What type of endocytosis

Columns: 1 2 3 4 6. Add to Cart Compare. Quick view. Proteinase K is an endoproteolytic enzyme that cleaves peptide bonds at the carboxylic sides of aliphatic, aromatic or hydrophobic amino acids. Proteinase K is classified as a nonspecific serine protease.

Potocytosis is used to bring small molecules into the cell and to transport these molecules through the cell for their release on the other side of the cell, a process called transcytosis.

Figure 3. In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds to the receptor on the external surface of the cell membrane.

A targeted variation of endocytosis employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances Figure 3. In receptor-mediated endocytosis, as in phagocytosis, clathrin is attached to the cytoplasmic side of the plasma membrane. If uptake of a compound is dependent on receptor-mediated endocytosis and the process is ineffective, the material will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration.

Some human diseases are caused by the failure of receptor-mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely. People with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear LDL particles from their blood.

Although receptor-mediated endocytosis is designed to bring specific substances that are normally found in the extracellular fluid into the cell, other substances may gain entry into the cell at the same site. Flu viruses, diphtheria, and cholera toxin all have sites that cross-react with normal receptor-binding sites and gain entry into cells. The reverse process of moving material into a cell is the process of exocytosis.

Exocytosis is the opposite of the processes discussed in the last section in that its purpose is to expel material from the cell into the extracellular fluid.

Waste material is enveloped in a membrane and fuses with the interior of the plasma membrane. By EM and immunogold labeling, flotillin appears to decorate small membrane invaginations distinct from clathrin- or caveolinlabeled pits. However, in a recent study in fibroblasts lacking caveolin-1, overexpression of flotillin isoforms, flotillin-1 and 2 could not induce flask-shaped invaginations, suggesting that factors more than flotillins may be necessary for the generation of the flotillin-decorated invaginations It is possible that flotillins generate specialized surface platforms that aid in the regulation of signaling of immunoreceptors and their endocytosis.

The trigger if any for flotillin-mediated endocytosis has recently been shown to be initiated by Fyn-mediated phosphorylation, suggesting that this endocytic process is regulated by non-receptor tyrosine kinase activation In summary, while flotillins may define a dynamin-independent endocytic process, the studies thus far raise interesting questions regarding the function of endogenous flotillins, their means of interacting with specific cargo and the mechanism of vesicle generation.

Role of tetraspanins Another protein platform that supports endocytosis is the family of tetraspanins. This mode of endocytosis is distinct from other dynamin-independent routes in that, it is not inhibited by perturbation of cholesterol levels in the cell , nor does it rely on cellular levels of clathrin, caveolin, and flotillin. However, exact steps in this endocytic mechanism, as well as the associated molecular players, are still unclear.

Recently, it was demonstrated that endocytosis of a tetraspanin, CD82, is dynamin-independent and occurs via a Cdcdependent GEEC-like pinocytic pathway It raises an interesting possibility that while tetraspanins regulate a distinct cholesterol-independent internalization route, they may utilize a different endocytic pathway for their own cellular entry Another such example, where inhibition of dynamin and cholesterol depletion does not affect uptake is the antagonist-induced internalization of nicotinic acetylcholine receptor These observations suggest that specialized protein-centric molecular platforms could assemble in the membrane to aid in this form of endocytosis.

There are now numerous examples of dynamin-independent endocytic systems 9 , , , that offer ample variety in terms of molecules employed and cargo endocytosed. Further study will enrich our understanding of principles behind mechanisms of endocytosis. Undoubtedly, there are a variety of endocytic pathways available at the PM. Based on the environmental adaptations and molecular divergence, different kingdoms seem to have evolved specializations in endocytosis. In prokaryotes, evidence for a well-defined endocytic system is missing.

A recent report using cryoelectron tomography showed various stages of the potential formation of membrane buds, including magnetosomes with inner membrane in Magnetospirillum magneticum These structures are reminiscent of endocytic membrane buds in higher organisms In unicellular eukaryotes, some subgroups have been shown to have spatially defined distribution of endocytic proteins, and specialized structures for endocytosis.

For example, in Paramecium , endocytosis is confined to the cytopharynx, in Trypanosoma , assembly of endocytic machinery is restricted to flagellar pocket and in euglenoids, the ingestion tubule serves as the site of endocytosis In unicellular eukaryotes such as fungi, owing to the availability of genetic, pharmacological and biochemical tools, endocytosis especially in the budding yeast, Saccharomyces cerevisiae has been dissected in great detail , as discussed earlier.

Emr and colleagues had isolated dim defective in internalization mutants, defective in internalizing FM dye A quantitative genome-wide screen performed in yeast recently, examined the internalization of the VAMP homologue, Snc1 The screen uncovered novel regulators of endocytosis. It also uncovered phenotypes for predicted endocytosis genes for which no defects had previously been reported. Apart from these fairly extensive studies in S. However, different species of Ustilago were found to contain homologs of endocytic proteins in their genome and could internalize lucifer yellow and the membrane marker FM, suggesting the operation of endocytic pathways.

These studies were carried out in cells after removal of the cell wall, and were therefore subject to validation in hyphae with an intact cell wall There seem to be differences in endocytic machinery in filamentous fungi and budding yeast. For example, internalization relies on ARF6 activity in A.

In invertebrates, Drosophila has been most extensively used to study endocytosis. A limited variety of endocytic routes are reported to operate in Drosophila , typically demonstrated in cultured hemocytes, isolated wing disc, oocyte nurse cells, and cultured neurons and pericardial cells for selected cell-surface proteins, primarily signaling receptors and bulk volume components , , Recently, increased usage of cultured cell lines derived from Drosophila , and advancement in techniques of genetic manipulation using dsRNA, have made it feasible to study endocytosis and other membrane-trafficking processes on a genome-wide scale 85 , In nematodes, typified by C.

The function of these genes was further validated in mammalian cells, where mutation in some of these genes replicated transport defects as were observed in C.

These studies also indicated that components of the molecular machinery underlying membrane trafficking may be conserved across systems. In a study that carried out extensive phylogenetic analysis of caveolin genes in metazoan which provided evidence of extensive gene duplication , it was observed that vertebrate caveolin-1 and caveolin-3 isoforms and an invertebrate caveolin Apis mellifera were able to form morphologically identical caveolae in caveolin-1 null mouse cells, while the C.

This indicates diversity of function in the caveolin gene family Primary and immortalized mammalian cell lines have proven to be an invaluable system to study endocytosis. While some internalization routes seem to be conserved in primary and immortalized cell lines, it is not certain whether their kinetics is conserved and if immortalization has consequences on overall endocytic activity of a cell type.

Genetic knockout of known endocytic proteins in mice allows for the derivation of primary fibroblasts that are now routinely used to dissect the process These fibroblasts serve as excellent tools to study the involvement of candidate proteins in endocytosis — this approach not only avoids complexities associated with overexpression of their dominant-negative isoforms and off-target effects, but also can reflect on the functional redundancy of a given protein in the system.

In plants, taking into account the cell wall as a physical barrier to PM accessibility, and the high intracellular vacuolar pressure that builds up turgor pressure, the very existence of endocytosis was debated for a long time. In a study using electron microscopy, PM-associated invaginations containing electron-dense tracer and clathrin coats were visualized More recently, uptake of FM dyes was demonstrated in plant cells , following which many studies reported endocytosis of membrane markers and specific proteins in plant root hair cells, pollen tube, and cultured tobacco cells Now there is accumulating evidence about the diversity of endosomal pathways and endocytic compartments in plants.

Although most of the studies used FM dyes to monitor internalization in plants, advancements in probing techniques are revealing a diversity of endocytic routes in this kingdom. Apart from clathrin-dependent receptor-mediated endocytosis, actin-sensitive fluid-phase endocytosis in inner cortex cells of maize root and PI3K-dependent bulk phase sucrose endocytosis in suspension culture cells of sycamore have been reported Blockade of clathrin-mediated endocytosis using ikarugamycin Ika did not inhibit uptake of positively charged nanogold particles, suggesting the operation of a clathrin-independent pathway However, in another study, Dhonukshe et al.

It is possible that a diversity of endocytic portals are conserved in plant and animal systems, and several modes of endocytosis operate in plant cells in a cell type- and tissue- specific manner. Given the diversity and complexity of endocytic routes available in different cellular context, we provide here only a simplified overview of these pathways. Detailed understanding of the functioning of an individual pathway is complex in terms of its associated molecules and mechanism, and will require an individual focus.

It is likely that there are many different biochemical and physical principles behind the formation of vesicles in cell membrane. For example, cell-surface proteins such as the GPI-APs, which lack cytosolic extensions, cannot directly associate with cytosolic coat and adaptor proteins. These proteins therefore need to utilize radically different principles for sorting, membrane deformation, and vesicle generation at PM. Similarly, much less is known about endocytosis of membrane lipids.

Dissection of the endocytic process of lipids will give insights into a novel way of vesicle formation. Also considering the multiplicity of functions of proteins, it will be interesting to investigate the overlapping functions of molecules known to be associated with specific trafficking pathways. The identification of an endocytic pathway as distinct has been primarily based on associated cargo proteins or lipids, and molecular regulators; the contribution of kinetics and detailed physical mechanism to such categorization is not generally available except in some well-characterized situations, namely clathrin-pit endocytosis or endocytosis by actin-dependent forces in yeast.

Considering the limited knowledge and vast diversity in molecular players that are employed for efficient functioning of endocytic trafficking pathways, it is difficult to clearly demarcate these mechanisms from one another.

On a cautionary note, it should be recognized that numerous cell lines have been utilized to study different types of endocytosis. These studies have helped uncover complex endocytic networks in a given cell system. The extent to which observations in one cell system may be extrapolated to another, or whether basic components of specific endocytic pathway are conserved across cell lines, is not completely clear.

The possibility that modes of immortalization of cell lines itself could have consequences on how the endocytic pathways are configured in these cells has not been addressed. Extensive work has been carried out to understand and elucidate the mechanism of endocytosis in some eukaryotes, and a considerable amount of information is available in some free-living eukaryotes and pathogens The integration of known information from complex endocytic systems in metazoan and comparison with uptake mechanisms in other eukaryotes could provide a means of understanding the scope of evolution, conservation, and redundancy in the internalization process across phyla.

Furthermore, whether specific endocytic modes are conserved in their entirety across phyla is also a subject of lively debate What is clear is that cells have many ways to endocytose material from the extracellular milieu, and perhaps as many reasons to do it by a particular mode.

Until we understand the basic principles behind cargo concentration, membrane deformation, and scission in each of these modes it will be difficult to provide any rationale for the proliferation of the mechanisms of entry.

Regardless, the endocytic process, in addition to its role in uptake of nutrients and fluid, influences diverse key processes in metazoans, such as establishment of cellular asymmetry and modulation of signaling. In addition to its physiological roles, the endocytic process is also exploited by various pathogens. Insights into the underlying mechanisms will reveal new targets for drug design, and selective employment of these entry routes for drug delivery will also allow for the manipulation of cellular endocytic pathways to alleviate effects of specific diseases.

Swanson JA. Shaping cups into phagosomes and macropinosomes. Nat Rev Mol Cell Biol ; 9 — Molecular mechanisms of phagocytic uptake in mammalian cells. Cell Mol Life Sci ; 65 — Phagosome maturation: going through the acid test. Fusion, fission, and secretion during phagocytosis. Physiology Bethesda ; 22 — CAS Google Scholar. Haas A. The phagosome: compartment with a license to kill.

Traffic ; 8 — Defining macropinocytosis. Traffic ; 10 — Macropinocytosis: regulated coordination of endocytic and exocytic membrane traffic events. J Cell Sci ; Pt 22 — SDPR induces membrane curvature and functions in the formation of caveolae. Nat Cell Biol ; 11 — Mechanisms of endocytosis. Annu Rev Biochem ; 78 — Studies of the macrophage complement receptor. Alteration of receptor function upon macrophage activation. J Exp Med ; — Phorbol esters cause sequential activation and deactivation of complement receptors on polymorphonuclear leukocytes.

J Immunol ; — Caron E, Hall A. Identification of two distinct mechanisms of phagocytosis controlled by different Rho GTPases. Science ; — Allen LA, Aderem A.

Molecular definition of distinct cytoskeletal structures involved in complement- and Fc receptor-mediated phagocytosis in macrophages. J Cell Biol ; — Pizarro-Cerda J, Cossart P. Subversion of cellular functions by Listeria monocytogenes. J Pathol ; — PLoS Biol ; 4 :e Cdc42, Rac1, and Rac2 display distinct patterns of activation during phagocytosis.

Mol Biol Cell ; 15 — Park H, Cox D. Mol Biol Cell ; 20 — Phosphatidylinositol-4,5-bisphosphate: actin dynamics and the regulation of ATP-dependent and -independent secretion.

Mol Pharmacol ; 67 — Phosphatidylinositol-4,5-bisphosphate hydrolysis directs actin remodeling during phagocytosis. A requirement for phosphatidylinositol 3-kinase in pseudopod extension.

J Biol Chem ; — Mol Biol Cell ; 18 — The phagosome proteome: insight into phagosome functions. Policard A, Bessis M. Distinct endocytotic pathways in epidermal growth factor-stimulated human carcinoma A cells. J Cell Biol ; 6 Pt 1 — Selective membrane exclusion in phagocytic and macropinocytic cups. J Cell Sci ; Pt 19 — Phorbol esters stimulate macropinocytosis and solute flow through macrophages. J Cell Sci ; 94 Pt 1 — Keller HU.

Diacylglycerols and PMA are particularly effective stimulators of fluid pinocytosis in human neutrophils. J Cell Physiol ; — Mercer J, Helenius A. Virus entry by macropinocytosis. 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.

A Sphingomonas bacterium interacting with epithelial cells. Res Microbiol ; — Human immunodeficiency virus type 1 enters brain microvascular endothelia by macropinocytosis dependent on lipid rafts and the mitogen-activated protein kinase signaling pathway. J Virol ; 76 — The coated pit and macropinocytic pathways serve distinct endosome populations. Developmental control of endocytosis in dendritic cells by Cdc Cell ; — Regulation of macropinocytosis by pactivated kinase Mol Biol Cell ; 11 — EMBO J ; 27 — Dynamin 2 mediates fluid-phase micropinocytosis in epithelial cells.

J Cell Sci ; Pt 23 — Isoform and splice-variant specific functions of dynamin-2 revealed by analysis of conditional knock-out cells. Mol Biol Cell ; 19 — Modulation of Rac localization and function by dynamin. Membrane phosphatidylserine regulates surface charge and protein localization. Chirality-induced budding: a raft-mediated mechanism for endocytosis and morphology of caveolae?

Biophys J ; 92 — Pearse BM. Clathrin: a unique protein associated with intracellular transfer of membrane by coated vesicles.

Yolk protein uptake in the oocyte of the mosquito Aedes Aegypti. J Cell Biol ; 20 — Kirchhausen, T, Clathrin. Annu Rev Biochem ; 69 — The structural era of endocytosis. Endocytosis by random initiation and stabilization of clathrin-coated pits. Ungewickell EJ, Hinrichsen L. Endocytosis: clathrin-mediated membrane budding.

Curr Opin Cell Biol ; 19 — Coupling between clathrin-coated-pit invagination, cortactin recruitment, and membrane scission observed in live cells. Young A. Structural insights into the clathrin coat. Semin Cell Dev Biol ; 18 — Kirchhausen T. Adaptors for clathrin-mediated traffic. Annu Rev Cell Dev Biol ; 15 — Habermann B.

The BAR-domain family of proteins: a case of bending and binding? EMBO Rep ; 5 — Kanaseki T, Kadota K. The 'vesicle in a basket'. A morphological study of the coated vesicle isolated from the nerve endings of the guinea pig brain, with special reference to the mechanism of membrane movements. Exocytosis involves the passage of a vesicle from the endoplasmic reticulum or Golgi apparatus, through the cytoplasm to the cell membrane, where it fuses and releases its contents.

Once the white blood cell has engulfed a foreign pathogen eliminate it, certain parts of the pathogen are no longer needed. The macrophage gets rid of this waste material through exocytosis, during which vesicles carry out the unwanted pathogen material.

Why is bulk transport important for cells? What is endocytosis? Endocytosis definition and purposes Endocytosis is the process by which cells take in substances from outside of the cell by engulfing them in a vesicle.

Endocytosis serves many purposes, including: Taking in nutrients for cellular growth, function and repair: Cells need materials like proteins and lipids to function. Capturing pathogens or other unknown substances that may endanger the organism: When pathogens like bacteria are identified by the immune system, they are engulfed by immune cells to be destroyed. Disposing of old or damaged cells: Cells must be safely disposed of when they stop functioning properly to prevent damage to other cells.

These cells are eliminated through endocytosis. Types of endocytosis There are two types of endocytosis: phagocytosis and pinocytosis. The steps of endocytosis The following is an outline of the basic steps of the two types of endocytosis.

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