The dynamic nature of the Golgi complex

The intracellular transport of newly synthesized G protein of vesicular stomatitis virus is blocked at 20 degrees C and this spanning membrane glycoprotein accumulates in the last Golgi compartment, the trans Golgi-network (TGN). Previous morphological evidence suggested that the TGN enlarged significantly under this condition. In the present study we have used stereological procedures to estimate the volume and surface area of the Golgi stack and the TGN of baby hamster kidney cells under different conditions. The results indicate that the increase in the size of the TGN at 20 degrees C is accompanied by a significant decrease in the surface area and volume of the preceding Golgi compartments. A similar effect is also seen in uninfected cells at 20 degrees C, as well as during normal (37 degrees C) infection with Semliki Forest virus. In the latter case, however, the decrease in the size of the Golgi stack and the increase in that of the TGN is not accompanied by inhibition of transport from the Golgi complex to the cell surface. The results indicate that the Golgi stack and the TGN are dynamic and interrelated structures that are capable of rapid alteration in total surface area in response to changes in the rates of membrane transport.     

Transepithelial transport of a viral membrane glycoprotein implanted into the apical plasma membrane of Madin-Darby canine kidney cells. II. Immunological quantitation

The envelope of vesicular stomatitis virus was fused with the apical plasma membrane of Madin-Darby canine kidney cells by low pH treatment. The fate of the implanted G protein was then followed using a protein A- binding assay, which was designed to quantitate the amount of G protein in the apical and the basolateral membranes. The implanted G protein was rapidly internalized at 31 degrees C, whereas at 10 degrees C no uptake was observed. Already after 15 min at 31 degrees C, a fraction of the G protein could be detected at the basolateral membrane. After 60 min 25-48% of the G protein was basolateral as measured by the protein A-binding assay. At the same time, 25-33% of the implanted G protein was detected at the apical membrane. Internalization of G protein was not affected by 20 mM ammonium chloride or by 10 microM monensin. However, the endocytosed G protein accumulated in intracellular vacuoles and redistribution back to the plasma membrane was inhibited. We conclude that the implanted G protein was rapidly internalized from the apical surface of Madin-Darby canine kidney cells and a major fraction was routed to the basolateral domain.     

Post-Golgi membrane traffic: brefeldin A inhibits export from distal Golgi compartments to the cell surface but not recycling

Recent studies using the fungal metabolite brefeldin A (BFA) have provided important insights into the dynamics and the organization of the ER/Golgi membrane system. Here we examined the effect of BFA on the functional integrity of the distal part of the secretory pathway, i.e., transport between trans-Golgi cisternae and the cell surface. To assay export via the constitutive pathway, we followed the movement of vesicular stomatitis virus (VSV) G glycoprotein that had been accumulated in the trans-Golgi network (TGN) by incubation of infected BHK-21 cells at 20 degrees C. Addition of BFA rapidly and reversibly inhibited cell surface transport of G protein. The block to secretion was not due to redistribution of externalized G protein to internal pools. It was also not due to collapse of TGN to the ER, since VSV G protein blocked in treated cells resided in compartments that were distinct from the ER/Golgi system. Similar effects were found with a bulk-flow marker: BFA blocked constitutive secretion of glycosaminoglycan chains that had been synthesized and sulfated in the trans-Golgi cisternae. To examine export via the regulated secretory pathway, we assayed secretion of [35S]SO4 labeled secretogranin II from PC12 cells, a marker that has been used to study secretory granule budding from the TGN (Tooze, S. A., U. Weiss, and W. B. Huttner. 1990. Nature [Lond.]. 347:207-208). BFA potently inhibited secretion of sulfated secretogranin II induced by K+ depolarization. Inhibition was at the level of granule formation, since BFA had no effect on regulated secretion from preformed granules. Taken together, the results suggest that BFA blocks export via both the constitutive and the regulated pathways. In contrast, endocytosis and recycling of VSV G protein were not blocked by BFA, consistent with previous studies that endocytosis is unaffected (Misumi, Y., Y. Misumi, K. Miki, A Takatsuki, G. Tamura, and Y. Ikehara. 1986. J. Biol. Chem. 261:11398-11403). These and earlier results suggest that the exo/endocytic pathway of mammalian cells consist of two similar but distinct endomembrane systems: an ER/Golgi system and a post-Golgi system. BFA prevents forward transport without affecting return traffic in both systems.     

Morphological localization of a major lysosomal membrane glycoprotein in the endocytic membrane system

We have raised specific polyclonal immunoglobulin G (IgG) against a major lysosomal membrane sialoglycoprotein (LGP107) taken from rat liver and have prepared a conjugate of its Fab' fragment with horseradish peroxidase (HRP-anti LGP107 Fab') as a probe for the subcellular antigen. Electron immunocytochemistry in primary cultured rat hepatocytes showed that LGP107 resided primarily within lysosomes and was associated with luminal amorphous materials as well as limiting membranes. In addition, LGP107 was shown to be substantially distributed throughout the endocytic vacuolar system. The glycoprotein was found clustered in coated pits at the cell surface and localized along the surrounding membranes in endocytic vesicles. When cultured cells were exposed to HRP-anti LGP107 Fab', the antibody which was bound to its antigen within the coated pits was internalized via a system of endocytic vesicles and transported to lysosomes. During 20 min of incubation at 37 degrees C, the HRP tracer appeared at an early stage in small vesicles and moved progressively to larger vesicles, including multivesicular bodies. After 1 h, the tracer could be clearly seen in lysosomes heterogeneous in shape and size. The existence of LGP107 in endocytic compartments and the uptake of anti LGP107 antibody by hepatocytes were not blocked by prior treatment of the cells with cycloheximide and excess amounts of anti LGP107 IgG. These data suggest that LGP107 circulates between the cell surface and lysosomes through the endocytic membrane traffic in hepatocytes.     

Brefeldin A redistributes resident and itinerant Golgi proteins to the endoplasmic reticulum

Brefeldin A (BFA) has been reported to block protein transport from the ER and cause disassembly of the Golgi complex. We have examined the effects of BFA on the transport and processing of the vesicular stomatitis virus G protein, a model integral membrane protein. Delivery of G protein to the cell surface was reversibly blocked by 6 micrograms/ml BFA. Pulse-label experiments revealed that in the presence of BFA, G protein became completely resistant to endoglycosidase H digestion. Addition of sialic acid, a trans-Golgi event, was not observed. Despite processing by cis- and medial Golgi enzymes, G protein was localized by indirect immunofluorescence to a reticular distribution characteristic of the ER. By preventing transport of G protein from the ER with the metabolic inhibitor carbonyl cyanide m-chlorophenylhydrazone or by use of the temperature-sensitive mutant ts045, which is restricted to the ER at 40 degrees C, we showed that processing of G protein occurred in the ER and was not due to retention of newly synthesized Golgi enzymes. Rather, redistribution of preexisting cis and medial Golgi enzymes to the ER occurred as soon as 2.5 min after addition of BFA, and was complete by 10-15 min. Delivery of Golgi enzymes to the ER was energy dependent and occurred only at temperatures greater than or equal to 20 degrees C. BFA also induced retrograde transport of G protein from the medial Golgi to the ER. Golgi enzymes were completely recovered from the ER 10 min after removal of BFA. These findings demonstrate that BFA induces retrograde transport of both resident and itinerant Golgi proteins to the ER in a fully reversible manner.     

Protein 4.1 is involved in a structural thermotropic transition of the red blood cell membrane detected by a spin-labeled stearic acid

Proteins involved in a structural transition in red blood cell membranes detected at 8 +/- 1.5 degrees C by a stearic acid spin-label have been investigated. Calcium loading of red blood cells with ionophore A23187 caused the disappearance of the 8 degrees C transition. Protein 4.1 appears to be the most susceptible protein to Ca2+ treatment. Antibodies specific for spectrin, band 3 (43K cytoplasmic domain), and protein 4.1 have been utilized as specific probes to modify membrane thermotropic properties. The 8 degrees C transition was eliminated by anti-4.1 protein antibodies but was not modified by the other antibodies. To further characterize the protein(s) involved in the transition, ghosts were subjected to sequential extraction of skeletal proteins. The extraction of band 6, spectrin, and actin did not modify the 8 degrees C transition. In contrast, high-salt extraction (1 M KCl) of spectrin-actin-depleted vesicles, a procedure that extracts proteins 2.1 and 4.1, was able to eliminate the 8 degrees C transition. Rebinding of purified protein 4.1 to the high salt extracted vesicles restored the 8 degrees C transition. These results indicate the involvement of protein 4.1 in the transition and suggest a functional membrane association of this protein. The binding of protein 4.1 to the membrane seems to contribute significantly to the thermotropic properties of red blood cells.     

Proteolytic processing of pro-ACTH/endorphin begins in the Golgi complex of pituitary corticotropes and AtT-20 cells

The intracellular sites where proteolytic processing of pro-ACTH/endorphin or POMC take place have not yet been reliably identified. We have used affinity-purified antisera that recognize only the products of POMC processing and immunoelectron microscopy to identify the compartments of rat pituitary corticotropes and mouse AtT-20 cells in which cleavage occurs. Immunoperoxidase labeling of cryostat sections and immunogold labeling of ultrathin frozen sections were used for localization of the processing sites. By both procedures we detected processed peptides in Golgi cisternae and secretion granules. Within the Golgi, labeling was limited to the last or transmost cisterna and was most concentrated in its dilated rims which contain condensing secretory protein. No labeling of other Golgi cisternae was seen. All Golgi cisternae were labeled, however, when antisera that recognize unprocessed POMC were used for immunolabeling. We conclude that in AtT-20 and rat pituitary cells: 1) processing of POMC through at least two endo- and exoproteolytic cleavage steps and alpha-amidation of joining peptide begin in the trans Golgi subcompartment; 2) no detectable processing takes place before POMC reaches the trans Golgi cisterna; and 3) this Golgi cisterna as well as secretion granules contain the active enzymes necessary for proteolytic processing and alpha-amidation.     

Polarized delivery of viral glycoproteins to the apical and basolateral plasma membranes of Madin-Darby canine kidney cells infected with temperature-sensitive viruses

The intracellular route followed by viral envelope glycoproteins in polarized Madin-Darby canine kidney cells was studied by using temperature-sensitive mutants of vesicular stomatitis virus (VSV) and influenza, in which, at the nonpermissive temperature (39.5 degrees C), the newly synthesized glycoproteins (G proteins) and hemagglutinin (HA), respectively, are not transported out of the endoplasmic reticulum. After infection with VSV and incubation at 39.5 degrees C for 4-5 h, synchronous transfer of G protein to the plasma membrane was initiated by shifting to the permissive temperature (32.5 degrees C). Immunoelectron microscopy showed that under these conditions the protein moved to the Golgi apparatus and from there directly to a region of the lateral plasma membrane near this organelle. G protein then seemed to diffuse progressively to basal regions of the cell surface and, only after it had accumulated in the basolateral domain, it began to appear on the apical surface near the intercellular junctions. The results of these experiments indicate that the VSV G protein must be sorted before its arrival at the cell surface, and suggest that passage to the apical domain occurs only late in infection when tight junctions are no longer an effective barrier. In complementary experiments, using the temperature-sensitive mutant of influenza, cultures were first shifted from the nonpermissive temperature (39.5 degrees C) to 18.5 degrees C, to allow entrance of the glycoprotein into the Golgi apparatus (see Matlin, K.S., and K. Simons, 1983, Cell, 34:233-243). Under these conditions HA accumulated in Golgi stacks and vesicles but did not reach the plasma membrane. When the temperature was subsequently shifted to 32.5 degrees C, HA rapidly appeared in discrete regions of the apical surface near, and often directly above, the Golgi elements, and later diffused throughout this surface. To ensure that the anti-HA antibodies had access to lateral domains, monolayers were treated with a hypertonic medium to dilate the intercellular spaces. Some labeling was then observed in the lateral plasma membranes soon after the shift, but this never increased beyond 1.0 gold particle/micron, whereas characteristic densities of labeling in apical surfaces soon became much higher (approximately 10 particles/micron). Our results suggest that the bulk of HA follows a direct pathway leading from the Golgi to regions of the apical surface close to trans-Golgi cisternae.     

Assembly pathway of newly synthesized LamB protein an outer membrane protein of Escherichia coli K-12

The assembly of newly induced LamB protein (phage lambda receptor) was investigated in an operon fusion strain of Escherichia coli, in which the lamB gene is expressed under lac promoter control. The induction kinetics both for total cellular and for cell surface-exposed LamB protein were studied by immunochemical detection methods, using two distinct antisera directed against detergent-solubilized LamB trimers and completely denatured LamB monomers, respectively. Anti-trimer antibodies recognized both monomers and trimers, whereas anti-monomer antibodies only reacted with monomers. Provided appropriate solubilization conditions were used, both antisera were able to immunoprecipitate intracellular mature LamB protein quantitatively. Following induction, the first LamB antigenic determinants were detected after 60 to 80 seconds; detection of the newly synthesized protein by anti-monomer antibodies slightly preceded that by anti-trimer antibodies, a finding that could be partly explained by the observation that anti-monomer antibodies recognized a larger fraction of nascent LamB than did anti-trimer antibodies. Exposure of antigenic determinants at the cell surface was delayed for 30 to 50 seconds with respect to their synthesis. Therefore, either translocation or conformational changes must be rate-limiting in the series of processes that eventually convert the newly synthesized protein into its mature outer membrane state. LamB protein was found to occur in at least three clearly distinguishable states. State I is the LamB monomer, state II corresponds to a metastable trimer that dissociates in sodium dodecyl sulphate above 60 degrees C, and state III is the state LamB trimer that dissociates in sodium dodecyl sulphate only at temperatures above 90 degrees C. The chase kinetics of these states showed that conversion of newly synthesized LamB monomers to stable LamB trimers occurred in two stages: state I monomers were chased into metastable state II trimers rapidly (t 1/2 = 20 s), whereas stabilization of state II trimers to state III trimers was a relatively slow (t 1/2 = 5.7 min) process. Based on our results, a timing sequence in the assembly of outer membrane LamB protein is proposed.     

An enzymatic assay reveals that proteins destined for the apical or basolateral domains of an epithelial cell line share the same late Golgi compartments.

The expression of viral envelope proteins on the plasma membrane domains of the epithelial cell line, MDCK, is polar. Influenza virus infection of these cells leads to expression of the viral haemagglutinin and neuraminidase glycoproteins on the apical domain of the plasma membrane while vesicular stomatitis virus (VSV) infection yields basolateral expression of the sialic acid-bearing G protein. We have exploited the ability of the influenza neuraminidase to desialate the G protein of VSV to test for contact between these proteins during their intracellular transport to separate plasma membrane domains. We were able to select for VSV-G protein expression in doubly-infected cells because VSV protein production was accelerated in cells pre-infected with influenza virus. During double infection the envelope proteins of both viruses displayed the same polar localization as during single infection but the VSG-G protein was undersialated due to the action of the influenza neuraminidase. Incubation of singly-infected cells at 20 degrees C blocked the transport of VSV-G protein to the cell surface and resulted in increased sialation of the protein over that seen at 37 degrees C. This suggests that G protein is held in contact with the sialyl transferase at this temperature. 20 degrees C incubations of doubly-infected cells also produced the undersialated G protein characteristic of interaction with the neuraminidase. We conclude that most of the newly synthesised basolaterally-directed G protein is in physical contact with the majority of the neuraminidase through the terminal steps of Golgi processing.     

Transformation of Golgi membrane into the envelope of herpes simplex virus in rat anterior pituitary cells

Envelopment of herpes simplex virus type-1 (HSV-1) was investigated in relation to membrane differentiation in dissociated anterior pituitary cells. The number of cells stained positively with anti-HSV-1 serum was increased from 16 h to 31 h post infection. During this period, electron microscopy revealed that a number of nucleocapsids (unenveloped particles) were accumulated in the Golgi area, where they frequently became surrounded by a double membrane of short Golgi cisternae or by one with a Golgi associated endoplasmic reticulum lysosome (GERL)-like structure. The inner membrane of the cisterna surrounding the nucleocapsids showed regional specialization which was characterized by increased thickness and electron opacity. Acid phosphatase activity, a marker for GERL or trans Golgi cisternae, appeared in the cytoplasmic short cisternae surrounding the nucleocapsids, whereas glucose-6-phosphatase activity, a marker for the nuclear envelope or for endoplasmic reticulum, was not demonstrated in such cisternae. Monoclonal antibody against glycoprotein gD revealed that gD was localized in the trans Golgi membrane as well as in the envelope of the virion. The antibody-binding sites were highly concentrated in the area where Golgi membranes showed increased opacity. Furthermore, nucleocapsids were surrounded exclusively by gD-positive cisternal (Golgi or Golgi-derived) membranes. Thus, our results indicate that the envelope of HSV is derived from trans Golgi cisterna (GERL), and that some viral components, including gD, destined for the envelope may be assembled initially in the Golgi membrane, which is thereby transformed into the envelope of the virus.     

Mutational changes in the vesicular stomatitis virus glycoprotein affect the requirement of carbohydrate in morphogenesis.

The role of carbohydrate in the morphogenesis of vesicular stomatitis virus was studied, using the antibiotic tunicamycin to inhibit glycosylation. It has been reported previously (Gibson et al., J. Biol. Chem. 254:3600-3607, 1979) that the San Juan strain of vesicular stomatitis virus requires carbohydrate for efficient migration of the glycoprotein (G) to the cell surface and for virion formation, whereas the prototype or Orsay strain of vesicular stomatitis virus is less stringent in its carbohydrate requirement at 30 degrees C. However, there are many differences between the two strains. We found that mutational changes within the G protein of the same strain of virus (prototype or Orsay) alters the requirement for carbohydrate at 30 degrees C. Group V or G protein mutants tsO45 and tsO44, like their prototype parent, did not require carbohydrate for efficient morphogenesis. In contrast, the G protein of another group V mutant, tsO110, was totally dependent upon carbohydrate addition for migration to the cell surface. Furthermore, no tsO110 particles were released in the absence of glycosylation. The wild-type prototype strain did require carbohydrate at 39.5 degrees C for insertion of the G protein into the plasma membrane and virion formation. However, a pseudorevertant of tsO44 (tsO44R), unlike the prototype parent, no longer exhibited this temperature-sensitive requirement for carbohydrate. At 39.5 degrees C in the presence of tunicamycin, tsO44R-infected cells released normal yields of particles and the unglycosylated G reached the cell surface very efficiently. In contrast to tsO110, which absolutely requires carbohydrate, mutational change in the tsO44R G protein has eliminated the requirement for carbohydrate. Thus, simple mutational changes, as opposed to many changes in the molecule, are sufficient to alter the carbohydrate requirement.     

The peritrophic membrane as a barrier: its penetration by Plasmodium gallinaceum and the effect of a monoclonal antibody to ookinetes

We studied the point at which a monoclonal antibody (mAb C5) to a surface protein (Pgs25) on Plasmodium gallinaceum ookinetes blocked the infection of Aedes aegypti mosquitoes. The antibody did not block the development of zygotes to ookinetes in vitro. Development of ookinetes to oocysts in the mosquito was blocked to the same extent whether zygotes grew to ookinetes in the presence of mAb C5 or the antibody was added after the ookinetes had reached full development. When ookinetes developed in vitro in the presence of mAb C5, antibody remained on the surface of the parasite for the next 50 hr and did not block attachment to the peritrophic membrane. When ookinetes were fed to mosquitoes, two subpopulations of mosquitoes were observed (high numbers of oocysts per midgut and low numbers of oocysts per midgut). mAb C5 reduced the number of oocysts per midgut in the subpopulation that had low numbers of oocysts. The subpopulation that had high numbers of oocysts was unaffected by antibody, indicating that the antibody did not block invasion of the midgut epithelium. When mAb C5 was fed with gametocytes, the parasites invaded the epithelium at the same time (between 30 and 35 hr after the blood meal) as in controls, although at a markedly reduced rate. The ultrastructural observations were consistent with a block of parasites within the peritrophic membrane and not with a block at the epithelium, as parasites were not seen to accumulate within the space between the peritrophic membrane and the epithelium. The mechanism by which mAb C5 to Pgs25 of P. gallinaceum blocks the penetration of the peritrophic membrane remains undefined. We present evidence that the parasite modifies the peritrophic membrane during penetration, an observation first made for Babesia microti during penetration of the peritrophic membrane in Ixodes ticks. Ookinetes in the absence of antibodies appeared to disrupt the layers of the peritrophic membrane, suggesting an enzymatic mechanism for penetration.     

Monoclonal antibodies to either domain of ovotransferrin block binding to transferrin receptors on chick reticulocytes

Monoclonal antibodies produced to both chicken ovotransferrin and to the isolated N- and C-terminal half-molecule domains of ovotransferrin have been used to probe the interaction of ovotransferrin with its specific receptor on chick embryo red blood cells. Two antibodies to epitopes on the N-terminal domain and one antibody to an epitope on the C-terminal domain were able to block the binding of 125I-labeled diferric ovotransferrin to the receptor. When the cellular surface receptors were first saturated with ovotransferrin at 0 degrees C, none of these antibodies bound to the cell-associated ovotransferrin. This suggests that the antibodies are to epitopes which lie very near to, or in the regions of, the two domains which interact with receptor. The same three antibodies also blocked the binding to the receptor of ovotransferrin associated in situ from the isolated N- and C-terminal half-molecule domains. A fourth antibody did not block binding to receptor of 125I-labeled diferric ovotransferrin or the associated domains; furthermore, it was able to bind to ovotransferrin bound to the cell surface at 0 degrees C. This antibody thus appears to recognize an epitope remote from the receptor binding region of ovotransferrin. Additional evidence for the requirement of the presence of both domains of ovotransferrin to effect binding to the transferrin receptor on chick reticulocytes was obtained with a fifth antibody which recognized only the N-terminal half-molecule domain but not holo-ovotransferrin. Although this antibody had no effect on the binding of 125I-labeled ovotransferrin to cells, it blocked binding to receptor of the associated domains of ovotransferrin, presumably by inhibiting the association of the two domains.     

Electron tomography of RabA4b- and PI-4Kβ1-labeled trans Golgi network compartments in Arabidopsis

The trans Golgi network (TGN) of plant cells sorts and packages Golgi products into secretory (SV) and clathrin-coated (CCV) vesicles. We have analyzed of TGN cisternae in Arabidopsis root meristem cells by cell fractionation and electron microscopy/tomography to establish reliable criteria for identifying TGN cisternae in plant cells, and to define their functional attributes. Transformation of a trans Golgi cisterna into a Golgi-associated TGN cisterna begins with cisternal peeling, the formation of SV buds outside the plane of the cisterna and a 30-35% reduction in cisternal membrane area. Free TGN compartments are defined as cisternae that have detached from the Golgi to become independent organelles. Golgi-associated and free TGN compartments, but not trans Golgi cisternae, bind anti-RabA4b and anti-phosphatidylinositol-4 kinase (PI-4K) antibodies. RabA4b and PI-4Kβ1 localize to budding SVs in the TGN and to SVs en route to the cell surface. SV and CCV release occurs simultaneously via cisternal fragmentation, which typically yields ～30 vesicles and one to four residual cisternal fragments. Early endosomal markers, VHA-a1-green fluorescent protein (GFP) and SYP61-cyan fluorescent protein (CFP), colocalized with RabA4b in TGN cisternae, suggesting that the secretory and endocytic pathways converge at the TGN. pi4k1/pi4k2 knockout mutant plants produce SVs with highly variable sizes indicating that PI-4Kβ1/2 regulates SV size.     

Effect of temperature on alanine uptake by membrane vesicles isolated from a psychrophilic marine bacterium.

The effect of temperature on the membranes of Ant-300, a psychrophilic marine bacterium, was studied by measuring alanine uptake by isolated membrane vesicles. Uptake was observed from 0 to 35 degrees C. The maximum initial rate of uptake occurred at 25 degrees C although more alanine was ultimately taken up at temperatures from 10 to 20 degrees C. An ARRHENIUS plot of these data shows a single infection point at 7.8 degrees C. Within 10 min, over 50% of the alpha-aminoisobutyric acid taken up by whole cells at 5 degrees C was lost after a temperature shift to 25 degrees C. Vesicles preloaded with alanine at 5 degrees C did not become leaky when shifted to 25 degrees C. In addition, exposure of the vesicles to 25 degrees C for 30 min did not affect subsequent alanine uptake at 5 degrees C. The data obtained suggest that the loss of the uptake and permeability control functions of membranes from psychrophilic bacteria at elevated temperatures is not due to degeneration of the membrane itself, but rather to a control or regulatory mechanism associated with whole cells.     

Monoclonal antibodies against a specific surface determinant on malarial (Plasmodium knowlesi) merozoites block erythrocyte invasion

Twelve hybridoma cell lines secreting monoclonal antibodies against Plasmodium knowlesi merozoites have been produced. Antibodies from 3 of the 12 lines agglutinated merozoites. The 2 monoclonal antibodies (13C11 and 16F8) that markedly agglutinated merozoites blocked merozoite invasion of erythrocytes. Of these 2 lines, the one that induced the most agglutination also blocked invasion most effectively. The third monoclonal antibody (53B3) caused minimal agglutination of merozoites and did not block invasion, nor did the other 9 nonagglutinating antibodies. The 2 blocking monoclonal antibodies bound to antigens around the entire surface of merozoites, as demonstrated by immunoelectron microscopy, and precipitated a single biosynthetically labeled protein of apparent m.w. of 250,000. None of the nonagglutinating lines precipitated this protein. Monoclonal antibodies 13C11 and 16F8 reacted with a common antigenic determinant on a Malaysian and a Philippine strain of P. knowlesi in that they blocked invasion and precipitated a 250,000 m.w. protein from both. Sera from immune monkeys also precipitated this 250,000 m.w. protein.     

An efficient method for introducing defined lipids into the plasma membrane of mammalian cells

An efficient method has been devised to introduce lipid molecules into the plasma membrane of mammalian cells. This method has been applied to fuse lipid vesicles with the apical plasma membrane of Madin-Darby canine kidney cells. The cells were infected with fowl plague or influenza N virus. 4 h after infection, the hemagglutinin (HA) spike glycoprotein of the virus was present in the apical plasma membrane of the cells. Lipid vesicles containing egg phosphatidylcholine, cholesterol, and an HA receptor (ganglioside) were then bound to the cells at 0 degrees C. More than 85% of the vesicles were released by external neuraminidase at 0 degrees C or by simply warming the cells to 37 degrees C for 10 s, probably because of the action of the viral neuraminidase at the cell surface. However, when the cells were warmed to 37 degrees C in a pH 5.3 medium for 30 s, 50% of the bound vesicles could no longer be released by external neuraminidase. This only occurred when the HA protein had been cleaved into its HA1 and HA2 subunits. When we used influenza N virus, whose HA is not cleaved in Madin-Darby canine kidney cells, cleavage with external trypsin was required. The fact that the HA protein has fusogenic properties at low pH only in its cleaved form suggests that fusion of the vesicles with the plasma membrane had taken place. Further confirmation for fusion was obtained using an assay based on the decrease of energy transfer between two fluorescent phospholipids in a vesicle upon fusion of the vesicle with the plasma membrane (Struck, D. K., D. Hoekstra, and R. E. Pagano. 1981. Biochemistry, 20:4093-4099).