker Hand Contamination

Background

Determining MDRO (multidrug-resistant organism) transmission routes in intensive care units (ICUs) can be complex and require the evaluation of multiple potential MDRO sources, including patients, the environment, and healthcare worker (HCW) hands. The objective of this study was to determine MDRO carriage in patients in two separate ICUs, and simultaneous environmental and HCW hand contamination from associated rooms.

Methods

Patient (P), environmental (E), and HCW hand (H) samples were collected from hospital A (1183 H, 1253 E, 729 P) and hospital B (699 H, 1372 E, 437 P) over approximately 5 weeks in each unit. Environmental and HCW hand samples were collected using a cellulose sponge. HCW hand samples were collected prior to any hand hygiene. Patient samples were collected from the axilla, groin, and perianal areas with a flocked swab with patient consent. All samples were tested semi-quantitatively for Clostridium difficile (Cdiff), vancomycin-resistant enterococci (VRE), and cefotaxime-resistant Enterobacteriaceae (Cef-R-Ent) by selective culture. Cdiff isolates representative of each P/E/H cluster were tested for Cdiff toxin testing by PCR.

Results

Cdiff, VRE, and Cef-R-Ent were detected in patients, patient rooms, and on HCW hands in both facilities (Table 1). Cdiff was more prevalent in Facility A, while Cef-R-Ent was more prevalent in Facility B. The prevalence of VRE was minimal in both facilities. Cdiff toxin gene testing revealed that 17% of the Cdiff isolate clusters tested positive for toxin genes. In Facility A, the prevalence of a given MDRO was similar regardless of sample type, but was more widely varied between sample types in Facility B. Prevalence of MDROs on HCW hands and in the environment was typically higher in Facility A compared with Facility B. Individual patient positives were frequently linked to positive HCW hand and environmental cultures.

Conclusion

We discovered a low prevalence of all MDROs in both facilities, with most positive cultures associated with patients who were not on MDRO precautions. HCW hand and environmental MDRO prevalence was generally similar for each MDRO, regardless of patient prevalence, supporting previously reported links on HCW hand contamination and hospital room surfaces.

Disclosures

All authors: No reported disclosures.

Vital Signs: Estimated Effects of a Coordinated Approach for Action to Reduce Antibiotic-Resistant Infections in Health Care Facilities - United States

BACKGROUND

Treatments for health care-associated infections (HAIs) caused by antibiotic-resistant bacteria and Clostridium difficile are limited, and some patients have developed untreatable infections. Evidence-supported interventions are available, but coordinated approaches to interrupt the spread of HAIs could have a greater impact on reversing the increasing incidence of these infections than independent facility-based program efforts.

METHODS

Data from CDC's National Healthcare Safety Network and Emerging Infections Program were analyzed to project the number of health care-associated infections from antibiotic-resistant bacteria or C. difficile both with and without a large scale national intervention that would include interrupting transmission and improved antibiotic stewardship. As an example, the impact of reducing transmission of one antibiotic-resistant infection (carbapenem-resistant Enterobacteriaceae [CRE]) on cumulative prevalence and number of HAI transmission events within interconnected groups of health care facilities was modeled using two distinct approaches, a large scale and a smaller scale health care network.

RESULTS

Immediate nationwide infection control and antibiotic stewardship interventions, over 5 years, could avert an estimated 619,000 HAIs resulting from CRE, multidrug-resistant Pseudomonas aeruginosa, invasive methicillin-resistant Staphylococcus aureus (MRSA), or C. difficile. Compared with independent efforts, a coordinated response to prevent CRE spread across a group of inter-connected health care facilities resulted in a cumulative 74% reduction in acquisitions over 5 years in a 10-facility network model, and 55% reduction over 15 years in a 102-facility network model.

CONCLUSIONS

With effective action now, more than half a million antibiotic-resistant health care-associated infections could be prevented over 5 years. Models representing both large and small groups of interconnected health care facilities illustrate that a coordinated approach to interrupting transmission is more effective than historical independent facilitybased efforts.

IMPLICATIONS FOR PUBLIC HEALTH

Public health-led coordinated prevention approaches have the potential to more completely address the emergence and dissemination of these antibiotic-resistant organisms and C. difficile than independent facility-based efforts.

Does universal active MRSA surveillance influence anti-MRSA antibiotic use? A retrospective analysis of the treatment of patients admitted with suspicion of infection at Veterans Affairs Medical Centers between 2005 and 2010

OBJECTIVES

After the implementation of an active surveillance programme for MRSA in US Veterans Affairs (VA) Medical Centers, there was an increase in vancomycin use. We investigated whether positive MRSA admission surveillance tests were associated with MRSA-positive clinical admission cultures and whether the availability of surveillance tests influenced prescribers' ability to match initial anti-MRSA antibiotic use with anticipated MRSA results from clinical admission cultures.

METHODS

Analyses were based on barcode medication administration data, microbiology data and laboratory data from 129 hospitals between January 2005 and September 2010. Hospitalized patient admissions were included if clinical cultures were obtained and antibiotics started within 2 days of admission. Mixed-effects logistic regression was used to examine associations between positive MRSA admission cultures and (i) admission MRSA surveillance test results and (ii) initial anti-MRSA therapy.

RESULTS

Among 569,815 included admissions, positive MRSA surveillance tests were strong predictors of MRSA-positive admission cultures (OR 8.5; 95% CI 8.2-8.8). The negative predictive value of MRSA surveillance tests was 97.6% (95% CI 97.5%-97.6%). The diagnostic OR between initial anti-MRSA antibiotics and MRSA-positive admission cultures was 3.2 (95% CI 3.1-3.4) for patients without surveillance tests and was not significantly different for admissions with surveillance tests.

CONCLUSIONS

The availability of nasal MRSA surveillance tests in VA hospitals did not seem to improve the ability of prescribers to predict the necessity of initial anti-MRSA treatment despite the high negative predictive value of MRSA surveillance tests. Prospective trials are needed to establish the safety and effectiveness of using MRSA surveillance tests to guide antibiotic therapy.

Plants and fungi in the era of heterogeneous plasma membranes

Examples from yeast and plant cells are described that show that their plasma membrane is laterally compartmented. Distinct lateral domains encompassing both specific lipids and integral proteins coexist within the plane of the plasma membrane. The compartments are either spatially stable and include distinct sets of proteins, or they are transiently formed to accomplish diverse functions. They are not related to lipid rafts or their clusters, as defined for mammalian cells. This review summarises only well-documented compartments of plasma membranes from plants and fungi, which have been recognised using microscopic approaches. In several cases, physiological functions of the membrane compartmentation are revealed.

Synthesis of an O-glycosylated cell surface protein induced in yeast by alpha factor

A number of cell surface glycoproteins can be specifically and completely released from intact cells of Saccharomyces cerevisiae with 0.5% mercaptoethanol. Among these proteins is one with a molecular mass of 22 kDa, which is synthesized only in haploid a cells treated with the peptide mating pheromone alpha factor. This protein could be radiolabeled in vivo with [2-(3)H]mannose, [(14)C]phenylalanine, and [(35)S]sulfate. Its synthesis and export to the cell surface were not inhibited by tunicamycin. beta-Elimination released almost all radioactivity from the [2-(3)H]mannose-labeled protein, 36% of its radioactivity being recovered subsequently as mannose and 43% as a dimannoside. Evidence is presented that the 22-kDa O-glycosylated protein is a mating-type specific a cell agglutinin.

Specific lipid requirements of membrane proteins--a putative bottleneck in heterologous expression

Membrane proteins are mostly protein-lipid complexes. For more than 30 examples of membrane proteins from prokaryotes, yeast, plant and mammals, the importance of phospholipids and sterols for optimal activity is documented. All crystallized membrane protein complexes show defined lipid-protein contacts. In addition, lipid requirements may also be transitory and necessary only for correct folding and intercellular transport. With respect to specific lipid requirements of membrane proteins, the phospholipid and glycolipid as well as the sterol content of the host cell chosen for heterologous expression should be carefully considered. The lipid composition of bacteria, archaea, yeasts, insects,Xenopus oocytes, and typical plant and mammalian cells are given in this review. A few examples of heterologous expression of membrane proteins, where problems of specific lipid requirements have been noticed or should be thought of, have been chosen.

Transpiration, a prerequisite for long-distance transport of minerals in plants?

The major "benefit" alleged to accrue from transpiration (the evaporative loss of water from plant surfaces) is that it is essential for the long-distance transport of mineral ions, but the possible interrelation between these two processes has rarely been tested. Transpiration was experimentally dissociated from mineral supply by growing sunflowers (Helianthus anuus) in hydroculture and providing mineral nutrients only during the nights. These plants grew as well as a control group that received nutrients only during the day and transpired 12-15 times more water during the exposure period. It thus appears that convective water transport in the xylem, brought about by root pressure and the resultant guttation, "growth water," and Münch's phloem counterflow is in itself sufficient for long-distance mineral supply and that transpiration is not required for this function.

The Chlorella hexose/H(+)-symporters

The physiology, molecular biology, and biochemistry of the inducible hexose uptake protein of Chlorella kessleri is reviewed. The protein encoded by the HUP1 gene is the most intensively studied membrane transporter of plants. Responsible for substrate accumulation up to 1500-fold, it translocates one proton together with one hexose, and the cell invests 1 ATP per sugar transported. Kinetics suggest that substrate accumulation is mainly brought about by a large delta Km (Kminside >> Kmoutside). The HUP1 protein (534aa) consists of 12 transmembrane helices of which at least helices I, V, VII, and XI interact with the sugar during translocation and participate in lining the transport path through the membrane. The helix packing might very well be identical to the one suggested for the E. coli lac permease, although the mechanism for transport and proton coupling that has been suggested for lac permease (Kaback, 1997) certainly does not hold for the Chlorella symporter; both are distantly related members, however, of the MFS-family of transporters. HUP1 has been functionally expressed in Schizosaccharomyces pombe, Saccharomyces cerevisiae, Escherichia coli, Volvox carteri, and in Xenopus oocytes.

Construction of phosphatidylethanolamine-less strain of Saccharomyces cerevisiae. Effect on amino acid transport

A triple yeast mutant was constructed which lacks BST1, the gene for sphingosine lyase, besides the phosphatidylserine decarboxylases PSD1 and PSD2. In this yeast mutant, which can only be grown in the presence of exogenous ethanolamine, phosphatidylethanolamine can be depleted to very low levels. Under those conditions, respiration as well as glucose and 3-O-methylglucose uptake proceed unaffected. Plasma membrane ATPase is as active in these cells as that of control cells grown in the presence of ethanolamine. Drastically decreased, however, are H+/amino acid symporters. The activities of arginine (Can1p), proline (Put4p) and general amino acid permease (Gap1p) are decreased more than 20-fold. Amino acid transport in yeast is dependent on coupling to the proton motive force. It can be envisaged that phosphatidylethanolamine might play a role in this process or in the early steps of the secretion pathway common for all amino acid permeases or, eventually, it could affect the transport proteins directly at the plasma membrane Transformation of the triple mutant with a CEN plasmid harbouring BST1 wild-type gene totally reversed its phenotype to that observed in the double mutant.

The C-terminal tetrapeptide HWFW of the Chlorella HUP1 hexose/H(+)-symporter is essential for full activity and an alpha-helical structure of the C-terminus

C-terminal tails of plant hexose/H(+)-symporters of the major facilitator superfamily contain a highly conserved motif of four amino acids: HWFW. A deletion of these four amino acids in the Chlorella HUP1 protein leads to a decrease in transport activity by a factor of 3-4. The mutated tail is highly sensitive to trypsin; it does not show alpha-helical conformation in contrast to the wild type C-terminal peptide with an alpha-helical content of at least 15%. The production of monoclonal antibody 416B8 recognizing an epitope within the central loop of HUP1 protein has been a prerequisite for the experiments described.

Properties of a reconstituted eukaryotic hexose/proton symporter solubilized by structurally related non-ionic detergents: specific requirement of phosphatidylcholine for permease stability

Overexpression of the hexose/proton symporter HUP1 from Chlorella kessleri in S. cerevisiae permits a one-step purification via a biotinylation domain. Milligram amounts of the protein are obtained starting from 2 l of yeast culture. The HUP1 protein is used as a model eukaryotic membrane protein of the 'major facilitator superfamily' (MFS) to study specific lipid requirements for activity and stability. Testing two series of detergents revealed that n-nonyl-beta-D-glucoside (NG) and n-octyl-beta-D-glucoside (OG) solubilize the HUP1 protein efficiently. Only the use of NG resulted in long-term stabilization of the HUP1 protein in the absence of external lipids. When affinity purified protein was extracted with organic solvents, a stoichiometric amount of phosphatidyl choline, phosphatidyl ethanolamine and ergosterol in the ratio of close to 2:1 was detected. These lipids were only observed, however, when the protein purification was carried out in the presence of NG; no lipids were copurified with the HUP1 protein in the presence of OG. Of the three lipids copurified, phosphatidyl choline showed a crucial role in ensuring maximal HUP1 permease activity and stability when added back to the OG-protein. The requirement of phosphatidylcholine documents a specific effect of lipids on vectorial transport mediated by a eukaryotic membrane protein of the MFS family.

Role of NaOH-extractable cell wall proteins Ccw5p, Ccw6p, Ccw7p and Ccw8p (members of the Pir protein family) in stability of the Saccharomyces cerevisiae cell wall

The Saccharomyces cerevisiae cell wall contains more than 20 identified mannoproteins. Some of them can be released from the wall by hot SDS/mercaptoethanol treatment and are, therefore, considered as disulphide-linked or non-covalently attached to wall structural components. A number of covalently linked cell wall proteins are released after SDS extraction. They can be divided into these extractable by glucanases and those which can be released with 30 mM NaOH. The SDS-extractable proteins either possess enzymatic activities or are homologues of enzymes, mainly glucanases. Nothing is known, however, about the function of covalently linked proteins. In order to investigate the role of NaOH-extractable cell wall proteins, genes encoding all four identified members of this family of Pir proteins, CCW5, CCW6, CCW7 and CCW8, were disrupted and the phenotype of the mutants obtained was examined. They grew somewhat more slowly, were larger and irregularly shaped, and showed pronounced susceptibility to cell wall synthesis inhibitors like Calcofluor white and Congo red. In addition, the triple and the quadruple deletants had a decreased mating ability. All these properties were more obvious the more of these genes were disrupted, indicating that probably all members of this protein family are at least functionally equivalent in the cell wall.

O-Glycosylation of Axl2/Bud10p by Pmt4p is required for its stability, localization, and function in daughter cells

Cells of the yeast Saccharomyces cerevisiae choose bud sites in a manner that is dependent upon cell type: a and alpha cells select axial sites; a/alpha cells utilize bipolar sites. Mutants specifically defective in axial budding were isolated from an alpha strain using pseudohyphal growth as an assay. We found that a and alpha mutants defective in the previously identified PMT4 gene exhibit unipolar, rather than axial budding: mother cells choose axial bud sites, but daughter cells do not. PMT4 encodes a protein mannosyl transferase (pmt) required for O-linked glycosylation of some secretory and cell surface proteins (Immervoll, T., M. Gentzsch, and W. Tanner. 1995. Yeast. 11:1345-1351). We demonstrate that Axl2/Bud10p, which is required for the axial budding pattern, is an O-linked glycoprotein and is incompletely glycosylated, unstable, and mislocalized in cells lacking PMT4. Overexpression of AXL2 can partially restore proper bud-site selection to pmt4 mutants. These data indicate that Axl2/Bud10p is glycosylated by Pmt4p and that O-linked glycosylation increases Axl2/ Bud10p activity in daughter cells, apparently by enhancing its stability and promoting its localization to the plasma membrane.

Expression of eukaryotic plasma membrane transporter HUP1 from Chlorella kessleri in Escherichia coli

To study the effect of sterols on the activity of the eukaryotic plasma membrane transporter, the hexose-proton symporter HUP1 from the unicellular alga Chlorella kessleri was expressed in Escherichia coli, a prokaryotic microorganism containing virtually no sterols. Under certain conditions, the recombinant protein was partially active in this prokaryotic organism. The heterologously produced HUP1p was purified from membrane fractions of E. coli and reconstituted in an in vitro system. The presence of ergosterol during solubilization, purification and reconstitution resulted in an increased activity of the reconstituted protein. Its activity, however, was 5-6 times lower as compared to the activity of HUP1p produced in Saccharomyces cerevisiae membranes and solubilized, purified, and reconstituted under the same conditions as above.

Deletion of new covalently linked cell wall glycoproteins alters the electrophoretic mobility of phosphorylated wall components of Saccharomyces cerevisiae

The incorporation of radioactive orthophosphate into the cell walls of Saccharomyces cerevisiae was studied. 33P-labeled cell walls were extensively extracted with hot sodium dodecyl sulfate (SDS). Of the remaining insoluble radioactivity more than 90% could be released by laminarinase. This radioactive material stayed in the stacking gel during SDS-polyacrylamide gel electrophoresis but entered the separating gel upon treatment with N-glycosidase F, indicating that phosphate was linked directly or indirectly to N-mannosylated glycoproteins. The phosphate was bound to covalently linked cell wall proteins as mannose-6-phosphate, the same type of linkage shown previously for soluble mannoproteins (L. Ballou, L. M. Hernandez, E. Alvarado, and C. E. Ballou, Proc. Natl. Acad. Sci. USA 87:3368-3372, 1990). From the phosphate-labeled glycoprotein fraction released by laminarinase, three cell wall mannoproteins, Ccw12p, Ccw13p and Ccw14p, were isolated and identified by N-terminal sequencing. For Ccw13p (encoded by DAN1 [also called TIR3]) and Ccw12p the association with the cell wall has not been described before; Ccw14p is identical with cell wall protein Icwp (I. Moukadiri, J. Armero, A. Abad, R. Sentandreu, and J. Zueco, J. Bacteriol. 179:2154-2162, 1997). In ccw12, ccw13, or ccw14 single or double mutants neither the amount of radioactive phosphate incorporated into cell wall proteins nor its position in the stacking gel was changed. However, the triple mutant brought about a shift of the 33P-labeled glycoprotein components from the stacking gel into the separating gel. The disruption of CCW12 results in a pronounced sensitivity of the cells to calcofluor white and Congo red. In addition, the ccw12 mutant shows a decrease in mating efficiency and a defect in agglutination.

Protein O-mannosylation

Protein O-mannosylation, originally observed in fungi, starts at the endoplasmic reticulum with the transfer of mannose from dolichyl activated mannose to seryl or threonyl residues of secretory proteins. This reaction is catalyzed by a family of protein O-mannosyltransferases (PMTs), which were first characterized in Saccharomyces cerevisiae. The identification of this evolutionarily conserved PMT gene family has led to the finding that protein O-mannosylation plays an essential role in a number of physiologically important processes. Focusing on the PMT gene family, we discuss here the main aspects of the biogenesis of O-linked carbohydrate chains in S. cerevisiae, Candida albicans, and other fungi. We summarize recent work utilizing pmt mutants that demonstrates the impact of protein O-mannosylation on protein secretion, on maintenance of cell wall integrity, and on budding. Further, the occurrence of PMT orthologs in higher eukaryotes such as Arabidopsis, Drosophila and mammals is reported and discussed.

New potential cell wall glucanases of Saccharomyces cerevisiae and their involvement in mating

Biotinylation of intact Saccharomyces cerevisiae cells with a nonpermeant reagent (Sulfo-NHS-LC-Biotin) allowed the identification of seven cell wall proteins that were released from intact cells by dithiothreitol (DTT). By N-terminal sequencing, three of these proteins were identified as the known proteins beta-exoglucanase 1 (Exg1p), beta-endoglucanase (Bgl2p), and chitinase (Cts1p). One protein was related to the PIR protein family, whereas the remaining three (Scw3p, Scw4p, and Scw10p [for soluble cell wall proteins]) were found to be related to glucanases. Single knockouts of these three potential glucanases did not result in dramatic phenotypes. The double knockout of SCW4 and the homologous gene SCW10 resulted in slower growth, significantly increased release of proteins from intact cells by DTT, and highly decreased mating efficiency when these two genes were disrupted in both mating types. The synergistic behavior of the disruption of SCW4 and SCW10 was partly antagonized by the disruption of BGL2. The data are discussed in terms of a possible counterplay of transglucosidase and glucosidase activities.

[Human cloning. The biological fundamentals and an ethical-legal assessment]

Following a description of the cloning process and how this might be used in humans, the authors examine the possibility of human cloning in the light of recognised ethical principles. They also address the question of whether current national and international laws are sufficient to prevent such practices.

Alteration of substrate affinities and specificities of the Chlorella Hexose/H+ symporters by mutations and construction of chimeras

The cDNAs HUP1 and HUP2 of Chlorella kessleri code for monosaccharide/H+ symporters that can be functionally expressed in Schizosaccharomyces pombe. By random mutagenesis three HUP1 mutants with an increased Km value for D-glucose were isolated. The 40-fold increase in Km of the first mutant is due to the amino acid exchange N436I in putative transmembrane helix XI. Two substitutions were found in a second (G97C/I303N) and third mutant (G120D/F292L), which show a 270-fold and 50-fold increase in Km for D-glucose, respectively. An investigation of the individual mutations revealed that the substitutions I303N and F292L (both in helix VII) cause the Km shifts seen in the corresponding double mutants. These mutations together with those previously found support the hypothesis that helices V, VII, and XI participate in the transmembrane sugar pathway. Whereas for most mutants obtained so far the Km change for D-glucose is paralleled by a corresponding change for other hexoses tested, the exchange D44E exclusively alters the Km for D-glucose. Moreover the pH profile of this mutant is shifted by more than 2 pH units to alkaline values, indicating that the activity of the transporter may require deprotonation of the corresponding carboxyl group. Chimeric transporters were constructed to study the 100-fold lower affinity for D-galactose of the HUP1 symporter as compared with that of the HUP2 protein. A crucial determinant for the differential D-galactose recognition was shown to be associated with the first external loop. The effect could be pinpointed to a single amino acid change: replacement of Asn-45 of HUP1 with isoleucine, the corresponding amino acid of HUP2, yields a transporter with a 20 times higher affinity for D-galactose. The reverse substitution (I47N) decreases the affinity of HUP2 for D-galactose 20-fold.

Post-translational fate of CAN1 permease of Saccharomyces cerevisiae

To study the post-translational fate of arginine permease (Can1p), the gene coding for this transport protein was placed behind a constitutive promoter of plasma membrane ATPase (PMA1) and furnished with a Myc tag. In exponential-phase cells the amount of Can1p is constant, although turnover can be demonstrated. A rapid decrease in transport activity during the early stationary phase is paralleled by a corresponding net degradation of the protein. The amount of Can1p present in exponential cells grown on various nitrogen sources is the same, except in arginine-grown cells, in which the amount of the protein is markedly lower. This occurs solely when arginine serves as nitrogen source but not as an immediate consequence of, for example, arginine addition to cells growing on other nitrogen sources. it was demonstrated that Can1p is phosphorylated. Since Can1p expression under the PMA1 promoter is glucose-dependent, the amount of the permease expressed in high-glucose-grown cells is higher than in low-glucose-grown ones. Only a part of the Can1p overexpressed in high-glucose-grown cells is phosphorylated, while in low-glucose-grown cells the phosphorylated form probably represents the majority of Can1p. The permease phosphorylation or dephosphorylation is not related to transinhibition.

Specific labelling of cell wall proteins by biotinylation. Identification of four covalently linked O-mannosylated proteins of Saccharomyces cerevisiae

Intact Saccharomyces cerevisiae cells were biotinylated with the non-permeable sulfosuccinimidyl-6-(biotinamido) hexanoate reagent. Twenty specifically labelled cell wall proteins would be extracted and visualized on SDS gels via streptavidin/horseradish peroxidase. Nine cell wall proteins were released by SDS extraction under reducing conditions and were designated Scw1-9p for (soluble cell wall proteins); five proteins were released from SDS-extracted cell walls by laminarinase (Ccw1-5p for covalently linked cell wall proteins) and six with mild (30 mM-NaOH, 4 degrees C, 14 h) alkali treatment (Ccw6-11p). N-terminal sequences of the Ccw proteins 6, 7, 8 and 11 showed that these cell wall proteins are members of the PIR gene family (predicted proteins with internal repeats), CCW6 being identical to PIR1 and CCW8 to PIR3. Single gene disruptions of all four genes did not yield a phenotype. In the CCW11 disruption the Ccw11p as well as the laminarinase-extracted Ccw5 protein was missing. The new cell wall proteins are O-mannosylated, contain a Kex2 processing site, but no C-terminal GPI anchor sequence.

Characterization of two new genes down-regulated by alpha-factor

To detect genes directly down-regulated by alpha-factor, 55,000 plaque-forming units of a Saccharomyces cerevisiae lambda gt10 gene bank were differentially screened with cDNA of cells treated with alpha-factor for 20 min. Two new genes were detected in this way, called alpha0.5 and alpha0.6. The former is transiently down-regulated by alpha-factor; it is very highly transcribed in late exponential-phase cells. The gene, located on the right arm of chromosome XIII, codes for a 59 amino-acid protein with a signal peptide. The protein has been shown with an antibody to be present in the membrane fraction. The gene has also been cloned as HOR7 (hyperosmolarity-responsive protein; Hirayama et al., 1995). No other homologous sequences have been detected in the yeast genome. alpha0.6, located on the right arm of chromosome XII, corresponds to the open reading frame YLR110c; it codes for a 133 amino-acid protein containing a signal peptide. Its derived amino-acid sequence is homologous to the N-terminal half of the SED1 gene product. SED1, when overexpressed, is able to suppress a defect in the HDEL receptor coded for by the ERD2 gene (Hardwick and Pelham, 1994); however, alpha0.6 is not able to do so. The disruption of alpha0.5 or alpha0.6 does not lead to a special phenotype.

Protein-O-glycosylation in yeast: protein-specific mannosyltransferases

S. cerevisiae contains at least six genes (PMT1-6) for dolicholphosphate-D-mannose: protein-O-D-mannosyltransferases. The in vivo mannosylation of seven O-mannosylated yeast proteins has been analyzed in a number of pmt mutants. The results clearly indicate that the various protein O-mannosyltransferases have different specificities for protein substrates. Five of the proteins tested (chitinase, a-agglutinin, Kre9p, Bar1p, Pir2p/hsp 150) are mainly underglycosylated in pmt1 and pmt2 mutants, whereby qualitative differences exist among the various proteins. Two of the O-mannosylated proteins (Ggp1p and Kex2p) are not at all affected in pmt1 and pmt2 mutants but are clearly underglycosylated when PMT4 is mutated. Although the PMT4 gene product is shown to be responsible for O-mannosylating a Ser-rich region of Ggp1p in vivo, a penta-seryl-peptide is not an in vitro substrate for this transferase. A PMT3 mutation does affect O-mannosylation of chitinase only in the genetic background of a pmt1pmt2 double mutation, indicating that PMT1 and PMT2 can compensate for a deleted PMT3 gene.

Purification of the Chlorella HUP1 hexose-proton symporter to homogeneity and its reconstitution in vitro

A prokaryotic biotin acceptor domain was fused to the carboxy terminal end of the Chlorella hexose-proton symporter. The plant symporter is biotinylated in vivo when expressed in Schizosaccharomyces pombe. The extended biotinylated transport protein is fully active, catalyzes accumulation of D-glucose analogs and restores growth of a glucose-uptake-deficient yeast strain. Crude membranes were solubilized with octyl-beta-D-glucoside in the presence of Escherichia coli L-alpha-phosphatidylethanolamine. Biotinylated symporter was purified to homogeneity by biotinavidin affinity chromatography. The symporter protein was reconstituted together with cytochrome-c oxidase prepared from beef heart mitochondria into proteo-liposomes. Cytochrome-c oxidase is a redox-driven H(+)-pump generating a proton motive force (inside negative and alkaline) while transferring electrons from cytochrome-c to oxygen; this energy is used by the symporter to accumulate D-glucose at least 30-fold. In the absence of the driving force the transport protein facilitates diffusion of D-glucose until the concentration equilibrium is reached. It was shown that maximal transport activity depends highly on the amount of co-reconstituted cytochrome-c oxidase and that the symporter possesses 10% of its in vivo turnover number under optimized in vitro transport conditions.

The PMT gene family: protein O-glycosylation in Saccharomyces cerevisiae is vital

The transfer of mannose to seryl and threonyl residues of secretory proteins is catalyzed by a family of protein mannosyltransferases coded for by seven genes (PMT1-7). Mannose dolichylphosphate is the sugar donor of the reaction, which is localized at the endoplasmic reticulum. By gene disruption and crosses all single, double and triple mutants of genes PMT1-4 were constructed. Two of the double and three of the triple mutants were not able to grow under normal conditions; three of these mutants could grow, however, when osmotically stabilized. The various mutants were extensively characterized concerning growth, morphology and their sensitivity to killer toxin K1, caffeine and calcofluor white. O-Mannosylation of gp115/Gas1p was affected only in pmt4 mutants, whereas glycosylation of chitinase was mainly affected in pmt1 and pmt2 mutants. The results show that protein O-glycosylation is essential for cell wall rigidity and cell integrity and that this protein modification, therefore, is vital for Saccharomyces cerevisiae.

MEMBRANE TRANSPORT CARRIERS

▪ Abstract  Plant and fungal membrane proteins catalyzing the transmembrane translocation of small molecules without directly using ATP or acting as channels are discussed in this review. Facilitators, ion-cotransporters, and exchange translocators mainly for sugars, amino acids, and ions that have been cloned and characterized from Saccharomyces cerevisiae and from various plant sources have been tabulated. The membrane topology and structure of the most extensively studied carriers (lac permease of Escherichia coli, Glut1 of man, HUP1 of Chlorella) are discussed in detail as well as the kinetic analysis of specific Na+ and H+ cotransporters. Finally, the knowledge concerning regulatory phenomena of carriers—mainly of S. cerevisiae—is summarized.

Importance of the first external loop for substrate recognition as revealed by chimeric Chlorella monosaccharide/H+ symporters

It had been shown previously by heterologous expression in Schizosaccharomyces pombe, that the two monosaccharide/H+ symporters HUP1 and HUP2 of Chlorella kessleri differ significantly concerning their substrate specificity: HUP1 transports predominantly D-glucose while HUP2 prefers D-galactose. Several chimeric transporters were constructed and their substrate specificities determined. Surprisingly, it is sufficient to replace the first part of the external loop 1 of the HUP1 symporter by the corresponding portion of HUP2 to improve transport and also to decrease the Km value for D-galactose. Additional data indicating the importance of the first loop for substrate recognition and binding are discussed.

Fungal glycoproteins and their biosynthetic pathway as potential targets for antifungal agents

The yeast cell wall as a good antifungal target is discussed in general. More specifically the reaction, catalyzed by Dol-P-Man: protein O-D-mannosyltransferase is proposed as a new potential target. Six genes responsible for this endoplasmic reticulum-localized reaction have been cloned and characterized so far. Triple disruptions of these genes are either lethal or the corresponding cells have to be osmotically stabilized to survive. No inhibitors of this reaction are as yet known.

Protein O-glycosylation in Saccharomyces cerevisiae: the protein O-mannosyltransferases Pmt1p and Pmt2p function as heterodimer

The protein O-mannosyltransferases Pmt1p and Pmt2p are catalyzing the O-glycosylation of serine and threonine residues in the endoplasmic reticulum of yeast. Deletion of each of these proteins by disruption of the corresponding gene leads to a dramatic decrease of mannosyltransferase activity in vitro. With an anti-Pmt1p immunoaffinity column a complex of Pmt1p and a second protein was purified; this protein turned out to be Pmt2p. Overexpression of Pmt1p or Pmt2p, respectively, does not increase mannosyltransferase activity in vitro. Overexpression of both mannosyltransferases together, however, raises in vitro activity threefold. These data indicate that Pmt1p and Pmt2p function as a complex catalyzing protein O-glycosylation in yeast.

PMT3 and PMT4, two new members of the protein-O-mannosyltransferase gene family of Saccharomyces cerevisiae

Two genes PMT3 and PMT4 were identified by polymerase chain reaction of genomic DNA using primers derived from regions of high homology between the products of three genes PMT1, PMT2 of Saccharomyces cerevisiae and part of a PMT1 related sequence of Kluyveromyces lactis. Pmt1p and Pmt2p are mannosyltransferases involved in the transfer of a mannosyl residue from dolichyl phosphate-D-mannose (Dol-P-Man) to seryl and threonyl residues in proteins. The products encoded by the PMT3 and PMT4 genes have almost identical hydropathy profiles in comparison to PMT1 and PMT2: a hydrophobic N- and C-terminal third each with multiple potential transmembrane helices and a central hydrophilic part. The predicted Pmt3p contains 753 amino acids, four potential N-glycosylation sites and it is significantly homologous to Pmt1p, Pmt2p and Pmt4p. Pmt4p contains 762 amino acids and two potential N-glycosylation sites. Northern blot analysis showed a single mRNA transcript of PMT3 and PMT4 of 2.8 kb. Thus PMT3 and PMT4 are two new members of the PMT gene family. The pmt4 null mutant the pmt3 pmt4 double null mutant, but not pmt3 null mutant, showed a significant shift of chitinase due to under glycosylation of the enzyme. The triple disruption pmt2 pmt3 pmt4 and the quadruple disruption result in a lethal phenotype.

Protein O-glycosylation in yeast. The PMT2 gene specifies a second protein O-mannosyltransferase that functions in addition to the PMT1-encoded activity

The PMT2 gene from Saccharomyces cerevisiae was identified as FUN25, a transcribed open reading frame on the left arm of chromosome I (Ouellette, B. F. F., Clark, M. W. C., Keng, T., Storms, R. G., Zhong, W., Zeng, B., Fortin, N., Delaney, S., Barton, A., Kaback, D.B., and Bussey, H. (1993) Genome 36, 32-42). The product encoded by the PMT2 gene shows significant similarity with the dolichyl phosphate-D-mannose:protein O-D-mannosyltransferase, Pmt1p (EC 2.4.1.109), which is required for initiating the assembly of O-linked oligosaccharides in S. cerevisiae (Strahl-Bolsinger, S., Immervoll, T., Deutzmann, R., and Tanner, W. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 8164-8168). The PMT2 gene encodes a new protein O-D-mannosyltransferase. Yeast cells carrying a PMT2 disruption show a diminished in vitro and in vivo O-mannosylation activity and resemble mutants with a nonfunctional PMT1 gene. Strains bearing a pmt1 pmt2 double disruption show a severe growth defect but retain residual O-mannosylation activity indicating the presence of at least one more protein-O-mannosyltransferase.

A new Dol-P-Man:protein O-D-mannosyltransferase activity from Saccharomyces cerevisiae

The deletion of the protein mannosyltransferase 1 gene (PMT1) of Saccharomyces cerevisiae results in viable cells. O-Mannosylation of proteins is reduced to about half of the value in comparison to wild-type cells. In order to distinguish between the the PMT1 gene product (= Pmt1p) and residual transferase activity, an in vitro assay to measure Dol-P-Man:protein mannosyltransferase activity in cells deleted for PMT1 has been developed. The transferase activity of these cells exhibits a pH optimum of 6.5 as compared to pH 7.5 for Pmt1p. The Km value of the residual enzyme activity for the hexapeptide YNPTSV is 7 times higher than that of Pmt1p and shows a clear preference for the seryl residue. Differences in substrate affinities as well as in seryl/threonyl depend on the specific sequence of the peptides used in the enzyme assay. The new enzyme activity shows a significantly lower thermal stability as compared to Pmt1p.

Subcellular localization of the inducible Chlorella HUP1 monosaccharide-H+ symporter and cloning of a Co-induced galactose-H+ symporter

The unicellular green alga Chlorella kessleri can induce monosaccharide-H+ symport catalyzing the energy-dependent transport of D-glucose (D-Glc) and several other pentoses and hexoses across the plasmalemma. The gene coding for the inducible HUP1 monosaccharide-H+ symporter has been cloned and the protein has been characterized previously. The data presented in this paper demonstrate that the presence of the HUP1 gene product alone is not sufficient to cover the broad substrate specificity of monosaccharide transport in induced Chlorella cells. Two other HUP genes are shown to be co-induced in Chlorella in response to D-Glc in the medium. The cloning of HUP2 and HUP3 cDNA and genomic sequences is described, both being very homologous to HUP1. Modification of the 5' untranslated sequences of full-length cDNA clones of HUP2 and HUP3 allowed the functional expression of both transporters in Schizosaccharomyces pombe. HUP2 was shown to be a galactose-H+ symporter, whereas the substrate specificity of the HUP3 gene product is very similar to that of the HUP1 protein. However, HUP3 does not seem to be induced to high levels in Glc-treated Chlorella cells. Results are also presented proving that the product of the HUP1 gene is localized in the plasmalemma of D-Glc-induced Chlorella cells and is absent in plasma membranes of noninduced cells. Incubation of thin sections of Chlorella cells with anti-HUP1 antibodies and a fluorescence-labeled, second antibody yielded a ring of fluorescence on the surface of Glc-induced Chlorella cells.

Hexose/H+ symporters in lower and higher plants

A well-studied transporter of plant cells is the hexose/H+ symporter of the unicellular alga Chlorella kessleri. Its properties, studied in vivo, are briefly summarized. In part, they are atypical and it has been suggested that this porter acts in an asymmetric way. Three genes coding for Chlorella hexose transport activity have been identified (HUP1, HUP2 and HUP3). HUP1 cDNA expressed in a mutant of Schizosaccharomyces pombe not transporting any D-glucose has been studied in detail. Several mutants with changed Km values for substrate were obtained, some by random polymerase chain reaction mutation and selection for decreased sensitivity towards the toxic sugar 2-deoxyglucose, some by site-directed mutagenesis. The amino acids affected clustered in the centre of the putative transmembrane helices V, VII and XI. Large families of hexose transporter genes are found in higher plants (Arabidopsis, Chenopodium, Ricinus). Their functional role is discussed. Finally, the progress made in studying plant transporters in a vesicle system energized by cytochrome c oxidase is summarized.

Ethanol-responsive genes in neural cells include the 78-kilodalton glucose-regulated protein (GRP78) and 94-kilodalton glucose-regulated protein (GRP94) molecular chaperones

Previously we found that ethanol increases expression of the constitutive 70-kDa heat shock protein (Hsc70) in NG108-15 neuroblastoma x glioma cells. We suggested that known ethanol actions on cellular protein trafficking may relate to Hsc70 induction because Hsc70 functions as a molecular chaperone. Here we use a subtractive hybridization protocol to isolate ethanol-responsive genes (EtRGs). Northern blot hybridization verified ethanol-induced increases in mRNA abundance for five cDNA clones isolated from ethanol-treated NG108-15 neuroblastoma x glioma cells. DNA sequence analysis identified one EtRG as 94-kDa glucose-regulated protein (GRP94), a member of the "glucose-responsive" subgroup of stress proteins. Other identified EtRGs included an insulin-induced growth-response protein gene and an intracisternal A-type particle gene. Sequence analysis of the remaining two EtRGs showed no homology in DNA sequence databases. All EtRGs showed wide tissue expression, except SL64, which was not detected in Northern blot analyses of adult mouse or rat tissues. Ethanol also increased mRNA abundance for 78-kDa glucose-regulated protein (GRP78), a molecular chaperone known to function in glycoprotein trafficking and usually coordinately regulated with GRP94. However, ethanol induced GRP94 more than GRP78, a pattern distinct from those of other inducers of these genes. All EtRGs, including GRP94 and GRP78, showed similar ethanol concentration-dependent increases in mRNA abundance. In contrast, thapsigargin and other inducers of glucose-responsive proteins increased GRP94 and GRP78 mRNA levels without altering expression of other EtRGs. Our studies demonstrate that several molecular chaperones constitute a subset of EtRGs. Ethanol appears to regulate these EtRGs by a unique mechanism, rather than one shared by classical inducers of stress proteins.

Mating type-specific cell-cell recognition of Saccharomyces cerevisiae: cell wall attachment and active sites of a- and alpha-agglutinin

Mating type-specific agglutination of Saccharomyces cerevisiae a and alpha cells depends on the heterophilic interaction of two cell surface glycoproteins, the gene products of AG alpha 1 and AGA2. Evidence is presented with immunogold labelling that the alpha-agglutinin is part of the outer fimbrial cell wall coat. The a-agglutinin is bound via two S-S bridges (Cys7 and Cys50) to a cell wall component, most probably the gene product of AGA1. His273 of alpha-agglutinin has previously been shown to be essential for a- and alpha-agglutinin interaction and a model based on two opposing ion-pairs had been proposed. By site-directed mutagenesis this possibility has now been excluded. With the help of various peptides, either chemically synthesized, obtained by proteolysis of intact glycosylated a-agglutinin or prepared from a fusion protein expressed in Escherichia coli, the biologically active region of a-agglutinin was located at the C-terminus of the molecule. A peptide consisting of the C-terminal 10 amino acids (GSPIN-TQYVF) was active in nanomolar concentrations. Saccharide moieties, therefore, are not essential for the mating type-specific cell-cell interaction; glycosylated peptides are, however, four to five times more active than non-glycosylated ones. Comparisons of the recognition sequences of the S. cerevisiae agglutinins with that of the Dictyostelium contact site A glycoprotein (gp80), as well as with those of the various families of cell adhesion molecules of higher eucaryotes, have been made and are discussed.

Km mutants of the Chlorella monosaccharide/H+ cotransporter randomly generated by PCR

The HUP1 gene codes for the monosaccharide/H+ cotransporter protein of Chlorella kessleri. The gene is functionally expressed in Schizosaccharomyces pombe. This heterologous system has been used to screen for Km mutants of the Chlorella symporter. Since S. pombe transformed with HUP1 cDNA showed a 1000-fold increase in sensitivity toward the toxic sugar analogue 2-deoxyglucose, we screened for transformants with a decreased 2-deoxyglucose sensitivity. The transformants were produced with HUP1 cDNA randomly mutagenized by PCR. From 73 transformants with decreased 2-deoxyglucose sensitivity, four mutants with increased Km values for D-glucose were obtained. The amino acid exchanges responsible for the increased Km values are located in the center of the putative transmembrane helices V (Q179E), VII (Q298R), and XI (V433L/N436Y). Q179N and Q299N had previously been shown by directed mutagenesis to affect the Km value of the transporter for D-glucose. The drastic mutational changes Q298R and N436Y gave rise to very high Km values; however, the corresponding conservative amino acid changes Q298N or N436Q obtained by directed mutagenesis also result in Km values increased by a factor of 10 or 20, respectively. The data therefore support the proposal that at least helices V, VII, and XI may line the sugar translocation path and determine its specificity. These results are discussed in relation to other sugar transporters and to the interaction of the yeast hexokinase B with D-glucose as known from published crystal structures.

The HUP1 gene product of Chlorella kessleri: H+/glucose symport studied in vitro

An in vitro system was established to measure secondary active transport mediated by plant H+ symporters. For this purpose plasma membranes of Schizosaccharomyces pombe cells transformed with the HUP1 gene coding for the H+/hexose symporter of Chlorella kessleri were fused with cytochrome-c oxidase containing proteoliposomes. After energization with ascorbate/TMPD/cytochrome c these vesicles built up a protonmotive force of > 130 mV consisting mainly of a membrane potential of > 100 mV (inside negative). Energized vesicles accumulated D-glucose in a pH-dependent way up to 30-fold which was not the case with control vesicles prepared from cells transformed with the plasmid not containing the HUP1 gene. The Km value for D-glucose uptake was 5 x 10(-5) M. The pH-dependence of accumulation was not due to a difference in protonmotive force, but reflected the pH-dependence of the carrier activity, i.e., the accumulation was determined by kinetic and by thermodynamic parameters. In the system both components of protonmotive force delta psi and delta pH can be manipulated individually, which allows to evaluate to what extent they contribute to sugar accumulation. The results indicate that under certain conditions the internal pH may be a limiting factor for D-glucose accumulation.