Baseline Characteristics of the 2015-2019 First Year Student Cohorts of the NIH Building Infrastructure Leading to Diversity (BUILD) Program

Objective

The biomedical/behavioral sciences lag in the recruitment and advancement of students from historically underrepresented backgrounds. In 2014 the NIH created the Diversity Program Consortium (DPC), a prospective, multi-site study comprising 10 Building Infrastructure Leading to Diversity (BUILD) institutional grantees, the National Research Mentoring Network (NRMN) and a Coordination and Evaluation Center (CEC). This article describes baseline characteristics of four incoming, first-year student cohorts at the primary BUILD institutions who completed the Higher Education Research Institute, The Freshmen Survey between 2015-2019. These freshmen are the primary student cohorts for longitudinal analyses comparing outcomes of BUILD program participants and non-participants.

Design

Baseline description of first-year students entering college at BUILD institutions during 2015-2019.

Setting

Ten colleges/universities that each received <$7.5mil/yr in NIH Research Project Grants and have high proportions of low-income students.

Participants

First-year undergraduate students who participated in BUILD-sponsored activities and a sample of non-BUILD students at the same BUILD institutions. A total of 32,963 first-year students were enrolled in the project; 64% were female, 18% Hispanic/Latinx, 19% African American/Black, 2% American Indian/Alaska Native and Native Hawaiian/Pacific Islander, 17% Asian, and 29% White. Twenty-seven percent were from families with an income <$30,000/yr and 25% were their family's first generation in college.

Planned Outcomes

Primary student outcomes to be evaluated over time include undergraduate biomedical degree completion, entry into/completion of a graduate biomedical degree program, and evidence of excelling in biomedical research and scholarship.

Conclusions

The DPC national evaluation has identified a large, longitudinal cohort of students with many from groups historically underrepresented in the biomedical sciences that will inform institutional/national policy level initiatives to help diversify the biomedical workforce.

IMPACT OF OPEN ENROLLMENT IN COURSE-BASED UNDERGRADUATE RESEARCH EXPERIENCES WITH AT-RISK STUDENT POPULATIONS

Participation in authentic research activities, particularly mentored undergraduate research experiences, at the University of Texas at El Paso has long been associated with student success measures such as graduation and matriculation to strong graduate programs in STEM. However, these opportunities typically are available to upper division students, despite evidence suggesting that the first (Freshman) year at university is determinant for individuals to complete STEM degrees. To expand the number of research opportunities and to extend them preferentially to new, entering students, we established the Freshman Year Research Intensive Sequence (FYRIS) in 2015, a course sequence consisting of a research foundations course and one or two laboratory courses redesigned by faculty into small, special topic Course-based Undergraduate Research Experiences (CUREs). CUREs provide authentic research experiences with similar early-, middle-, and late-term benefits to those found in traditional mentored experiences. Several of these benefits can be conceptualized as "hubs", which derive from earlier benefits, while facilitating later positive outcomes. Self-efficacy is one such hub, while retention and persistence in science enrollment represent late-phase positive outcomes. In this report, we examined self-efficacy of FYRIS participants in surveys administered at the start and end of each course to assess their confidence in conducting fundamental and specific research activities in the foundations and research driven courses, respectively. Specific items from a validated survey were used in addition to items developed for each course based on specific learning objectives. Retention was measured across three years of assessment of participants and non-participants, controlling for key scholastic characteristics. Results on retention rates after FYRIS vary depending on whether students fully or partially participated in the course-sequence. Results will be presented for three cohorts of students: 2015-16, 2016-17, and 2017-18.

BUILDing SCHOLARS: enhancing diversity among U.S. biomedical researchers in the Southwest

Background and purpose

With funding from the National Institutes of Health, BUILDing SCHOLARS was established at The University of Texas at El Paso with the goal of implementing, evaluating and sustaining a suite of institutional, faculty and student development interventions in order to train the next generation of biomedical researchers from the U.S. Southwest region, where the need is dire among underserved communities. The focus is on supporting the infrastructure necessary to train and mentor students so they persist on pathways across a range of biomedical research fields. The purpose of this article is to highlight the design and implementation of BUILDing SCHOLARS' key interventions, which offer a systemic student training model for the U.S. Southwest. In-depth reporting of evaluation results is reserved for other technical publications.

Program and key highlights

BUILDing SCHOLARS uses a comprehensive regional approach to undergraduate training through a multi-institution consortium that includes 12 research partners and various pipeline partners across Texas, New Mexico, and Arizona. Through faculty collaborations and undergraduate research training, the program integrates social and behavioral sciences and biomedical engineering while emphasizing seven transdisciplinary nodes of biomedical research excellence that are common across partner institutions: addiction, cancer, degenerative and chronic diseases, environmental health, health disparities, infectious diseases, and translational biomedicine. Key interventions aim to: (1) improve institutional capacities by expanding undergraduate research training infrastructures; (2) develop an intra- and cross-institutional mentoring-driven "community of practice" to support undergraduate student researchers; (3) broaden the pool of student participants, improve retention, and increase matriculation into competitive graduate programs; and (4) support faculty and postdoctoral personnel by training them in research pedagogy and mentoring techniques and providing them with resources for increasing their research productivity. Student training activities focus on early interventions to maximize retention and on enabling students to overcome common barriers by addressing their educational endowments, science socialization, network development, family expectations, and material resources. Over the long term, BUILDing SCHOLARS will help increase the diversity of the biomedical research workforce in the U.S. by meeting the needs of students from the Southwest region and by serving as a model for other institutions.

Lipidomic analysis reveals that phosphatidylglycerol and phosphatidylethanolamine are newly generated phospholipids in an early-divergent protozoan, Giardia lamblia

The pathogenic protozoan Giardia lamblia is known to not synthesize membrane lipids de novo. Therefore, it is possible that lipids in the small intestine, where trophozoites colonize, play key roles in regulating the growth and differentiation of this important pathogen. The focus of the current study is to conduct a complete lipidomic analysis and to test the hypothesis that Giardia has some ability to generate new phospholipids (PLs). Using mass spectrometry, now we show that phosphatidylglycerols (PGs) are major PLs followed by phosphatidylcholines (PCs) and phosphatidylethanolamines (PEs) in non-encysting and encysting trophozoites, as well in cysts. The fatty acids attached to these PLs consist mostly of palmitate, palmitoleate, oleate, and linoleate. Results also indicate that PGs and PEs, unlike PCs, are not present in bovine bile and serum, the major sources of lipids of the culture medium, and that they could therefore be produced by fatty acid and headgroup remodeling reactions, circumventing the synthesis of entirely new PLs via de novo pathways. Genomic and transcriptional analyses show the presence of giardial phosphatidylglycerolphosphate synthase (gpgps) and phosphatidylserine decarboxylase (gpsd) genes, which are expressed throughout the life cycle. Bioinformatic and phylogenetic analyses further indicated that both genes are of prokaryotic origin and that they have undergone duplication in the course of evolution. Our studies suggest that the abundance of PG in Giardia is unique among eukaryotes and that its synthesis thus could serve as a potential target for developing new therapies against this waterborne parasite.

Genomic minimalism in the early diverging intestinal parasite Giardia lamblia

The genome of the eukaryotic protist Giardia lamblia, an important human intestinal parasite, is compact in structure and content, contains few introns or mitochondrial relics, and has simplified machinery for DNA replication, transcription, RNA processing, and most metabolic pathways. Protein kinases comprise the single largest protein class and reflect Giardia's requirement for a complex signal transduction network for coordinating differentiation. Lateral gene transfer from bacterial and archaeal donors has shaped Giardia's genome, and previously unknown gene families, for example, cysteine-rich structural proteins, have been discovered. Unexpectedly, the genome shows little evidence of heterozygosity, supporting recent speculations that this organism is sexual. This genome sequence will not only be valuable for investigating the evolution of eukaryotes, but will also be applied to the search for new therapeutics for this parasite.

Transcriptional analysis of three major putative phosphatidylinositol kinase genes in a parasitic protozoan, Giardia lamblia

The current investigation evaluates the expression of phosphatidylinositol kinase (PIK) genes in the parasitic protozoan, Giardia lamblia. The G. lamblia Genome Database revealed the presence of two putative phosphatidylinositol-3-kinase (gPI3K) and one phosphatidylinositol-4-kinase (gPI4K) genes resembling the catalytic subunit of eukaryotic PIKs. Primers, designed to amplify mRNA of these three genes, were used to measure transcription by quantitative reverse-transcriptase polymerase chain reactions. Results suggest that all three PIK genes are expressed in non-encysting and encysting trophozoites. The relative levels of the mRNA were highest in parasites cultured in pre-encysting medium that contained no bile. Two inhibitors of PI3K, LY 294002 and wortmannin were found to inhibit the growth of the trophozoite in culture. However, wortmannin was more effective than LY294002. Altogether, the present study indicates that Giardia is capable of expressing PIKs that are necessary for the growth and differentiation of this pathogen.

Clathrin-dependent pathways and the cytoskeleton network are involved in ceramide endocytosis by a parasitic protozoan, Giardia lamblia

Although identified as an early-diverged protozoan, Giardia lamblia shares many similarities with higher eukaryotic cells, including an internal membrane system and cytoskeleton, as well as secretory pathways. However, unlike many other eukaryotes, Giardia does not synthesize lipids de novo, but rather depends on exogenous sources for both energy production and organelle or membrane biogenesis. It is not known how lipid molecules are taken up by this parasite and if endocytic pathways are involved in this process. In this investigation, we tested the hypothesis that highly regulated and selective lipid transport machinery is present in Giardia and necessary for the efficient internalization and intracellular targeting of ceramide molecules, the major sphingolipid precursor. Using metabolic and pathway inhibitors, we demonstrate that ceramide is internalized through endocytic pathways and is primarily targeted into perinuclear/endoplasmic reticulum membranes. Further investigations suggested that Giardia uses both clathrin-dependent pathways and the actin cytoskeleton for ceramide uptake, as well as microtubule filaments for intracellular localization and targeting. We speculate that this parasitic protozoan has evolved cytoskeletal and clathrin-dependent endocytic mechanisms for importing ceramide molecules from the cell exterior for the synthesis of membranes and vesicles during growth and differentiation.

The Giardia genome project database

The Giardia genome project database provides an online resource for Giardia lamblia (WB strain, clone C6) genome sequence information. The database includes edited single-pass reads, the results of BLASTX searches, and details of progress towards sequencing the entire 12 million-bp Giardia genome. Pre-sorted BLASTX results can be retrieved based on keyword searches and BLAST searches of the high throughput Giardia data can be initiated from the web site or through NCBI. Descriptions of the genomic DNA libraries, project protocols and summary statistics are also available. Although the Giardia genome project is ongoing, new sequences are made available on a bi-monthly basis to ensure that researchers have access to information that may assist them in the search for genes and their biological function. The current URL of the Giardia genome project database is www.mbl.edu/Giardia.

Novel protein-disulfide isomerases from the early-diverging protist Giardia lamblia

Protein-disulfide isomerase is essential for formation and reshuffling of disulfide bonds during nascent protein folding in the endoplasmic reticulum. The two thioredoxin-like active sites catalyze a variety of thiol-disulfide exchange reactions. We have characterized three novel protein-disulfide isomerases from the primitive eukaryote Giardia lamblia. Unlike other protein-disulfide isomerases, the giardial enzymes have only one active site. The active-site sequence motif in the giardial proteins (CGHC) is characteristic of eukaryotic protein-disulfide isomerases, and not other members of the thioredoxin superfamily that have one active site, such as thioredoxin and Dsb proteins from Gram-negative bacteria. The three giardial proteins have very different amino acid sequences and molecular masses (26, 50, and 13 kDa). All three enzymes were capable of rearranging disulfide bonds, and giardial protein-disulfide isomerase-2 also displayed oxidant and reductant activities. Surprisingly, the three giardial proteins also had Ca(2+)-dependent transglutaminase activity. This is the first report of protein-disulfide isomerases with a single active site that have diverse roles in protein cross-linking. This study may provide clues to the evolution of key functions of the endoplasmic reticulum in eukaryotic cells, protein disulfide formation, and isomerization.

Sequence survey of the Giardia lamblia genome

The parasitic protozoan Giardia lamblia represents one of the earliest diverging lineages in the evolutionary history of eukaryotic organisms as well as an important human pathogen. A representative sampling of gene sequences from this early diverging protozoan could provide insights into genotypic and phenotypic innovations associated with the origin of eukaryotes. Currently, known giardial gene sequences are heavily biased toward a few gene families, including variant surface proteins (VSPs), structural proteins, and ribosomal RNA genes. One-pass sequences of Giardia genomic DNA were obtained using vector flanking priming sequences on the ends of cosmids in two independent libraries. Comparisons of 2304 of these sequences against the GenBank database identified 205 potential giardial genes with BLAST scores P(n) < 10(9). These coding regions encompass a wide range of metabolic, repair, and signaling enzymes, and include some genes not predicted by our current understanding of Giardia biochemistry. The efficiency of identification of putative genes is consistent with earlier findings that coding regions in the Giardia genome are densely packed and do not appear to contain introns. Our current results suggest that direct genome sequencing is an efficient method for identifying giardial genes for evolutionary and biochemical studies.

Giardia lamblia: evidence for carrier-mediated uptake and release of conjugated bile acids

Giardia lamblia trophozoites colonize the human small intestine, where they are exposed to high concentrations of conjugated bile acids. Previous work has shown that bile acids enhance trophozoite survival, multiplication, and differentiation into the cyst stage. Therefore, experiments were performed to test whether carrier-mediated uptake of conjugated bile acids is present in this primitive parasite. Uptake of both cholyltaurine (C-tau) and cholylglycine (C-gly) was increased manyfold after culturing trophozoites in medium lacking bile acids. Absence of uptake at 4 degrees C and inhibition by other conjugated bile acids provided additional evidence for carrier-mediated uptake. Uptake of C-tau was greater than that of C-gly under all experimental conditions and appeared to be mediated by a different carrier. The major evidence for different carriers is that C-tau uptake was Na(+)-dependent, while C-gly uptake was not. In addition, C-tau uptake was more strongly inhibited by DTNB and several organic anions than C-gly uptake. Radiolabeled C-tau and C-gly were each released rapidly from trophozoites at 37 degrees C but not at 4 degrees C, suggesting that release of conjugated bile acids was also carrier-mediated. These findings are consistent with the notion that multiple transporters for conjugated bile acids are present in a lower eukaryote. We speculate that intracellular bile acids may facilitate lipid trafficking and membrane biosynthesis.

Ultrastructural effects of lactoferrin binding on Giardia lamblia trophozoites

Lactoferrin and its derived N-terminal peptide may be important host defenses against Giardia lamblia. We showed earlier that lactoferrin and the derived peptides have potent giardicidal activity in vitro. Using indirect immunofluorescence, we now demonstrate binding of lactoferrin and the peptides to the live trophozoite surface. Iron strongly inhibited binding of lactoferrin, and decreased binding of the peptides, while certain divalent metal ions decreased binding of all forms by about half. Lactoferrin and the peptides caused striking and complex morphologic changes in the trophozoite plasmalemma, endomembranes and cytoskeleton, and increased the electron density of the lysosome-like peripheral vacuoles.

Developmentally regulated transcripts and evidence of differential mRNA processing in Giardia lamblia

Although encystation and excystation are crucial to transmission of Giardia lamblia, little is known about the regulation of these very distinct differentiation processes. Fingerprinting of giardial mRNA populations throughout the time course of differentiation demonstrated complex patterns in mRNA differential display. Certain transcripts appeared or increased, while others decreased or disappeared at specific times, in response to physiologic stimuli that mimic key stages in parasite descent through the host gastrointestinal tract. This approach has allowed the direct identification of critical stages in differentiation, as well as isolation of genes which may be crucial to the development of G. lamblia. One stage-specific single copy gene (ENC6) whose transcript is greatly upregulated during encystation was analyzed further. Partial sequence analysis revealed no correspondence with known genes. 3'-rapid amplification of cDNA ends (3'-RACE) analysis of ENC6 transcripts at various times of encystation revealed two polyadenylation sites. The more proximal site, 10 nucleotides past the single classic AGTAAA sequence, was utilized only during encystation and its transcript increased approximately 16-fold during the first 24 h of encystation. In contrast, a slightly divergent polyadenylation site 288 nucleotides downstream from the open reading frame (ORF) was used during both vegetative growth and encystation, although its transcript was present at low levels. These studies are the first evidence of differential mRNA processing in G. lamblia and suggest a potential role of the 3'-untranslated region (3'-UTR) in modulating gene expression during differentiation of this primitive eukaryote.

Giardicidal activity of lactoferrin and N-terminal peptides

Human and bovine lactoferrins and their derived N-terminal peptides were giardicidal in vitro. Fe3+, but not Fe2+, protected trophozoites from both native lactoferrin and peptides, although the latter lack iron-binding sites. Other divalent metal ions protected only against native lactoferrin. Log-phase cells were more resistant to killing than stationary-phase cells. These studies suggest that lactoferrin, especially in the form of the N-terminal peptides, may be an important nonimmune component of host mucosal defenses against Giardia lamblia.

Specialized surface adaptations of Giardia lamblia

Although Giardia lamblia trophozoites were first described by Von Leeuwenhoek in his own diarrheic stool, relatively little is known of the basic biology of this common parasite or the pathophysiology of giardiasis. In particular, there is little specific information about trophozoite properties that cause diarrhea, as neither toxins nor conventional virulence factors have been identified. Therefore, parasite adaptations that promote cyst survival in the external environment and infection and trophozoite persistence in the small intestine, may be viewed as key virulence properties. This review focuses on unusual surface structures of the trophozoite and cyst forms that enable Giardia to be such a successful parasite.

Killing of Giardia lamblia by cryptdins and cationic neutrophil peptides

Antimicrobial polypeptides such as the defensins kill a wide range of organisms, including bacteria, fungi, viruses, and tumor cells. Because of the recent finding that intestinal defensins, also known as cryptdins, are synthesized by the Paneth cells of the small intestinal crypts and released into the lumen, we asked whether defensins and other small cationic antimicrobial peptides could kill the trophozoites of Giardia lamblia, which colonize the small intestine. Four mouse cryptdins, two neutrophil defensins (HNP-1 [human] and NP-2 [rabbit]), and the unique tryptophan-rich bovine neutrophil polypeptide indolicidin each had some antigiardial activity against trophozoites in vitro. Cryptdins 2 and 3, indolicidin, and NP-2 each reduced viability by more than 3 log units in 2 h, and killing by all peptides was dose and time dependent. Exposure of trophozoites to peptides frequently resulted in cell aggregation and dramatic changes in morphology. The mechanism of binding and lysis appeared to involve charge interactions, since 150 mM NaCl as well as millimolar levels of Ca2+ and Mg2+ inhibited killing by most of the peptides. Our studies show that G. lamblia is sensitive to defensins and indolicidin and suggest that these small polypeptides could play a role in nonimmune host defenses.

GP49, an invariant GPI-anchored antigen of Giardia lamblia

Giardia lamblia is a primitive protozoan and a major cause of waterborne enteric disease throughout tropical and temperate zones. The ability to grow the infective trophozoites in culture as well as the discovery of the method of in vitro encystation made it possible to study the biology of this primitive protozoan and to characterize the surface antigens. Giardia trophozoites are exposed to high concentrations of fatty acids in the human small intestine. This raises the possibility that intestinal fatty acids may become incorporated into Giardia. Therefore, we determined the pattern of fatty acylation of Giardia surface molecules. By metabolic labeling with radiolabeled fatty acids we identified a single glycosylphosphatidylinositol (GPI)-anchored surface protein in Giardia. GP49 differs from the cysteine-rich variable surface antigens described previously. The presence of a GPI anchor in GP49 was supported by the metabolic incorporation of [14C]-ethanolamine, [3H]-myoinositol and fatty acids into the protein. This was confirmed by chemical and enzymatic cleavage experiments. Most interestingly, GP49 was found to be present in different isolates of Giardia and thus can be considered as an invariant surface antigen. Although the biological function of GP49 is not known, recently we have found that intact and soluble GP49 altered the electrolyte fluxes which regulate fluid secretion in the cultured human intestinal epithelial cell line, T84. These studies indicate that the GPI-anchored invariant antigen of Giardia may play an important role in the pathophysiology of giardiasis.

Giardia lamblia: post-translational processing and status of exposed cysteine residues in TSA 417, a variable surface antigen

Giardia lamblia develops and thrives within the harsh and variable environment of the human small intestine. To survive this environment, Giardia has evolved a unique family of antigenically variable, extremely cysteine-rich surface proteins. We have characterized the expression of one of these antigens, TSA 417, at the gene and protein levels. TSA 417 mRNA and protein were expressed at constant levels during both logarithmic growth and encystation. On the trophozoite surface, TSA 417 exists in two forms, an 85-kDa protein and a derived 66-kDa form. Both species have identical amino-terminal sequences that match the translated sequence of TSA 417 after removal of a predicted 17 amino acid signal peptide. Since TSA 417 is highly cysteine rich (ca. 12 mole%), we asked whether it was complexed with itself or with other molecules. Although we found no interchain disulfide bonds, there was substantial intrachain bonding that helped retain the gross structure of isolated TSA 417 after partial trypsin digestion. Because the hydropathy profile of TSA 417 suggested that most of the cysteine and all 29 of its conserved CXXC motifs are on the cell surface, we investigated whether TSA 417 might contain some of the free thiols previously shown to be on the trophozoite surface (Gillin et al., Molecular and Biochemical Parasitology, 13, 1-12, 1984). However, no free thiols were detected, either exposed on native TSA 417 or in the unfolded protein. The absence of free thiols and resistance to proteolytic digestion suggest that most of the cysteine residues are in intrachain bridges, probably either disulfide bonds or cysteine-metal complexes. This internal crosslinking may help explain the resistance of Giardia to proteases and other degradative enzymes in the intestinal fluid.

Cysteine-dependent zinc binding by membrane proteins of Giardia lamblia

The abundant, highly variable surface proteins (VSPs) which cover the surface of Giardia lamblia trophozoites compose a group of extremely cysteine (C)-rich proteins in which more than half of the cysteines are in the motif CXXC. Because of the constancy of these features among the known VSPs and the prominence of cysteine and particularly CXXC in proteins that bind zinc and other metals, we asked whether G. lamblia VSPs bind zinc in vitro. VSPs are the major protein component of Triton X-114 detergent-phase extracts of G. lamblia trophozoites and can be readily identified by surface iodination of intact cells. The partitioning of 65Zn binding into the Triton X-114 detergent phase and the correspondence between surface iodination and zinc binding patterns of four G. lamblia strains or sublines with different VSPs support the idea that VSPs bind zinc. The requirement for renaturation of blots with a reducing agent indicates that Zn2+ is coordinated by cysteine residues, rather than by other amino acids. Binding did not appear to be specific to zinc since it was inhibited by competition with other divalent metal ions. The abundance of the VSPs and the prevalence of metal binding motifs among all known variants suggest that they may play an important role in trophozoite survival and colonization in the host.

Identification of an apically-located antigen that is conserved in sporozoan parasites

Sporozoan parasites of the phylum Apicomplexa all possess common apical structures. The current study used a monoclonal antibody (mAb-E12) to identify a conserved antigen in the apical region of merozoites of seven species of Plasmodium (including rodent, primate and human pathogens), tachyzoites of Toxoplasma gondii, bradyzoites of Sarcocystis bovis, and sporozoites and merozoites of Eimeria tenella and E. acervulina. The antigen was also present in sporozoites of haemosporinid parasites. Immunofluorescence studies showed that the antigen was restricted to the apical 3rd of these invasive stages. Using immunoelectron microscopy, labeling was demonstrated in the region of the polar ring, below the paired inner membranes of the parasite pellicle, and near the subpellicular microtubules radiating from the polar ring of merozoites and sporozoites of E. tenella. The majority of the antigen could be extracted with 1% Triton-X 100, but a portion remained associated with the cytoskeletal elements. The molecule has a relative rate of migration (Mr) of 47,000 in Plasmodium spp. and 43-46,000 in coccidian species. Since the epitope recognized by mAb-E12 is highly conserved, restricted to motile stages, and appears to be associated with microtubules, this antigen could be involved in cellular motility and cellular invasion.

Isolation and expression of the gene for a major surface protein of Giardia lamblia

To study the interactions between the parasitic protozoan Giardia lamblia and its environment, we have cloned the gene that encodes the two major surface-labeled trophozoite protein species. Sequence analysis of this gene reveals a single open reading frame specifying a hydrophilic, cysteine-rich (11.8%) protein of 72.5-kDa molecular mass with an amino-terminal signal peptide and a postulated hydrophobic membrane-spanning anchor region near the carboxyl terminus. Most of the cysteine residues (58 of 84) are in the motif Cys-Xaa-Xaa-Cys, which is dispersed 29 times throughout the sequence. Antibodies against the recombinant protein react with the entire surface of live trophozoites, including flagella and adhesive disc. These antibodies inhibit trophozoite attachment, prevent growth, and immunoprecipitate the major approximately 66- and 85-kDa proteins from surface-labeled live trophozoites. The recombinant Escherichia coli also expresses polypeptides of approximately 66- and 85-kDa molecular mass, which are not fusion proteins. This suggests that the processing and/or conformational changes that lead to production of these two peptide species in E. coli reflect those that occur in Giardia. The abundance of cysteine residues suggests that the native proteins on the parasite surface may contain numerous disulfide bonds, which would promote resistance to intestinal fluid proteases and to the detergent activity of bile salts and would help to explain the survival of Giardia in the human small intestine.

Expression of Plasmodium berghei circumsporozoite antigen on the surface of exoerythrocytic schizonts and merozoites

The intracellular distribution of circumsporozoite (CS) antigen was traced by immunoelectron microscopy in cultures of Plasmodium berghei exoerythrocytic (EE) schizonts with monoclonal antibody (Mab) 3D11 to the immunodominant repeat region of the P. berghei CS protein. CS antigen was localized on the parasitophorous vacuole (PV) membrane and pellicular complex of recently invaded sporozoites and on electron-dense masses of sloughed CS antigen in the host cell cytoplasm. CS antigen persisted throughout the complete EE cycle of P. berghei on the surface of EE schizonts and was incorporated into the plasma membrane of budding EE merozoites. Erythrocytic merozoites were not labeled by Mab 3D11, indicating that these 2 populations of merozoites differ in antigenic composition. Significant internal labeling occurred in 50 hr EE schizonts in association with the limiting membranes of peripheral vesicles and short, tube-like structures attached to their outer surfaces. These vesicles contained an electron-dense flocculent material also present in the PV space. Association of CS antigen with the limiting membranes of these vesicles suggests that they either develop as endocytotic invaginations of the schizont plasma membrane or transport newly synthesized CS antigen from the endoplasmic reticulum and Golgi of developing EE schizonts to the parasite surface.

Ultrastructural localization of Plasmodium falciparum circumsporozoite protein in newly invaded hepatoma cells

The fate and disposition of the circumsporozoite (CS) protein of Plasmodium falciparum was investigated during hepatoma cell invasion with several sera raised against defined CS peptides, including both repeat and nonrepeat regions spanning approximately 60% of the P. falciparum CS gene product. Distribution of the protein, as revealed by immunoelectron microscopy, was limited to the surface of the sporozoite both before and after invasion. In particular, no CS protein antigen was detected in association with either the parasitophorous vacuole membrane or the host cell surface.

Plasmodium vivax: exoerythrocytic schizonts recognized by monoclonal antibodies against blood-stage schizonts

Exoerythrocytic parasites of Plasmodium vivax grown in human hepatoma cells in vitro were probed with monoclonal antibodies raised against other stages of P. vivax. Monoclonal antibodies specific for four independent antigens on blood-stage merozoites all reacted with exoerythrocytic schizonts and merozoites by immunostaining. The characteristic staining pattern of each monoclonal antibody was similar on both blood- and exoerythrocytic-stage parasites and appeared only in mature schizont segmenters. In contrast, a monoclonal antibody specific for the caveolar-vesicle complex of the infected host cell membrane and a second monoclonal antibody reacting with an unknown internal antigen did not appear to react with exoerythrocytic parasites. We confirm prior reports that monoclonal antibodies against the sporozoite immunodominant repeat antigen react with all exoerythrocytic-stage parasites, but note that as the exoerythrocytic parasite matures the immunostaining is concentrated in plaques reminiscent of germinal centers and apparently distinct from mature merozoites. These results indicate that mature merozoites from either exoerythrocytic or blood-stage parasites are antigenically very similar, but that stage-specific antigens may be found in specialized structures present only in a specific host cell type.

Localization of Plasmodium falciparum histidine-rich protein 1 in the erythrocyte skeleton under knobs

Plasmodium falciparum parasites that induce knobs in the host erythrocyte membrane (K+ phenotype) synthesize a 90 kDa histidine-rich protein (PfHRP-1), whereas knobless variants do not. A monoclonal antibody (mAb 89) to PfHRP-1, in combination with cryo-thin section immunoelectron microscopy, localized the antigen in the parasitophorous vacuolar space and vesicles within the erythrocyte cytosol. Additional immunoelectron microscopic studies showed that PfHRP-1 was also associated with submembranous electron-dense material under knobs and with microfilaments of the host erythrocyte skeletal network. Immunofluorescence and immunoelectron microscopy of intact, non-fixed K+ infected erythrocytes using mAb 89 and a rabbit antiserum raised against purified PfHRP-1, failed to identify any surface exposed epitopes. These antibodies also failed to block cytoadherence of infected erythrocytes to C32 melanoma cells or to affect macrophage phagocytosis of infected erythrocytes.

Plasmodium falciparum: elicitation by peptides and recombinant circumsporozoite proteins of circulating mouse antibodies inhibiting sporozoite invasion of hepatoma cells

The immunodominant epitope region of the circumsporozoite protein of Plasmodium falciparum sporozoites contains 37 tandem repeats of the tetrapeptide Asn-Ala-Asn-Pro and 4 repeats of Asn-Val-Asp-Pro. Synthetic peptides and recombinant proteins of the repeat region were used to immunize mice using different doses and adjuvants. Antisera were tested for inhibition of sporozoite invasion of cultured human hepatoma cells. Synthetic peptides and recombinant proteins elicited high levels of antibodies that inhibited sporozoite invasion when emulsified with complete Freund's adjuvant. Since recombinant proteins with alum elicited a better antibody response to sporozoite invasion than they did without adjuvant, it may be that a recombinant protein containing 32 tandem copies of the tetrapeptide repeat combined with alum could be a candidate malarial vaccine suitable for human trials.

Transport of an Mr approximately 300,000 Plasmodium falciparum protein (Pf EMP 2) from the intraerythrocytic asexual parasite to the cytoplasmic face of the host cell membrane

The profound changes in the morphology, antigenicity, and functional properties of the host erythrocyte membrane induced by intraerythrocytic parasites of the human malaria Plasmodium falciparum are poorly understood at the molecular level. We have used mouse mAbs to identify a very large malarial protein (Mr approximately 300,000) that is exported from the parasite and deposited on the cytoplasmic face of the erythrocyte membrane. This protein is denoted P. falciparum erythrocyte membrane protein 2 (Pf EMP 2). The mAbs did not react with the surface of intact infected erythrocytes, nor was Pf EMP 2 accessible to exogenous proteases or lactoperoxidase-catalyzed radioiodination of intact cells. The mAbs also had no effect on in vitro cytoadherence of infected cells to the C32 amelanotic melanoma cell line. These properties distinguish Pf EMP 2 from Pf EMP 1, the cell surface malarial protein of similar size that is associated with the cytoadherent property of P. falciparum-infected erythrocytes. The mAbs did not react with Pf EMP 1. In one strain of parasite there was a significant difference in relative mobility of the 125I-surface-labeled Pf EMP 1 and the biosynthetically labeled Pf EMP 2, further distinguishing these proteins. By cryo-thin-section immunoelectron microscopy we identified organelles involved in the transit of Pf EMP through the erythrocyte cytoplasm to the internal face of the erythrocyte membrane where the protein is associated with electron-dense material under knobs. These results show that the intraerythrocytic malaria parasite has evolved a novel system for transporting malarial proteins beyond its own plasma membrane, through a vacuolar membrane and the host erythrocyte cytoplasm to the erythrocyte membrane, where they become membrane bound and presumably alter the properties of this membrane to the parasite's advantage.

Synthetic peptides from the circumsporozoite proteins of Plasmodium falciparum and Plasmodium knowlesi recognize the human hepatoma cell line HepG2-A16 in vitro

Several lines of evidence have emphasized the importance of the malaria circumsporozoite (CS) protein as a factor in sporozoite invasion of the hepatocyte; however, the specific mechanism of cell recognition and invasion has not been explained. In this study we present evidence that a highly conserved region of the CS protein immediately adjacent to the repeat region, the N1 region, specifically recognizes receptors on the human hepatoma cell line HepG2-A16 under conditions where invasion by sporozoites can occur. Peptides consisting of sequences from the repeat region or of the more extensive N2 region showed no such specific association. Antibody against the N1 peptide could inhibit sporozoite invasion in vitro. Covalent coupling of radiolabeled N1 peptide to HepG2-A16 cells identified two hepatic cell proteins to be closely associated with the peptide. We suggest that these proteins could act as receptors or mediators, via the N1 region of the CS protein, for the P. falciparum sporozoite in the process of invasion of the hepatocyte.

Secretion of a malarial histidine-rich protein (Pf HRP II) from Plasmodium falciparum-infected erythrocytes

Plasmodium falciparum-infected erythrocytes (IRBCs) synthesize several histidine-rich proteins (HRPs) that accumulate high levels of [3H]histidine but very low levels of amino acids such as [3H]isoleucine or [35S]methionine. We prepared a monoclonal antibody which reacts specifically with one of these HRPs (Pf HRP II) and studied the location and synthesis of this protein during the parasite's intracellular growth. With the knob-positive Malayan Camp strain of P. falciparum, the monoclonal antibody identified a multiplet of protein bands with major species at Mr 72,000 and 69,000. Pf HRP II synthesis began with immature parasites (rings) and continued through the trophozoite stage. The Mr 72,000 band of Pf HRP II, but not the faster moving bands of the multiplet, was recovered as a water-soluble protein from the culture supernatant of intact IRBCs. Approximately 50% of the total [3H]histidine radioactivity incorporated into the Mr 72,000 band was extracellular between 2 and 24 h of culture. Immunofluorescence and cryothin-section immunoelectron microscopy localized Pf HRP II to several cell compartments including the parasite cytoplasm, as concentrated "packets" in the host erythrocyte cytoplasm and at the IRBC membrane. Our results provide evidence for an intracellular route of transport for a secreted malarial protein from the parasite through several membranes and the host cell cytoplasm.

Plasmodium falciparum and P. knowlesi: initial identification and characterization of malaria synthesized glycolipids

This is the first report establishing the existence of glycolipids synthesized by plasmodia, in particular Plasmodium falciparum. Trophozoites, schizonts, gametocytes, and gametes were metabolically labeled in vitro with [3H]glucosamine, [3H]galactose, [3H]glucose, [3H]mannose, [3H]fucose, [32P]inorganic phosphate, or [35S]sulfate, and total lipid extracts analyzed by high-performance thin-layer chromatography and autoradiography or fluorography. Parasites incorporated [3H]monosaccharides into distinctly different series of molecules previously undescribed. Three properties of [3H]glucosamine labeled molecules indicate they are glycolipids. First, labeled molecules have lipid solubility properties. Second, mobility on thin-layer chromatography was characteristic of glycolipids. Third, following acid hydrolysis, [3H]glucosamine was recovered from a total lipid extract of labeled parasites demonstrating that glucosamine is a constituent of some of these lipid molecules. Most of these glycolipids are neutral and alkali labile. The majority of these glycolipids differs from several synthesized phospholipids. None of these glycolipids was sulfated. Plasmodial glycolipid synthesis occurs concomitantly with glycoprotein synthesis, and both increase during schizogony. Many of these glycolipids appear to be identical among three strains of P. falciparum and between two species, P. falciparum and P. knowlesi. In contrast, there are stage specific differences in glycolipid synthesis among rings, schizonts, gametocytes, and a mixture of gametes plus zygotes of P. falciparum, examples of both erythrocytic and vector forms of the parasite.

Identification of isolate-specific proteins on sorbitol-enriched Plasmodium falciparum infected erythrocytes from Gambian patients

We have compared the surface radio-iodinated proteins of uninfected and Plasmodium falciparum-infected erythrocytes from natural infections of human patients. Cryopreserved infected blood from Gambian children with falciparum malaria was thawed, cultured to the middle trophozoite stage, and surface radio-iodinated. Trophozoite-infected cells were enriched about 10-fold on a Percoll gradient newly designed to separate cells based on their differential permeability to sorbitol. Infected blood was radio-iodinated and erythrocytes from the fraction enriched in parasitized cells and uninfected erythrocytes from the same sample obtained from the gradient and compared by SDS-PAGE and autoradiography. In each sample, parasitized erythrocytes contained one or more polypeptides of very high molecular weight (Mr 250 000-300 000) that were not found on uninfected erythrocytes from the same patient. These proteins were isolate-specific in size and number, suggesting that natural isolates contain a variable number of different P. falciparum phenotypes for this surface protein. In addition, these radio-iodinated surface proteins could not be extracted from the host cell membrane by the non-ionic detergent Triton X-100, but were extracted by SDS. The properties of these proteins suggest they are the equivalent for natural infections of the strain-dependent antigen previously described (Leech, Barnwell, Miller & Howard, 1984) on the surface of P. falciparum-infected Aotus erythrocytes. In addition, we observed a second parasite-dependent modification of labelled proteins on infected erythrocytes with the appearance of a new band of Mr 30 000. There were also variations in the pattern of radio-isotope labelled proteins on uninfected erythrocytes from different patients.

Use of a two-sited monoclonal antibody assay to detect a heat-stable malarial antigen in the sera of mice infected with Plasmodium yoelii

Antigens, circulating in the blood during malarial infections, have been implicated in immune protection, immunosuppression, and immune-complex formation. We used a monoclonal antibody (MAb 7H8) to identify an antigen (Ag-7H8) in the sera of mice infected with Plasmodium yoelii. The major form of the antigen has a molecular weight of approximately 120,000 in P. yoelii, with minor components of 220,000; 65,000 to 75,000; and 45,000. Ag-7H8 remains antigenic after boiling for 5 min. A two-sited assay was developed with MAb 7H8 that demonstrated that the Ag-7H8 has at least two similar epitopes per molecule. The two-sited assay was used to follow Ag-7H8 in the blood of mice during lethal (strain 17XL) and nonlethal (strain 17XNL) P. yoelii infections. Ag-7H8 appeared on days 6 and 7 after infection with 10(6) and 10(4) 17XL P. yoelii parasites, respectively, and remained until the animals died. It was in plasma samples between days 6 and 14 after 17XNL P. yoelii injections in several inbred strains of mice, regardless of the course of parasitemia. Thus, the kinetics of antigenemia correspond with early stages of infection and not with the number of circulating parasites. Indirect immunofluorescence assays demonstrated that MAb 7H8 detects a cross-reactive antigen in other malarial parasites, including Plasmodium berghei and Plasmodium falciparum. Thus, this two-sited assay may have general application for the serodiagnosis of malaria and may be beneficial in determining the relationship of circulating antigens to malarial immunity.

Antigenic diversity and size diversity of P. falciparum antigens in isolates from Gambian patients. II. the schizont surface glycoprotein of molecular weight approximately 200 000

A panel of monoclonal antibodies has been shown previously to identify both serologically diverse and serologically conserved epitopes on a major polymorphic surface protein of P. falciparum schizonts from culture-adapted isolates. The molecular nature of the antigen recognized by eight of these monoclonal antibodies was studied with three isolates analyzed directly from patients in The Gambia. Malarial (glyco) proteins were labelled by biosynthetic uptake of 3H-glucosamine or 3H-leucine during culture of ring-stage parasites from infected blood to the late-trophozoite/early-schizont stage (26-30 h). Those monoclonal antibodies which reacted positively with an isolate by indirect immunofluorescence also immunoprecipitated a single 3H-leucine or 3H-glucosamine labelled antigen of mol. wt approximately 200 000 from Triton X-100 extracts of the same isolate. Monoclonal antibodies which did not react by indirect immunofluorescence failed to immunoprecipitate this antigen. Although each of the three isolates studied in detail was very similar serologically with the panel of monoclonal antibodies specific for this mol. wt approximately 200 000 antigen, this protein could be distinguished with each isolate on the basis of its apparent size on SDS-polyacrylamide gel electrophoresis. The specifically immunoprecipitated antigen had a mol. wt of 204 000, 197 000 or 202 000, depending on the isolate. Size diversity of this malarial glycoprotein was also detected with seven other Gambian P. falciparum isolates. We conclude that natural isolates of P. falciparum express a major 3H-glucosamine labelled glycoprotein of mol. wt Mr approximately 200 000 which exhibits size diversity and expresses antigenically conserved as well as diverse epitopes as defined by the panel of monoclonal antibodies.

Characterization of permeation pathways in the plasma membrane of human erythrocytes infected with early stages of Plasmodium falciparum: association with parasite development

Human intraerythrocytic malarial parasites (Plasmodium falciparum) induce permeability changes in the membrane of their host cells. The differential permeability of infected erythrocytes at various stages of parasite growth, in combination with density gradient centrifugation, was used to fractionate parasitized cells according to their developmental stage. By this method it was possible to obtain cell fractions consisting essentially of erythrocytes infected with the youngest parasite stage (i.e., rings). These preparations were used for the measurement of transport of various solutes. It is shown that permeabilization of host erythrocyte membrane appears as early as 6 h after parasite invasion of the erythrocyte and increases gradually with parasite maturation. Since the selectivity for several different solutes and the enthalpy of activation of transport remain unaltered with maturation-related increase of permeability, it is concluded that the number of transport agencies in the host cell membrane increases with parasite maturation. Evidence is presented to indicate the need for parasite protein synthesis as an essential factor for the generation of the new permeability pathways.

Knob-positive and knob-negative Plasmodium falciparum differ in expression of a strain-specific malarial antigen on the surface of infected erythrocytes

We have investigated the expression of a strain-specific malarial antigen on the surface of erythrocytes infected with knobless (K-) variants of knob-positive (K+) strains of Plasmodium falciparum. Aotus blood infected with K+ or K- parasites derived from two independent geographical isolates (Malayan camp and Santa Lucia) was surface iodinated by the lactoperoxidase method. Infected and uninfected erythrocytes were then separated by a new procedure involving equilibrium density sedimentation on a Percoll gradient containing sorbitol. Strain-specific antigens were readily identified on the surface of erythrocytes infected with either of the K+ strains by their characteristic size and detergent solubility. These proteins were not detected on the surface of erythrocytes infected with either of the K- variants nor on uninfected erythrocytes isolated from K+- or K- -infected blood. These results are consistent with a role for the strain-specific surface antigen in cytoadherence of P. falciparum-infected erythrocytes. Our findings represent the second biochemical difference (with the knob-associated histidine-rich protein) between K+ and K- P. falciparum.

Endocytosis in Entamoeba histolytica. Evidence for a unique non-acidified compartment

Our studies on endocytosis in Entamoeba histolytica trophozoites suggest that there are two vacuolar compartments in this organism. The first compartment consists of large vacuoles (greater than 2 microns diameter). As measured by the fluid phase markers, fluorescein isothiocyanate (FITC)-dextran and horseradish peroxidase (HRP), this compartment is a rapid equilibrium with the external milieu and is constantly exchanging (1-2 h) its contents with the external medium. The contents of these vacuoles are not acidified. This together with the absence of degradation of fluid phase markers clearly differentiates these vacuoles from lysosomes of eucaryotes. By labeling externally disposed peptides on the surface membrane of trophozoites with 125I, we could show that the surface membrane was rapidly internalized over a 2-h period and then reached a plateau. All major 125I surface proteins, with the exception of a set of peptides in the 40,000 molecular weight range, were interiorized and approximately 60% of the total radiolabel were found to be in the internal membrane fraction at any given time. The kinetics of this process were similar to those for the uptake of fluid phase markers and are best explained by cycling of the surface membrane into the vacuolar compartment(s) and then back to the cell surface. The second vacuolar compartment consisted of small vesicles (less than 2 microns diameter) with acidified contents as indicated by acridine orange uptake. The endocytic nature of these vesicles was shown by their slow (days) labeling with FITC-dextran, and spectral analysis of internalized FITC-dextran confirmed that this second compartment is acidified (pH 5.2).

Identification of parasite proteins in a membrane preparation enriched for the surface membrane of erythrocytes infected with Plasmodium knowlesi

A subcellular fraction enriched in erythrocyte membranes has been isolated from rhesus monkey erythrocytes infected with Plasmodium knowlesi. Infected cells were lysed by centrifugation through a zone of hypotonic buffer and membranes isolated by equilibrium density gradient centrifugation in the same tube. The purified membrane fraction was shown to include the erythrocyte surface membrane by several methods: electron microscopy, identification of Coomassie Blue stained erythrocyte membrane proteins, identification of band 3 with a monoclonal antibody, and identification of radioiodinated cell surface proteins. The resulting ghosts were shown to be specifically reactive with monkey sera against the variant surface antigens of P. knowlesi by indirect immunofluorescence and membrane agglutination. No reactivity was seen with a monoclonal antibody (13C11) against the intracellular schizont surface. A number of metabolically labelled parasite proteins were enriched in this membrane function, including peptides of 277, 208, 173, 153, 134, 109, 80, 60 and 48 kDa and the variant surface antigens of variable molecular mass (180-207 kDa). These proteins were distinct from the major parasite proteins of total infected erythrocytes and isolated merozoites. The major glucosamine labelled glycoprotein of the internal schizont (230 kDa) was not found in this fraction. Moreover, no fragment of this parasite glycoprotein was found in this membrane fraction, indicating that no part of this molecule is transported to the erythrocyte surface. In contrast, the variant antigen of P. knowlesi, known to be on the erythrocyte surface, could be readily identified as peptides unique to specific cloned parasite lines. We propose that the other nine parasite proteins found within this membrane fraction represent a starting point for the identification of other parasite proteins transported to the surface membrane of the infected erythrocyte.

Entamoeba histolytica: a eukaryote without glutathione metabolism

Entamoeba histolytica was found to grow normally without producing glutathione and the main enzymes of glutathione metabolism, indicating that glutathione is not essential for many eukaryotic processes. This parasitic amoeba is an unusual eukaryote whose special features may help define the crucial functions of glutathione in those eukaryotes that do use it. Since Entamoeba histolytica lacks mitochondria and the usual aerobic respiratory pathways, the finding that it grows without glutathione and other evidence support the hypothesis that a primary function of glutathione in eukaryotes involves protection against oxygen toxicity associated with mitochondria and suggest that eukaryotes may have acquired glutathione metabolism at the time that they acquired mitochondria.

Localization of the major Plasmodium falciparum glycoprotein on the surface of mature intraerythrocytic trophozoites and schizonts

The subcellular location of the major malarial glycoprotein in erythrocytes infected with schizonts of Plasmodium falciparum has been studied by two methods. In the first, glycoproteins were labelled with [3H]glucosamine or [3H]isoleucine during in vitro culture. Trypsin treatment of intact infected erythrocytes caused no major qualitative or quantitative changes in [3H]glucosamine labelled glycoproteins or [3H]isoleucine labelled proteins separated by sodium dodecylsulfate polyacrylamide gel electrophoresis. However, in the presence of Triton X-100 the labelled glycoproteins and proteins were completely cleaved by trypsin. In the second method, two monoclonal antibodies which specifically immunoprecipitate the major 195 kDa glycoprotein failed to react on indirect immunofluorescence with intact non-fixed schizont-infected erythrocytes, but reacted strongly with saponin released schizonts indicating specificity for the surface of mature intracellular parasites. Immunoelectronmicroscopy using ferritin-conjugated secondary antibody confirmed the location of the epitope(s) recognized by these monoclonals on the surface of intracellular parasites. Ferritin particles were not associated with knob-bearing erythrocyte membranes. The results indicate that only a small proportion or none of the 195 kDa glycoprotein is on the surface of the infected erythrocyte and that the largest proportion is expressed on the surface of mature intraerythrocytic parasites.

Susceptibility of Entamoeba histolytica to oxygen intermediates

To explore the susceptibility of the extracellular protozoan, Entamoeba histolytica, to toxic oxygen intermediates, trophozoites were exposed to fluxes of O2, H2O2, and OH. generated enzymatically by the glucose oxidase and xanthine oxidase reactions. HM-1 trophozoites were resistant to O2, but were readily killed by H2O2 alone. OH. and 1O2 were not required for effective amebicidal activity. The addition of peroxidase and halide enhanced trophozoite killing by H2O2. Sonicates of amebae contained virtually no catalase and little glutathione peroxidase activity which may contribute to susceptibility to H2O2. Coupled with our previous studies with Toxoplasma gondii and Leishmania spp. these observations indicate that there is a broad spectrum of susceptibility of intra- and extracellular pathogenic protozoa to killing by oxygen intermediates.

Plasma membrane of Entamoeba histolytica

Axenically propagated Entamoeba histolytica (HK9:NIH strain) were employed as starting material for the isoation of plasma membrane by a novel procedure. In the absence of known enzymatic markers, the externally disposed polypeptides of intact amoebae were iodinated and the incorporated label used to monitor membrane separation and recovery. 12 major plasma membrane polypeptides (12 x 10(3)-200 x 10(3) mol wt) were labeled and separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Each of these was a glycoprotein. Preincubation of amoebae with concanavalin A stabilized the plasma membranes as large sheets, facilitating its separation by low-speed centrifugation. Dissociation of the lectin with alpha-methyl mannoside, followed by additional homogenization led to vesiculation and further purification. The isolated plasma membrane was recovered in high yield (28%) and enriched 30-fold in terms of incorporated iodide. All iodinated surface glycoproteins of the intact organism were present in the plasma membrane fraction. A Ca++-dependent ATPase was enriched in the plasma membrane to a similar extent, but over one-half of the total activity was associated with internal, unlabeled membranes, suggesting a dual localization of this activity. The isolated plasma membrane was enriched in cholesterol and had a cholesterol:molar ratio of 0.87. It also contained larger amounts of an unusual phospholipid--ceramide aminoethyl phosphonate--a phospholipase-resistant species.