Thyroid hormone effects on LKB1, MO25, phospho-AMPK, phospho-CREB, and PGC-1alpha in rat muscle

Expression of all of the isoforms of the subunits of AMP-activated protein kinase (AMPK) and AMPK activity is increased in skeletal muscle of hyperthyroid rats. Activity of AMPK in skeletal muscle is regulated principally by the upstream kinase, LKB1. This experiment was designed to determine whether the increase in AMPK activity is accompanied by increased expression of the LKB1, along with binding partner proteins. LKB1, MO25, and downstream targets were determined in muscle extracts in control rats, in rats given 3 mg of thyroxine and 1 mg of triiodothyronine per kilogram chow for 4 wk, and in rats given 0.01% propylthiouracil (PTU; an inhibitor of thyroid hormone synthesis) in drinking water for 4 wk (hypothyroid group). LKB1 and MO25 increased in the soleus of thyroid hormone-treated rats vs. the controls. In other muscle types, LKB1 responses were variable, but MO25 increased in all. In soleus, MO25 mRNA increased with thyroid hormone treatment, and STRAD mRNA increased with PTU treatment. Phospho-AMPK and phospho-ACC were elevated in soleus and gastrocnemius of hyperthyroid rats. Thyroid hormone treatment also increased the amount of phospho-cAMP response element binding protein (CREB) in the soleus, heart, and red quadriceps. Four proteins having CREB response elements (CRE) in promoter regions of their genes (peroxisome proliferator-activated receptor-gamma coactivator-1alpha, uncoupling protein 3, cytochrome c, and hexokinase II) were all increased in soleus in response to thyroid hormones. These data provide evidence that thyroid hormones increase soleus muscle LKB1 and MO25 content with subsequent activation of AMPK, phosphorylation of CREB, and expression of mitochondrial protein genes having CRE in their promoters.

Molecular technology and antigenic variation among intraerythrocytic hemoparasites: do we see reality?

Antigenic variation is one mechanism of immune evasion utilized by many microorganisms--encompassing such broad evolutionary groups as viruses, bacteria, and protozoa--to survive the onslaught of a specifically activated host immune system. Because of its importance to the survival of many infectious agents there is considerable interest in understanding this phenomenon. With knowledge of the molecular mechanisms by which these microbes deliberately manipulate their genomes, it may be possible to disrupt the molecular machinery of the responsible genetic mechanisms. Among intraerythrocytic parasites, genetic mechanisms that have been observed or postulated to control antigenic variation include segmental gene conversion, epigenetically controlled in situ transcriptional switching, alterations of chromosomal structure associated with transcriptional control, and recombination during sexual reproduction. Likely, more than one type of mechanism is used by all organisms that undergo antigenic variation. In this paper, both the observed mechanisms and some of the molecular technology used to detect these mechanisms are discussed. While often seemingly straightforward from a technical standpoint, sometimes subtle differences in the methods used to study this process may affect what is observed. Some examples of this phenomenon are discussed in the context of a small selection of intraerythrocytic parasites.

Antigenic variation in babesiosis:is there more than one ˈwhyˈ?

Many intraerythrocytic hemoparasites survive the host immune system through rapid antigenic variation. Among babesial parasites antigenic variation has been demonstrated convincingly only for Babesia bovis and Babesia rodhaini. The molecular basis for antigenic variation in babesial parasites and its possible connection with cytoadherence and sequestration are discussed.

Selection of Babesia bovis-infected erythrocytes for adhesion to endothelial cells coselects for altered variant erythrocyte surface antigen isoforms

Sequestration of Babesia bovis-infected erythrocytes (IRBCs) in the host microvasculature is thought to constitute an important mechanism of immune evasion. Since Ig is considered to be important for protection from disease, an in vitro assay of B. bovis sequestration was used to explore the ability of anti-B. bovis Ig to interfere with IRBC cytoadhesion, and to identify IRBC surface Ags acting as endothelial cell receptors. Bovine infection sera reactive with the IRBC surface inhibited and even reversed the binding of IRBCs to bovine brain capillary endothelial cells (BBECs). This activity is at least partially attributable to serum IgG. IgG isolated from inhibitory serum captured the variant erythrocyte surface ag 1 (VESA1) in surface-specific immunoprecipitations of B. bovis-IRBCs. Selection for the cytoadhesive phenotype concurrently selected for antigenic and structural changes in the VESA1 Ag. In addition, the anti-VESA1 mAb, 4D9.1G1, proved capable of effectively inhibiting and reversing binding of adhesive, mAb-reactive parasites to BBECs, and by immunoelectron microscopy localized VESA1 to the external tips of the IRBC membrane knobs. These data are consistent with a link between antigenic variation and cytoadherence in B. bovis and suggest that the VESA1 Ag acts as an endothelial cell ligand on the B. bovis-IRBC.

The ves multigene family of B. bovis encodes components of rapid antigenic variation at the infected erythrocyte surface

B. bovis, an intraerythrocytic protozoal parasite, establishes chronic infections in cattle in part through rapid variation of the polymorphic, heterodimeric VESA1 protein on the infected erythrocyte surface and sequestration of mature parasites. We describe the characterization of the ves1 alpha gene encoding the VESA1a subunit, thus providing a description of a gene whose product is involved in rapid antigenic variation in a babesial parasite. This three-exon gene, a member of a multigene family (ves), encodes a polypeptide with no cleavable signal sequence, a single predicted transmembrane segment, and a cysteine/lysine-rich domain. Variation appears to involve creation and modification or loss of a novel, transcribed copy of the gene.

Cytoadherence of Babesia bovis-infected erythrocytes to bovine brain capillary endothelial cells provides an in vitro model for sequestration

Babesia bovis, an intraerythrocytic parasite of cattle, is sequestered in the host microvasculature, a behavior associated with cerebral and vascular complications of this disease. Despite the importance of this behavior to disease etiology, the underlying mechanisms have not yet been investigated. To study the components involved in sequestration, B. bovis parasites that induce adhesion of the infected erythrocytes (IRBCs) to bovine brain capillary endothelial cells (BBEC) in vitro were isolated. Two clonal lines, CD7(A+I+) and CE11(A+I-), were derived from a cytoadherent, monoclonal antibody 4D9.1G1-reactive parasite population. This antibody recognizes a variant, surface-exposed epitope of the variant erythrocyte surface antigen 1 (VESA1) of B. bovis IRBCs. Both clonal lines were cytoadhesive to BBEC and two other bovine endothelial cell lines but not to COS7 cells, FBK-4 cells, C32 melanoma cells, or bovine brain pericytes. By transmission electron microscopy, IRBCs were observed to bind to BBEC via the knobby protrusions on the IRBC surface, indicating involvement of components associated with these structures. Inhibition of protein export in intact, trypsinized IRBCs ablated both erythrocyte surface reexpression of parasite protein and cytoadhesion. IRBCs allowed to recover surface antigen expression regained the ability to bind endothelial cells, demonstrating that parasite protein export is required for cytoadhesion. We propose the use of this assay as an in vitro model to study the components involved in B. bovis cytoadherence and sequestration.

Antigenic variation in Babesia bovis: how similar is it to that in Plasmodium falciparum?

Despite significant differences in some aspects of their life-cycles, the Apicomplexan parasites Babesia bovis and Plasmodium falciparum share many parallels. Significant among these are participation in rapid, clonal antigenic variation, and cyto-adherence and sequestration in the deep vasculature. Antigenic variation has long been thought to be primarily a mechanism of escape from antibody-mediated mechanisms of the host's immune system. In each species, the components demonstrated to participate in antigenic variation are parasite-derived proteins expressed on the infected erythrocyte's surface. Recently, the malarial component PfEMP1 has been found to be a multifunctional protein that is not only subject to antigenic variation, but also participates in cyto-adherence and rosetting (adhesion to uninfected erythrocytes). In the present review, the antigens elaborated on the surface of an erythrocyte infected with B. bovis, for immune evasion via antigenic variation, are described, and compared and contrasted with those from P. falciparum. The significance of the similarities between B. bovis and P. falciparum, and the potential for contributions to be made to our understanding of malaria through the study of babesiosis are discussed.

Characterization of a variant erythrocyte surface antigen (VESA1) expressed by Babesia bovis during antigenic variation

Babesia bovis, an intraerythrocytic, protozoal parasite of cattle, undergoes clonal antigenic variation (Allred DR, Cinque RM, Lane TJ, Ahrens KP. Infect Immun 1994;62:91-98). This ability could provide a mechanism by which the parasite escapes host immune defenses to establish chronic infection. Previous work identified two parasite-derived antigens of Mr 128,000 and 113,000 that were present on the surface of the infected erythrocyte and appeared to be associated with clonal antigenic variation (Allred DR, Cinque RM, Lane TJ, Ahrens KP. Infect Immun 1994;62:91 98). Two monoclonal antibodies (mAbs), 3F7.1H11 and 4D9.1G1, which recognize the variant erythrocyte surface antigen (VESA1) have been identified. These mAbs react only with the surface of erythrocytes infected with the B. bovis C9.1 clone in live-cell immunofluorescence assays. In both conventional and surface immunoprecipitations, the mAbs precipitate a variant antigen doublet that matches in mass the infected red blood cell (IRBC) surface antigens precipitated with bovine serum. In contrast, Western blot analysis revealed that only the Mr 128,000 polypeptide is recognized by the mAbs. Neither mAb recognizes antigenically variant progenitor or progeny parasite clones in any of the immunoassays, confirming the involvement of this antigen in rapid clonal antigenic variation. Failure to label this antigen with [9,10(n)-3H]myristic acid, [9,10(n)-3H]palmitic acid or D-[6-3H]glucosamine indicates that these polypeptides are neither N-glycosylated nor fatty acylated. Identity of the variant antigen recognized by the mAbs with that putatively identified with immune serum was confirmed by comparison of partial proteolytic digestion products. Unambiguous identification of the VESA1 antigen as a component of antigenic variation will facilitate characterization of the events leading to antigenic variation on the B. bovis-infected erythrocyte surface and its significance to parasite survival during chronic infection.

Immunochemical methods for identification of Babesia bovis antigens expressed on the erythrocyte surface

Intraerythrocytic parasites, such as Babesia bovis, modify the erythrocyte plasma membrane structurally, antigenically, and functionally. For such parasites the infected erythrocyte surface also is thought to be a primary site for interaction with the host immune system. These properties demand characterization of the various alterations to understand the overall host-parasite interaction, immunity to disease or infection, and bases for parasite persistence. A paucity of adequate methods exists for characterization of parasite-derived components of the parasitized erythrocyte surface. To facilitate such studies we developed or modified several techniques to detect, identify, and localize parasite-induced alterations on the B. bovis-infected erythrocyte surface. These methods, which we present here, should be adaptable to a variety of intraerythrocytic parasite-host combinations.

Immune Evasion by Babesia bovis and Plasmodium falciparum: Cliff-dwellers of the Parasite World

Erythrocyte-dwelling parasites, such as Babesia bovis and Plasmodium falciparum, are not accessible to the host immune system during most of their asexual reproductive cycle because they are intracellular. While intracellular, the host immune response must be directed toward the surface of the infected erythrocyte. Immune individuals mount protective antibody and cell-mediated responses which eliminate most of the parasites, yet some survive to establish chronic infections. In this review, David Allred discusses some of the mechanisms used by these parasites to evade individual immune mechanisms targeting the infected erythrocyte to survive in the hostile environment of an effective immune response.

Babesia bigemina: host factors affecting the invasion of erythrocytes

Babesia bigemina merozoites enter their host's erythrocytes by an unknown mechanism that likely involves parasite surface components. Identification of the parasite ligands involved in invasion is hampered by a lack of basic information about the invasion characteristics of Babesia bigemina. Therefore, restrictions on the species of red blood cells (RBC) that are susceptible to invasion were examined as well as the roles of erythrocyte ligands. An invasion assay and a proliferation assay were developed for this study. Unlike some other species of Babesia that infect cattle, B. bigemina failed to enter RBC from most animals that are not natural hosts, suggesting that a species restricted receptor mechanism mediates invasion. Two carbohydrates which are prominent on the surface of bovine erythrocytes, N-acetylglucosamine and N-acetylgalactosamine, when added to cultures, reduced the ability of B. bigemina merozoites to invade erythrocytes. Neuraminidase or trypsin treatment of bovine erythrocytes significantly decreased their susceptibility to invasion whereas chymotrypsin had little effect. These data imply that proteinaceous erythrocyte ligands and carbohydrate residues may be involved in the invasion process. Identification of a species-specific pattern of invasion and RBC treatments that render cells refractory to invasion may provide the basis for the characterization of B. bigemina erythrocyte binding molecules based on their differential binding to invasion competent and refractory cells.

Putative adhesins of Anaplasma marginale: major surface polypeptides 1a and 1b

Genes for the MSP1a and MSP1b subunits of the Anaplasma marginale surface antigen complex MSP1 were previously cloned and expressed in Escherichia coli. We report here the localization of MSP1a and MSP1b polypeptides on the surface of recombinant E. coli by using a live cell indirect immunofluorescent antibody assay. Recombinant E. coli cells expressing the msp1 alpha gene or the msp1 beta gene encoding the MSP1a and MSP1b polypeptide subunits, respectively, were shown by a culture recovery adhesion assay and by direct microscopic examination to specifically adhere to bovine erythrocytes. This adhesion was more than additive when both genes were coexpressed in a single recombinant construct. Similarly, these recombinants hemagglutinated bovine erythrocytes in a microtiter hemagglutination assay. Inhibition of recombinant E. coli adhesion to bovine erythrocytes and hemagglutination inhibition were observed in the presence of homologous monospecific polyclonal antiserum raised against purified MSP1a or MSP1b polypeptide. These data suggest that the MSP1a and MSP1b polypeptides have functions as adhesins on A. marginale initial bodies, probably during erythrocyte invasion.

Characterization of hemagglutinating components on the Anaplasma marginale initial body surface and identification of possible adhesins

Interaction of Anaplasma marginale initial bodies with the bovine erythrocyte surface was examined by a direct hemagglutination assay. Purified initial bodies were shown to specifically hemagglutinate bovine erythrocytes but not erythrocytes from nonhost animal species. Hemagglutination was inhibited by treatment of purified initial bodies with trypsin, alpha-chymotrypsin, or proteinase K but not by treatment with neuraminidase or sodium periodate. Treatment of bovine erythrocytes with alpha-chymotrypsin or neuraminidase partially inhibited hemagglutination of the treated cells by initial bodies. In contrast, no inhibition occurred after treatment of erythrocytes with trypsin, phospholipases, or sodium periodate or when monosaccharides and disaccharides were used as potential competitive inhibitors. Thus, the initial body receptor is probably a surface protein, whereas the bovine receptor may comprise both protein and carbohydrate. Hemagglutination was unaffected by treatment of initial bodies with monoclonal or polyclonal antibodies raised against the A. marginale 31-kDa (MSP4) major surface polypeptide or non-A. marginale proteins or by treatment with a monoclonal antibody to the A. marginale MSP1a neutralization-sensitive epitope. In contrast, antiserum raised against whole A. marginale initial bodies or monospecific antibodies raised against purified A. marginale major surface polypeptides with molecular sizes of 105 (MSP1a), 100 (MSP1b), 61, and 36 (MSP2) kDa completely or partially inhibited hemagglutination. These data confirm the proposed surface location of the proteins susceptible to inhibition and suggest that they mediate hemagglutination of bovine erythrocytes. We propose that these surface proteins are possible adhesins.

Polypeptides reactive with antibodies eluted from the surface of Babesia bovis-infected erythrocytes

A technique was sought that would enable identification of surface-exposed parasite antigens on Babesia bovis-infected erythrocytes (BbIE) that are not detectable by surface-specific immunoprecipitations. Antibodies which bind to the surface of BbIE were recovered from intact cells using a low pH wash procedure. The eluted antibodies were then used in conventional immunoprecipitation assays to identify parasite-synthesized polypeptides carrying epitopes that are exposed on the surface or are cross-reactive with such epitopes. The results of these experiments support our previous data, obtained using a surface-specific immunoprecipitation technique, in the identification of a repertoire of parasite-derived antigens on the surface of infected erythrocytes (Allred et al., 1991). In addition, two polypeptides of M(r) 68,000 and 185,000 were identified which react strongly with the eluted antibodies but which are not detected by surface-immunoprecipitation. These data illustrate the potential of this approach for identification of parasite polypeptides which carry epitopes exposed on, or cross-reactive with exposed epitopes of the infected erythrocyte surface.

Antigenic variation of parasite-derived antigens on the surface of Babesia bovis-infected erythrocytes

The hemoparasite Babesia bovis antigenically alters the bovine erythrocyte membrane surface by expression of isolate-specific, parasite-derived polypeptides. To determine whether antigenic variation also occurred on the infected erythrocyte surface, a calf was infected once with parasitized erythrocytes carrying the C9.1 clonal line of B. bovis. In vitro cultures then were established periodically from the peripheral blood and analyzed with sequentially collected sera from the same animal. The surface reactivity of infected erythrocytes cultured from the infected animal varied over time, on the basis of reactivity in live cell immunofluorescence, surface immunoprecipitation, and panning assays. Subclones C8 and H10, established from day 41 cultures, were analyzed immunochemically. A loss of immunoreactivity was observed in antigens corresponding to the 113- and 128-kDa parasite-derived antigens of clone C9.1, demonstrating epitopic variation in these antigens; the immunochemical recognition of these antigens paralleled the results of live cell immunofluorescence and panning assays. Concomitant size polymorphism suggested polypeptide structural variation of these antigens as well. Calves infected by inoculation of infected blood or by injection of cloned parasites from in vitro cultures rapidly developed antibodies which cross-reacted among the clonal variant lines, suggesting the presence of common as well as unique epitopes. These results demonstrate that antigenic variation occurs on the surface of B. bovis-infected erythrocytes and that the parasite-derived antigens of 113 and 128 kDa compose at least a part of the antigens undergoing variation.

Isolate-specific parasite antigens of the Babesia bovis-infected erythrocyte surface

Bovine erythrocytes taken from in vitro cultures of Babesia bovis parasites from Mexico and the United States were assayed for the presence of new epitopes on the erythrocyte surface. New surface-exposed epitopes were detected by means of a whole-cell antigen capture assay. These epitopes were subsequently demonstrated only on infected erythrocytes by immunofluorescence staining of intact, living cells. Parasite-synthesized antigens were identified on each isolate using a surface-specific immunoprecipitation technique to analyze metabolically-labeled infected erythrocytes. In the Mexico isolate these antigens were 120 kDa and 107 kDa, whereas in the United States isolate polypeptides of 135, 120 and 107 kDa were detected. In each of these assays, reaction of immune sera with the infected erythrocyte surface was found to be isolate-specific.

A nonsubjective assay for antigenic modifications of the Babesia bovis-parasitized erythrocyte surface

Intracellular protozoan parasites induce numerous alterations in the invaded host cell, including antigenic modifications of the host cell plasma membrane. We have developed a quantifiable, non-subjective assay for the detection of novel antigenic reactivities on the host cell surface using as a model system bovine erythrocytes infected with Babesia bovis. Infected erythrocytes, metabolically labeled with L-[35S]methionine, were sensitized by incubation with bovine immune serum, then were captured in microtiter plates coated with rabbit anti-bovine IgG antibody. This technique enabled specific capture of B. bovis-infected cells with immune infection sera raised against B. bovis but not with similar sera raised against Babesia bigemina. This assay should be easily applicable to the study of other parasitic diseases.

The msp1 beta multigene family of Anaplasma marginale: nucleotide sequence analysis of an expressed copy

A gene for the beta subunit of the immunoprotective surface antigen MSP-1 of Anaplasma marginale was previously cloned and expressed in Escherichia coli. A nucleic acid probe based on this gene detects A. marginale infection in carrier cattle and in the tick vector. We report here the sequence and structural features of the cloned msp1 beta gene and expressed polypeptide. The gene codes for a polypeptide of 756 amino acids that contains domains of tandemly repeated sequence and glutamine-rich regions at the N and C termini. The cloned copy is a member of a multigene family with multiple restriction fragment length polymorphisms in isolates of this rickettsia from different geographical regions. The availability of the sequence will allow use of the polymerase chain reaction in diagnostic assays and the preparation and testing of different vaccine constructs in cattle.

Molecular basis for surface antigen size polymorphisms and conservation of a neutralization-sensitive epitope in Anaplasma marginale

Anaplasmosis is one of several tick-borne diseases severely constraining cattle production and usage in many parts of the world. Cattle can be protected from anaplasmosis by immunization with major surface protein 1, a surface protein of Anaplasma marginale carrying a neutralization-sensitive epitope. Marked size polymorphisms exist among different isolates of A. marginale in the AmF105 subunit of major surface protein 1, yet all isolates still contain the neutralization-sensitive epitope. To clarify the basis for these observations, the mspl alpha gene encoding AmF105 was cloned from four isolates and sequenced. The encoded polypeptides share a high degree of overall homology between isolates but contain a domain with various numbers of tandemly repeated sequences and three regions of clustered amino acid substitutions outside the repeat domain. The polypeptide size differences are completely explained by the variations in the numbers of tandem repeat units. We have mapped the neutralization-sensitive epitope to a sequence that is present within each repeat unit. These results identify a basis for size polymorphisms of the surface polypeptide antigen concomitant with B-cell epitope conservation in rickettsiae.

Detection and quantitation of Anaplasma marginale in carrier cattle by using a nucleic acid probe

Cattle which have recovered from acute infection with Anaplasma marginale, a rickettsial hemoparasite of cattle, frequently remain persistently infected with a low-level parasitemia and serve as reservoirs for disease transmission. To fully understand the role of these carriers in disease prevalence and transmission, it is essential that low levels of parasitemia can be accurately detected and quantitated. We have developed a nucleic acid probe, derived from a portion of a gene encoding a 105,000-molecular-weight surface protein, that can detect A. marginale-infected erythrocytes. The probe is specific for A. marginale and can detect 0.01 ng of genomic DNA and 500 to 1,000 infected erythrocytes in 0.5 ml of blood, which is equivalent to a parasitemia of 0.000025%. This makes the probe at least 4,000 times more sensitive than light microscopy. Hybridization of the probe with treated blood from animals proven to be carriers of anaplasmosis showed that parasitemia levels were highly variable among carriers, ranging from greater than 0.0025 to less than 0.000025%. Parasitemia levels of individual animals on different dates were also variable. These results imply that, at any given time, individuals within a group of cattle may differ significantly in their abilities to transmit disease.

Hierarchical Response of Light Harvesting Chlorophyll-Proteins in a Light-Sensitive Chlorophyll b-Deficient Mutant of Maize

The light-sensitive chlorophyll b (Chl b)-deficient oil yellow-yellow green (OY-YG) mutant of maize (Zea mays) grown under conditions of high light exhibits differential reductions in the accumulation of the three major Chl b-containing antenna complexes and characteristic changes in thylakoid architecture. When observed by freeze-fracture electron microscopy, the most notable changes in the OY-YG thylakoid structure are: (a) a major reduction in the number of 8 nanometer particles of the protoplasmic fracture face of stacked membrane regions (PFs) paralleled by a 60% reduction in the chlorophyll-proteins (CP) associated with the peripheral light harvesting complex (LHCII) for photosystem II (PSII) and which give rise to the LHCII oligomer/monomer (CPII(*)/CPII) bands on mildly dissociated green gels; (b) a sizable decrease in the proportion of 11 to 13 nanometer particles of the protoplasmic fracture face of unstacked membrane regions (PFu) that parallels the loss of light harvesting complex I (LHCI) antennae from photosystem I (PSI) centers and a 40% reduction of the band containing CP1 and LHCI (CPI(*)) on mildly dissociating green gels; (c) an unchanged or slightly increased average size of particles of the exoplasmic fracture face of stacked (or appressed) membrane regions (EFs) along with a relative increase in CP29, the postulated bound LHC of PSII, and of CP47 and CP43, PSII core antenna complexes. This latter result sets the OY-YG mutant apart from all other Chl b-deficient mutants studied to date, all of which possess EFs particles that are substantially reduced in size. Based on these findings, we postulate that the bound LHCII associated with EFs particles consists mostly of CP29 chlorophyll proteins and very little, if any, CPII(*)/CPII chlorophyll proteins. Indeed, the CPII(*)/CPII chlorophyll proteins may be exclusively associated with the ;peripheral' LHCII units that give rise to 8 nanometer PF particles. The differential effect of the Chl b deficiency on the accumulation of the three main antenna complexes (CPII(*)/CPII>CPI(*)>CP29) suggests, furthermore, that there is a hierarchy among Chl b-binding proteins, and that this hierarchy might be an integral part of long-term photoregulation mediating Chl b partitioning in the chloroplast.

Assocation of the 33 kDa extrinsic polypeptide (water-splitting) with PS II particles: immunochemical quantification of residual polypeptide after membrane extraction

Various washing procedures were tested on Triton-prepared PS II particles for their ability to remove the 33 kDa extrinsic polypeptide (33 kDa EP) associated with the water-splitting complex. Residual 33 kDa EP was evaluated by Coomassie blue staining of SDS gels of washed particles and by Western blotting with an antibody specific for the 33 kDa EP. A wash with 16 mM Tris buffer, pH 8.3, inhibited water-splitting activity but did not remove all the 33 kDa EP. Sequential washes with 30 mM octyl glucoside (pH 8.0 and 6.8), and a single wash with 0.8 M Tris were also ineffective in removing all the 33 kDa EP. Washing with 1 M CaCl2 was more effective in removing 33 kDa EP; while only a faint trace of protein was detectable by Coomassie-staining, immunoblotting revealed a considerable remainder. The treated particles retained some water-splitting activity. The two step procedure of Miyao and Murata (1984) involving 1 M NaCl and 2.3 M urea was most effective, removing all but a trace of antibody positive protein. Our finding suggests that (1) the degree of depletion of the 33 kDa EP cannot be judged on the basis of Coomassie stain alone, and (2) this extrinsic protein is very tightly associated with the membrane, perhaps via a hydrophilic portion of this otherwise hydrophilic protein. The results also suggest that the presence or absence of the 33 kDa protein per se is not the primary determinant of residual water splitting activity.

Implications of cytochrome b6/f location for thylakoidal electron transport

The cytochrome b6/f complex of higher plant chloroplasts is uniformly distributed throughout both appressed and nonappressed thylakoids, in contrast to photosystem II and photosystem I, the other major membrane protein complexes involved in electron transport. We discuss how this distribution is likely to affect interactions of the cytochrome b6/f complex with other electron transport components because of the resulting local stoichiometries, and how these may affect the regulation of electron transport.

Spatial organization of the cytochrome b6-f complex within chloroplast thylakoid membranes

The spatial distribution of the chloroplast thylakoid protein complex comprised of cytochromes f and b-563, and the Rieske iron-sulfur protein (Cyt b6-f) has been controversial because of conflicting results obtained by different techniques. We have combined the following biochemical and immunochemical techniques to approach this question: (1) French press disruption of thylakoids, followed by repeated two-phase aqueous polymer partitioning to separate inside-out grana from right-side-out stroma membrane fragments; (2) electrophoretic analysis followed by the 3,3',5,5'-tetramethylbenzidine stain for cytochrome hemes; (3) electroblot analysis with anti-Cyt b6-f antibodies; (4) agglutination of membrane fragments with anti-Cyt b6-f antibodies; and (5) post-embedment thin-section immunolabeling of chemically fixed or ultrarapidly frozen chloroplasts with anti-Cyt b6-f antibodies. Our results indicate that the complex is present in both of the isolated membrane fragment populations in similar amounts, with the bulk of the immunoreactive sites exposed to the thylakoidal lumen. Direct immunolabeling of thin-sectioned chloroplasts resulted in localization of the complex throughout the thylakoids, without specialized compartmentation. These results provide both the temporal and spatial resolution necessary for accurate localization of the complex. We concur with models proposing distribution of Cyt b6-f throughout all thylakoid membranes.

Dynamic rearrangements of erythrocyte membrane internal architecture induced by infection with Plasmodium falciparum

Cultured human erythrocytes infected with Plasmodium falciparum were studied by freeze-fracture electron microscopy. Special emphasis was placed upon the formation of the membrane surface excrescences (‘knobs’) found on red cells containing mature parasites. Knobs were visualized as conoid projections of the protoplasmic fracture face (PF) and depressions of the exoplasmic fracture face (EF). Knob formation was correlated with parasite growth and, on the basis of the organization of intramembranous particles (IMP) in the PF leaflet, a series of changes associated with parasite maturation was discerned: (1) a focal IMP cluster with minimal erythrocyte membrane elevation; (2) an elevated central IMP cluster surrounded by an IMP-free zone and concentric IMP ring; (3) maximal erythrocyte membrane deformation, concomitant with a loss of obvious IMP organization. Subtle changes in PF IMP organization were seen with knob formation and parasite maturation, including an apparent lateral partitioning of endogenous red cell membrane proteins between knobby or knob-free membrane areas in trophozoite-infected cells. IMP size distributions of the PF were shifted toward smaller particles in schizont-infected cells. Parasite development did not affect IMP densities in the PF; however, a decrease from 464 +/− 106 micron-2 to 374 +/− 94 micron-2 was seen in the EF of schizont-infected cells. IMP densities were similar over knobs and knob-free areas of either membrane leaflet, and there was no apparent EF IMP reorganization associated with the presence of knobs. These findings indicate that dynamic membrane changes are associated with knob formation and parasite maturation.

Lateral Distribution of the Cytochrome b(6)/f and Coupling Factor ATP Synthetase Complexes of Chloroplast Thylakoid Membranes

We have visualized directly the distribution of the cytochrome b(6)/f and coupling factor ATP synthetase complexes in thylakoid membranes of embedded, thin-sectioned, intact chloroplasts by using rabbit antibodies directed against each complex, followed by ferritin-conjugated goat anti- (rabbit immunoglobulin G) antibodies. The labeling patterns indicate that in spinach (Spinacia oleracea) chloroplasts the cytochrome b(6)/f complex is distributed laterally throughout both stacked grana and unstacked stroma membrane regions, whereas the coupling factor ATP synthetase complex is found exclusively in stroma thylakoids and in the marginal and end membranes of grana.

Developmental modulation of protein synthetic patterns by the human malarial parasite Plasmodium falciparum

Under conditions of in vitro culture, Plasmodium falciparum incorporated amino acids into particulate (membrane) and soluble proteins in a pattern which changed sequentially and which was dependent upon the stage of parasite maturation. Synchronized cultures pulse labeled with a mixture of 15 14C-labeled amino acids or [14C]histidine alone displayed stage-related patterns of polypeptide biosynthesis. Certain plasmodial proteins were associated with both particulate (membrane) and soluble fractions, whereas others appeared to be specific to a given fraction. Proteolysis of intact infected cells with pronase under conditions which removed 97 +/- 2.2% of the endogenous red cell acetylcholinesterase activity did not cause the apparent removal of any radiolabeled proteins; this suggests the absence of externally exposed, parasite-synthesized proteins in the infected red cell membrane. Such a result was consistent whether the radiolabel was [14C]histidine or the 14C-labeled amino acid mixture. These results indicate that specific modulation of parasite biosynthetic patterns occurs during the asexual reproductive cycle and is probably one mechanism whereby parasite differentiation occurs. Despite the formation of surface excrescences on infected red cells containing mature parasites, results of surface digestion experiments failed to demonstrate the presence of surface-exposed plasmodial proteins.

Scanning electron microscope-analysis of the protrusions (knobs) present on the surface of Plasmodium falciparum-infected erythrocytes

The nature of the surface deformations of erythrocytes infected with the human malaria parasite Plasmodium falciparum was analyzed using scanning electron microscopy at two stages of the 48-h parasite maturation cycle. Infected cells bearing trophozoite-stage parasites (24-36 h) had small protrusions (knobs), with diameters varying from 160 to 110 nm, and a density ranging from 10 to 35 knobs X micron-2. When parasites were fully mature (schizont stage, 40-44 h), knob size decreased (100-70 nm), whereas density increased (45-70 knobs X micron-2). Size and density of the knobs varied inversely, suggesting that knob production (a) occurred throughout intraerythrocytic parasite development from trophozoite to schizont and (b) was related to dynamic changes of the erythrocyte membrane. Variation in the distribution of the knobs over the red cell surface was observed during parasite maturation. At the early trophozoite stage of parasite development, knobs appeared to be formed in particular domains of the cell surface. As the density of knobs increased and they covered the entire cell surface, their lateral distribution was dispersive (more-than-random); this was particularly evident at the schizont stage. Regional surface patterns of knobs (rows, circles) were seen throughout parasite development. The nature of the dynamic changes that occurred at the red cell surface during knob formation, as well as the nonrandom distribution of knobs, suggested that the red cell cytoskeleton may have played a key role in knob formation and patterning.