Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “PINOCYTOSIS”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 721 records · Page 40Linked to original sources
PubMed1983

Relationship between axenic growth of Dictyostelium discoideum strains and their track morphology on substrates coated with gold particles.

Amoebae of Dictyostelium discoideum produce tracks with two distinct morphologies on gold-coated coverslips. The wild-type strain and other strains that feed only by phagocytosis produced indistinct, fuzzy tracks, whereas mutants capable of axenic growth produced clear, sharp tracks. The sharp track morphology was found to be a recessive phenotype that segregates with axenicity and probably requires a previously unidentified axenic mutation. Axenic and nonaxenic strains also differed in their ability to pinocytose. When the two types of cells were shifted from bacterial growth plates to nutrient media, within 24 h the axenic strain established a rapid rate of pinocytosis, approximately 100-fold higher than the low rate detectable for the nonaxenic strain. However, track formation did not appear to be directly related to endocytosis. Electron microscopic examination of cells during track formation showed that both axenic and nonaxenic strains accumulated gold particles on their surfaces, but neither strain internalized the gold to any significant degree. Observation of living cells revealed that axenic strains collected all particles that they contacted, whereas wild-type strains left many particles undisturbed. The size of the gold particle clusters discarded by the cells also contributed to track morphology.

Dictyostelium↗
PubMed1984

Evidence for both prelysosomal and lysosomal intermediates in endocytic pathways.

Horseradish peroxidase (HRP), an enzyme internalized by fluid phase pinocytosis, has been used to study the process by which pinosome contents are delivered to lysosomes in Chinese hamster ovary cells. Pinosome contents were labeled by allowing cells to internalize HRP for 3-5 min. Following various chase times, cells were either processed for HRP and acid phosphatase (AcPase) cytochemistry or homogenized and fractionated in Percoll gradients. In Percoll gradients, pinosomes labeled by a 3-5 min HRP pulse behaved as a vesicle population more dense than plasma membrane and less dense than lysosomes. In pulse-chase experiments, internalized HRP was chased rapidly (3-6 min chase) to a density position intermediate between the "initial" pinocytic vesicle population and lysosomes. With longer chase periods, a progressive accumulation of HRP in more dense vesicles was observed. Correspondence between the HRP distribution and lysosomal marker distribution was reached after a approximately 1-h chase. By electron microscope cytochemistry of intact cells, the predominant class of HRP-positive vesicles after pulse uptakes or a 3-min chase period was characterized by a peripheral rim of reaction product and was AcPase negative. After 10-120-min chase periods, the predominant class of HRP-positive vesicles was characterized by luminal deposits and HRP activity was frequently observed in multivesicular bodies. HRP-positive vesicles after a 10- or 30-min chase were AcPase-positive. No HRP activity was detected in Golgi apparatus. Together these observations indicate that progressive processing of vesicular components of the vacuolar apparatus occurs at both a prelysosomal and lysosomal stage.

Animals↗
PubMed1984

Internalization and degradation of macrophage Fc receptors bound to polyvalent immune complexes.

We have studied the Fc receptor-mediated pinocytosis of immunoglobulin G (IgG)-containing immune complexes by mouse macrophages. IgG complexes were formed from affinity-purified rabbit dinitrophenyl IgG and dinitrophenyl modified BSA at molar ratios of 2.5-10:1. Both the specificity of binding and the fate of internalized receptors were analyzed using monoclonal and polyclonal anti-Fc receptor antibodies. Based on the susceptibility of surface-bound ligand to release by proteolysis, we have found that at 37 degrees C, 125I-labeled IgG complexes were rapidly internalized (t1/2 less than 2 min) and delivered to lysosomes; acid-soluble 125I was detectable in the growth medium within 5-10 min of uptake. However, kinetic evidence indicated that Fc receptors were not efficiently re-used for multiple rounds of ligand uptake. Instead, macrophages that were exposed continuously to saturating concentrations of IgG complexes exhibited a selective and largely irreversible removal of Fc receptors from the plasma membrane. This loss of surface receptors correlated with an increased rate of receptor turnover, determined by immune precipitation of Fc receptors from 125I-labeled macrophages. Thus, in contrast to the results obtained in the accompanying paper (I. Mellman, H. Plutner, and P. Ukkonen, 1984, J. Cell Biol. 98:1163-1169) using a monovalent ligand, these data indicate that the interaction of Fc receptors with polyvalent complexes leads to the degradation of both ligand and receptor following their delivery to lysosomes.

Animals↗
PubMed1967

The response of cultured mammalian cells to diphtheria toxin. I. Amino acid transport, accumulation, and incorporation in normal and intoxicated sensitive cells.

The response to diphtheria toxin of two sensitive cell lines, KB and HeLa, was investigated. Inhibition of the incorporation of radioactively labeled amino acids into protein was the earliest detectable effect of diphtheria toxin. It was observed that, during the period of intoxication, the cell membrane was morphologically intact and retained its semi-permeable character, although it was rendered fragile and more easily disrupted by mechanical manipulations than the normal cell. The transport of amino acids continued even after intoxicated cells had ceased to synthesize protein, and the levels accumulated were equal to those of control cells. It was observed that cultural conditions, age, and handling of cells affected their response to toxin. In early log phase cells subjected to a minimum of handling before application of the toxin, the normally observed latent period preceding detectable effects was reduced to 15 min for KB cells and 30 min for HeLa cells, shorter times than previously reported. The data are consistent with the hypothesis that diphtheria toxin enters susceptible cells, possibly by pinocytosis, and there acts upon cytoplasmic sites of protein synthesis.

Amino Acids↗
PubMed1970

Morphology and peroxidase cytochemistry of mouse promonocytes, monocytes, and macrophages.

Mouse promonocytes have been identified and studied in cultures of bone marrow cells. These cells have a diameter of 14-20 micro, and in stained preparations reveal a large, indented or folded nucleus, and basophilic, finely granular cytoplasm. The living promonocyte viewed by phase contrast shows additional features: nucleoli, small dense bodies, and vesicles in the cytoplasm adjacent to the nuclear hilus, and slight membrane ruffling. Prominent ultrastructural components of promonocytes include a well developed Golgi apparatus, small numbers of centrosomal granules and vacuoles, extensive ribosomal aggregates, and finger-like projections of the cell surface. Promonocytes engage in pinocytosis and phagocytosis, but they are less active in these functions than are peripheral blood monocytes of peritoneal macrophages. Promonocytes are positive for peroxidase, the reaction product being localized to granules most of which are centrally situated in the cell. Monocytes in blood or in inflammatory peritoneal exudates display much smaller numbers of peroxidase-positive granules, and various types of mature mouse macrophages are peroxidase negative.

Animals↗
PubMed1975

Mass isolation and culture of rat kupffer cells.

Collagenase perfusion of the liver followed by pronase treatment of the cell suspension thus obtained gave a quantitative recovery of viable nonparenchymal liver cells (NPC). From these NPC, Kupffer (K) cells can be purified by attachment to tissue culture dishes. Tail vein injection of carbon 1-2 h before liver perfusion permitted stepwise calculation as well as visualization of carbon-containing K cells. When these K cells have been put into tissue culture medium with serum and incubated overnight, they exhibit typical macrophage characteristics. Phase-contrast and transmission electron microscopy showed typical macrophage morphology and scanning electron microscopy revealed well-spread cells with cytoplasmic projections and ruffled membranes. Endocytosis studies using radioactive colloidal gold and inert latex particles also indicated that these cells are highly active in pinocytosis and phagocytosis. Further characterization of K cells is the identification of Fc receptor on their membranes. Studies on lysosomal enzymes showed that purified K cells possess higher specific activities in beta-glucuronidase, acid DNase, and cathepsin D than in purified parenchymal cells.

Animals↗
PubMed1983

Characterization of nonlymphoid cells derived from rat peripheral lymph.

Mesenteric lymphadenectomy in rats is followed by union of peripheral and central lymphatics, allowing the collection of intestine-derived peripheral lymph cells via the thoracic duct for several days. These cells include a proportion of nonlymphoid cells (NLC) that show irregular and heterogeneous surface morphology including long pseudopodia and veils. They stain variably for nonspecific esterase and acid phosphatase and are ATPase-positive. Their nuclei are irregular and some contain cytoplasmic inclusions, some of which show peroxidase activity and/or contain DNA. NLC have a range of densitites generally lower than that of lymphocytes. Freshly collected NLC express the leukocyte-common antigen (defined by monoclonal antibody MRC Ox 1) and Ia antigens (I-A and I-E subregion products defined by monoclonal antibodies) but they show a relative lack of other surface markers normally found on rat B or T lymphocytes (W3/13, W3/25, MRC Ox 12 (sIg), MRC Ox 19) or rat macrophages (FcR, C'R, mannose R, W3/25). In general NLC are only weakly adherent to glass or plastic. Although a subpopulation of NLC appear to have had a phagocytic past, freshly collected NLC fail to phagocytose a variety of test particles in vitro. NLC also appear incapable of pinocytosis in vitro. This heterogeneity may represent distinct subpopulations of NLC or different stages in the development of a single cell lineage. Direct cannulation of mesenteric lacteals shows that the majority of NLC are derived from the small intestine and their precursors appear to be present both in lamina propria and Peyer's patches. Kinetic studies, following irradiation or intravenous tritiated thymidine, show that the majority of NLC turn over rapidly in the intestine with a modal time of 3-5 d. Studies with bone marrow chimeras show that they are derived from a rapidly dividing precursor present in normal bone marrow. NLC occur at very low frequencies in normal thoracic duct lymph at all times following cannulation. The evidence presented suggests that NLC closely resemble mouse lymphoid dendritic cells. This conclusion is supported by evidence already obtained showing that NLC are potent stimulators of the semi-allogeneic rat primary mixed leukocyte reaction. In addition to the ceils resembling dendritic cells rare monocytoid cells are found in thoracic duct lymph of lymphadenectomized specific pathogen-free rats. The proportion of these cells increases greatly when the animals are conventionally housed. It seems probable that the physiological function of NLC is to act as accessory cells in the lymph nodes to which they normally drain. Methods for enriching NLC and thus facilitating analysis of their functions are discussed.

Animals↗
PubMed1983

Fc receptor modulation in mononuclear phagocytes maintained on immobilized immune complexes occurs by diffusion of the receptor molecule.

We describe a method for synchronously assembling antigen-antibody complexes underneath macrophages adherent to an antigen-coated surface. We have used this method to study the mechanism of Fc receptor (FcR) disappearance that occurs when resident and thioglycollate-elicited mouse macrophages are cultured on immune complex-coated surfaces. Erythrocytes opsonized with IgG (E(IgG) and a monoclonal antibody (2.4G2 IgG) directed against the trypsin-resistant FcR (FcRII) were used as indicators of the presence and distribution of FcRII molecules on the macrophage plasma membrane. Inhibitors of aerobic (NaCN) and anerobic (2-deoxyglucose, NaF) glycolysis and pinocytosis, of protein biosynthesis (cycloheximide), and of cytoskeletal function (cytochalasin B and D, colchicine, podophyllotoxin, taxol) did not reduce the rate or extent of FcRII modulation. Moreover, treatment of the macrophages with 0.1-0.5% formaldehyde did not reduce the extent of FcRII modulation as measured by the disappearance of E(IgG) binding sites. FcRII modulation was markedly slowed when the temperature was decreased to 2-4 degrees C. These results prove that FcRII modulation is governed by diffusion of the receptor in the plasma membrane. From the speed of FcRII disappearance from the macrophage's upper surface we calculate that the receptor has a diffusion coefficient at 37 degrees C of 2.5 X 10(-9) cm2/s. This finding indicates that FcRII, in its unligated form, is not linked to the macrophage's cytoskeleton, and that the receptor is capable of accommodating spatially to any distribution of ligands on a particle's surface.

Animals↗
PubMed1995

Dendritic cells use macropinocytosis and the mannose receptor to concentrate macromolecules in the major histocompatibility complex class II compartment: downregulation by cytokines and bacterial products.

We have previously demonstrated that human peripheral blood low density mononuclear cells cultured in granulocyte/macrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-4 develop into dendritic cells (DCs) that are extremely efficient in presenting soluble antigens to T cells. To identify the mechanisms responsible for efficient antigen capture, we studied the endocytic capacity of DCs using fluorescein isothiocyanate-dextran, horseradish peroxidase, and lucifer yellow. We found that DCs use two distinct mechanisms for antigen capture. The first is a high level of fluid phase uptake via macropinocytosis. In contrast to what has been found with other cell types, macropinocytosis in DCs is constitutive and allows continuous internalization of large volumes of fluid. The second mechanism of capture is mediated via the mannose receptor (MR), which is expressed at high levels on DCs. At low ligand concentrations, the MR can deliver a large number of ligands to the cell in successive rounds. Thus, while macropinocytosis endows DCs with a high capacity, nonsaturable mechanism for capture of any soluble antigen, the MR gives an extra capacity for antigen capture with some degree of selectivity for non-self molecules. In addition to their high endocytic capacity, DCs from GM-CSF + IL-4-dependent cultures are characterized by the presence of a large intracellular compartment that contains high levels of class II molecules, cathepsin D, and lysosomal-associated membrane protein-1, and is rapidly accessible to endocytic markers. We investigated whether the capacity of DCs to capture and process antigen could be modulated by exogenous stimuli. We found that DCs respond to tumor necrosis factor alpha, CD40 ligand, IL-1, and lipopolysaccharide with a coordinate series of changes that include downregulation of macropinocytosis and Fc receptors, disappearance of the class II compartment, and upregulation of adhesion and costimulatory molecules. These changes occur within 1-2 d and are irreversible, since neither pinocytosis nor the class II compartment are recovered when the maturation-inducing stimulus is removed. The specificity of the MR and the capacity to respond to inflammatory stimuli maximize the capacity of DCs to present infectious non-self antigens to T cells.

Antigens↗
PubMed1999

RacF1, a novel member of the Rho protein family in Dictyostelium discoideum, associates transiently with cell contact areas, macropinosomes, and phagosomes.

Using a PCR approach we have isolated racF1, a novel member of the Rho family in Dictyostelium. The racF1 gene encodes a protein of 193 amino acids and is constitutively expressed throughout the Dictyostelium life cycle. Highest identity (94%) was found to a RacF2 isoform, to Dictyostelium Rac1A, Rac1B, and Rac1C (70%), and to Rac proteins of animal species (64-69%). To investigate the role of RacF1 in cytoskeleton-dependent processes, we have fused it at its amino-terminus with green fluorescent protein (GFP) and studied the dynamics of subcellular redistribution using a confocal laser scanning microscope and a double-view microscope system. GFP-RacF1 was homogeneously distributed in the cytosol and accumulated at the plasma membrane, especially at regions of transient intercellular contacts. GFP-RacF1 also localized transiently to macropinosomes and phagocytic cups and was gradually released within <1 min after formation of the endocytic vesicle or the phagosome, respectively. On stimulation with cAMP, no enrichment of GFP-RacF1 was observed in leading fronts, from which it was found to be initially excluded. Cell lines were obtained using homologous recombination that expressed a truncated racF1 gene lacking sequences encoding the carboxyl-terminal region responsible for membrane targeting. These cells displayed normal phagocytosis, endocytosis, and exocytosis rates. Our results suggest that RacF1 associates with dynamic structures that are formed during pinocytosis and phagocytosis. Although RacF1 appears not to be essential, it might act in concert and/or share functions with other members of the Rho family in the regulation of a subset of cytoskeletal rearrangements that are required for these processes.

Amino Acid Sequence↗
PubMed1998

Overexpression of a novel rho family GTPase, RacC, induces unusual actin-based structures and positively affects phagocytosis in Dictyostelium discoideum.

Rho family proteins have been implicated in regulating various cellular processes, including actin cytoskeleton organization, endocytosis, cell cycle, and gene expression. In this study, we analyzed the function of a novel Dictyostelium discoideum Rho family protein (RacC). A cell line was generated that conditionally overexpressed wild-type RacC three- to fourfold relative to endogenous RacC. Light and scanning electron microscopy indicated that the morphology of the RacC-overexpressing cells [RacC WT(+) cells] was significantly altered compared with control cells. In contrast to the cortical F-actin distribution normally observed, RacC WT(+) cells displayed unusual dorsal and peripheral F-actin-rich surface blebs (petalopodia, for flower-like). Furthermore, phagocytosis in the RacC WT(+) cells was induced threefold relative to control Ax2 cells, whereas fluid-phase pinocytosis was reduced threefold, primarily as the result of an inhibition of macropinocytosis. Efflux of fluid-phase markers was also reduced in the RacC WT(+) cells, suggesting that RacC may regulate postinternalization steps along the endolysosomal pathway. Treatment of cells with Wortmannin and LY294002 (phosphatidylinositol 3-kinase inhibitors) prevented the RacC-induced morphological changes but did not affect phagocytosis, suggesting that petalopodia are probably not required for RacC-induced phagocytosis. In contrast, inactivating diacylglycerol-binding motif-containing proteins by treating cells with the drug calphostin C completely inhibited phagocytosis in control and RacC WT(+) cells. These results suggest that RacC plays a role in actin cytoskeleton organization and phagocytosis in Dictyostelium.

Androstadienes↗
PubMed2004

Cellular distribution and functions of wild-type and constitutively activated Dictyostelium PakB.

Dictyostelium PakB, previously termed myosin I heavy chain kinase, is a member of the p21-activated kinase (PAK) family. Two-hybrid assays showed that PakB interacts with Dictyostelium Rac1a/b/c, RacA (a RhoBTB protein), RacB, RacC, and RacF1. Wild-type PakB displayed a cytosolic distribution with a modest enrichment at the leading edge of migrating cells and at macropinocytic and phagocytic cups, sites consistent with a role in activating myosin I. PakB fused at the N terminus to green fluorescent protein was proteolyzed in cells, resulting in removal of the catalytic domain. C-terminal truncated PakB and activated PakB lacking the p21-binding domain strongly localized to the cell cortex, to macropinocytic cups, to the posterior of migrating cells, and to the cleavage furrow of dividing cells. These data indicate that in its open, active state, the N terminus of PakB forms a tight association with cortical actin filaments. PakB-null cells displayed no significant behavioral defects, but cells expressing activated PakB were unable to complete cytokinesis when grown in suspension and exhibited increased rates of phagocytosis and pinocytosis.

Actins↗
PubMed2005

Helicobacter pylori VacA cytotoxin: a probe for a clathrin-independent and Cdc42-dependent pinocytic pathway routed to late endosomes.

The vacuolating cytotoxin VacA is a major virulence factor of Helicobacter pylori, a bacterium responsible for gastroduodenal ulcers and cancer. VacA associates with lipid rafts, is endocytosed, and reaches the late endocytic compartment where it induces vacuolation. We have investigated the endocytic and intracellular trafficking pathways used by VacA, in HeLa and gastric AGS cells. We report here that VacA was first bound to plasma-membrane domains localized above F-actin structures that were controlled by the Rac1 GTPase. VacA was subsequently pinocytosed by a clathrin-independent mechanism into cell peripheral early endocytic compartments lacking caveolin 1, the Rab5 effector early endosomes antigen-1 (EEA1) and transferrin. These compartments took up fluid-phase (as evidenced by the accumulation of fluorescent dextran) and glycosylphosphatidylinositol-anchored proteins (GPI-APs). VacA pinocytosis was controlled by Cdc42 and did not require cellular tyrosine kinases, dynamin 2, ADP-ribosylating factor 6, or RhoA GTPase activities. VacA was subsequently routed to EEA1-sorting endosomes and then sorted to late endosomes. During all these different endocytic steps, VacA was continuously associated with detergent resistant membrane domains. From these results we propose that VacA might be a valuable probe to study raft-associated molecules, pinocytosed by a clathrin-independent mechanism, and routed to the degradative compartment.

ADP-Ribosylation Factor 6↗
PubMed1984

A descriptive study of the blood vessels of the sciatic nerve in the rat, man and other mammals.

The gross blood supply and internal vascular organization of the sciatic nerve in the rat (and other mammals including man) have been studied by injection, dissection, and microradiographic methods, in thick sections stained for alkaline phosphatase and semithin sections, and by electron microscopy. In the rat, an anastomotic vascular connection from the arteria comitans to the medial femoral circumflex artery is illustrated. In all species studied, the microvascular network of the nerve has an unusually large calibre and wide spacing, particularly when compared with that of skeletal muscle. Arterioles and venules are present within fascicles; the arterioles are thin-walled, with only rudimentary internal elastic laminae. Endoneurial capillaries are large, continuous and nonfenestrated, with prominent (often multiple) basal laminae and an unusually complete pericyte investment. Endothelial junctions appear to vary in type; not all are tight. Pinocytosis is common. The extensively interconnected vascular network of the nerve fascicle may act as an adaptable in situ reservoir for blood in the precise maintenance of the endoneurial milieu.

Alkaline Phosphatase↗
PubMed1976

Intestinal permeability to large particles in normal and protein-deficient adult rats.

Intestinal absorption of ferritin and virus particles was assessed in control and protein-deficient rats using light and electron microscopic procedures. Rats weighing 100 to 130 grams were divided into two groups and fed either a 0.5% of 18% lactalbumin diet. At monthly intervals following diet initiation, three rats from each group were administered ferritin (MW-650,000) or adenovirus Type 5 (MW is greater than 1,000,000) via ligated jejunal loops either 15 or 30 minutes prior to loop excision and processing for electron microscopy. Morphological evaluation of jejunal tissues revealed that both control and protein-deficient rats absorbed ferritin and virus particles via pinocytosis and in both situations the exogenous particles were believed to be present in lysosmal bodies of typical absorptive cells. Neither ferritin molecules nor virus particles were identified elsewhere in the jejunal mucosa of control rats. Rats deprived of sufficient dietary protein for four months or longer, however, demonstrated exogenous tracer materials in intercellular spaces of the epithelium and lamina propria. Since these severely protein-deprived rats also demonstrated deterioration of apical intercellular junctions, the possibility exists that large particles moved directly between cells of the epithelial lining and subsequently into underlying connective tissue and vascular compartments for conveyance (as antigens) to other body regions.

Adenoviridae↗
PubMed1978

Cytochemical study on uptake of exogenous peroxidase by Vx2 carcinoma cells transplanted into the rabbit.

When horse-radish peroxidase (HPO) was administered iv to Vx2 carcinoma-bearing rabbits, an HPO reaction product was detected in the lumina of blood vessels and the extracellular spaces between tumor cells in the first few minutes after administration. HPO was also seen in vesicles in tumor cells. Fifteen minutes to 1 hour after administration, the HPO reaction product was found mainly in the large membrane-bound vacuoles. Within 6--12 hours, the HPO activity gradually diminished in large membrane-bound vacuoles (lysosomes). In conclusion, exogenous HPO was rapidly incorporated into Vx2 carcinoma cells by pinocytosis, and then pinocytotic vesicles were fused with lysosomes.

Animals↗
PubMed1982

Edema and vascular permeability in cerebral ischemia: comparison between ischemic neuronal damage and infarction.

The respective influences of ischemic neuronal damage and infarction on the development of abnormal blood-brain barrier (BBB) permeability and cerebral edema were evaluated in a rat model of temporary four-vessel occlusion in which ischemic neuronal damage with only infrequent infarction is produced. Survival times ranged from 40 minutes to 5 days after ischemia. Evans blue and horseradish peroxidase (HRP) were given before sacrifice. The majority of brain showed moderate ischemic neuronal damage inthe striatum. In these areas there was neither leakage of Evans blue nor extravasation of HRP. Astrocytic processes were moderately swollen. Large, grossly-visible unilateral infarcts were present in only 5 animals, and all showed abnormal BBB permeability of HRP which occurred via enchanced pinocytosis, and occasionally via diffuse leakage through necrotic vessels. Astrocytic processes were markedly swollen and their plasma membranes were disrupted. Whole brain and regional water content in a parallel series of animals were measured from 15 minutes (min) to 48 hours (h) postischemia. They showed a transient, 1% increase in whole brain water content from 15 to 60 min postischemia, but no increase in regional water content at any postischemic interval. These studies suggest that ischemia produces BBB permeability to large molecules, and sustained cerebral edema only when the process damages blood vessels and astrocytes; neuronal necrosis alone is insufficient.

Animals↗
PubMed1975

Ultrastructural aspects of BK virus uptake and replication in human fibroblasts.

After exposure of human embryonic fibroblasts to BK virus, virus particles adsorbed to the plasma membrane were engulfed by pinocytosis or captured by vesicles, possibly originating from the endoplasmic reticulum, within 2 h after infection. Most of the virus particles were then transported into lysosomes or into the nucleus, while a small amount of virus was found free in the cytoplasm. Virus particles entered the nucleus between 2 and 12 h after infection, were still detectable in the nucleus at 24 h after infectionand becamse morphologically undiscernible at 30 h after infection, suggesting that a nuclear uncoating mechanism was active between 24 and 30 h after infection. Virus progeny started to appear in the nucleus of infected cells at 4 days after infection, but not until 7 to 8 days after infection did the virus escape into the cytoplasm and cell degeneration became evident. The possible explanations for the long replicative cycle of BK virus are discussed.

Adsorption↗