Cell Membrane [Extra Quality]

TGF-beta signaling controls a plethora of cellular responses and figures prominently in animal development. Recent cellular, biochemical, and structural studies have revealed significant insight into the mechanisms of the activation of TGF-beta receptors through ligand binding, the activation of Smad proteins through phosphorylation, the transcriptional regulation of target gene expression, and the control of Smad protein activity and degradation. This article reviews these latest advances and presents our current understanding on the mechanisms of TGF-beta signaling from cell membrane to the nucleus.

Red cell osmotic fragility was assessed by a battery of tests, and their sensitivity ranged from 48% to 95% independently of the cytoskeletal abnormality and of the amount of protein deficiency3. The association of the AGLT and the NaCl test on incubated blood reached a sensitivity of 99%, even considering atypical patients, i.e. those with normal reticulocyte counts and/or no spherocytes in the peripheral blood. Interestingly, in splenectomised patients the percent positivity of all the osmotic fragility tests was increased, as compared with not splenectomised cases; in particular, both the AGLT and the NaCl test on incubated blood reached 100% sensitivity. Moreover, surgery allowed the identification of the membrane defect in all the previously unclassified cases (4 with spectrin deficiency, 3 with spectrin/ankyrin deficiency, and 1 with band 3 deficiency).

Cell Membrane

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It is a semipermeable lipid bilayer found in all cells. It contains a wide variety of biological molecules, primarily proteins and lipids, which are involved in a vast array of cellular processes such as cell adhesion, ion channel conductance and cell signaling. The plasma membrane also serves as the attachment point for both the intracellular cytoskeleton and, if present, the extracellular cell wall.

Cellmembranes protect and organize cells. All cells have an outer plasma membranethat regulates not only what enters the cell, but also how much of any givensubstance comes in. Unlike prokaryotes, eukaryotic cells also possess internalmembranes that encase their organelles and control the exchange of essentialcell components. Both types of membranes have a specialized structure thatfacilitates their gatekeeping function.

Figure 1: The lipid bilayer and the structure and composition of a glycerophospholipid molecule(A) The plasma membrane of a cell is a bilayer of glycerophospholipid molecules. (B) A single glycerophospholipid molecule is composed of two major regions: a hydrophilic head (green) and hydrophobic tails (purple). (C) The subregions of a glycerophospholipid molecule; phosphatidylcholine is shown as an example. The hydrophilic head is composed of a choline structure (blue) and a phosphate (orange). This head is connected to a glycerol (green) with two hydrophobic tails (purple) called fatty acids. (D) This view shows the specific atoms within the various subregions of the phosphatidylcholine molecule. Note that a double bond between two of the carbon atoms in one of the hydrocarbon (fatty acid) tails causes a slight kink on this molecule, so it appears bent. 2010 Nature Education All rights reserved.

Altogether, lipids account for about half the mass of cell membranes. Cholesterol molecules, although less abundant than glycerophospholipids, account for about 20 percent of the lipids in animal cell plasma membranes. However, cholesterol is not present in bacterial membranes or mitochondrial membranes. Also, cholesterol helps regulate the stiffness of membranes, while other less prominent lipids play roles in cell signaling and cell recognition.

Figure 2: The glycerophospholipid bilayer with embedded transmembrane proteins 2010 Nature Education All rights reserved. In addition to lipids, membranes are loaded with proteins. In fact, proteins account for roughly half the mass of most cellular membranes. Many of these proteins are embedded into the membrane and stick out on both sides; these are called transmembrane proteins. The portions of these proteins that are nested amid the hydrocarbon tails have hydrophobic surface characteristics, and the parts that stick out are hydrophilic (Figure 2).

At physiological temperatures, cell membranes are fluid; at cooler temperatures, they become gel-like. Scientists who model membrane structure and dynamics describe the membrane as a fluid mosaic in which transmembrane proteins can move laterally in the lipid bilayer. Therefore, the collection of lipids and proteins that make up a cellular membrane relies on natural biophysical properties to form and function. In living cells, however, many proteins are not free to move. They are often anchored in place within the membrane by tethers to proteins outside the cell, cytoskeletal elements inside the cell, or both.

Cell membranes serve as barriers and gatekeepers. They are semi-permeable, which means that some molecules can diffuse across the lipid bilayer but others cannot. Small hydrophobic molecules and gases like oxygen and carbon dioxide cross membranes rapidly. Small polar molecules, such as water and ethanol, can also pass through membranes, but they do so more slowly. On the other hand, cell membranes restrict diffusion of highly charged molecules, such as ions, and large molecules, such as sugars and amino acids. The passage of these molecules relies on specific transport proteins embedded in the membrane.

Membrane transport proteins are specific and selective for the molecules they move, and they often use energy to catalyze passage. Also, these proteins transport some nutrients against the concentration gradient, which requires additional energy. The ability to maintain concentration gradients and sometimes move materials against them is vital to cell health and maintenance. Thanks to membrane barriers and transport proteins, the cell can accumulate nutrients in higher concentrations than exist in the environment and, conversely, dispose of waste products (Figure 3).

Other transmembrane proteins have communication-related jobs. These proteins bind signals, such as hormones or immune mediators, to their extracellular portions. Binding causes a conformational change in the protein that transmits a signal to intracellular messenger molecules. Like transport proteins, receptor proteins are specific and selective for the molecules they bind (Figure 4).

Figure 4: Examples of the action of transmembrane proteinsTransporters carry a molecule (such as glucose) from one side of the plasma membrane to the other. Receptors can bind an extracellular molecule (triangle), and this activates an intracellular process. Enzymes in the membrane can do the same thing they do in the cytoplasm of a cell: transform a molecule into another form. Anchor proteins can physically link intracellular structures with extracellular structures. 2010 Nature Education All rights reserved.

Peripheral membrane proteins are associated with the membrane but are not inserted into the bilayer. Rather, they are usually bound to other proteins in the membrane. Some peripheral proteins form a filamentous network just under the membrane that provides attachment sites for transmembrane proteins. Other peripheral proteins are secreted by the cell and form an extracellular matrix that functions in cell recognition.

Incontrast to prokaryotes, eukaryotic cells have not only a plasma membrane thatencases the entire cell, but also intracellular membranes that surround variousorganelles. In such cells, the plasma membrane is part of an extensive endomembrane system that includes theendoplasmic reticulum (ER), the nuclear membrane, the Golgi apparatus, andlysosomes. Membrane components are exchanged throughout the endomembrane systemin an organized fashion. For instance, the membranes of the ER and the Golgiapparatus have different compositions, and the proteins that are found in thesemembranes contain sorting signals, which are like molecular zip codes thatspecify their final destination.

Antibodies against specific organelles, the cell membrane, or cytoskeletal components allow you to explore protein localization in situ. Also, you can use them in western blot analyses to confirm the proper fractionation of cell lysates.

Cadherin: a transmembrane protein that mediates calcium-dependent cell-cell adhesion. The Ca2+ binding domains of cadherins are highly conserved, enabling the creation of antibodies that are effective across all members of the cadherin superfamily.

Vimentin: class-III intermediate filaments found in various non-epithelial cells, especially mesenchymal cells. Vimentin is attached to the nucleus, endoplasmic reticulum, and mitochondria, either laterally or terminally.

Desmin: class-III intermediate filaments found in muscle cells. In adult striated muscle, they form a fibrous network connecting myofibrils to each other and the plasma membrane from the periphery of the Z-line structures.

Cytokeratin: intermediate filaments present in all epithelial cells and several non-epithelial cells. These may regulate the activity of kinases, such as PKC and SRC, via binding to integrin beta-1 (ITB1) and the receptor of activated protein kinase C (RACK1/GNB2L1).

Endoplasmic reticulum (ER): found in eukaryotic cells and is made of membrane sacs called cisternae. Rough ER (where ribosomes are bound) is a site of protein synthesis. Smooth ER is a site for lipid and carbohydrate metabolism. The ER forms part of a network of membranes with the Golgi and lysosomes.

Golgi apparatus: serves as a molecular assembly line in which membrane proteins undergo extensive post-translational modification. The Golgi is part of a network of membranes with the ER and lysosomes.

Mitochondria: cytoplasmic organelles found in almost all eukaryotic cells, comprising an outer membrane, a folded inner membrane, and a matrix. Mitochondria are the cellular powerhouses, generating ATP through oxidative phosphorylation, and play a role in apoptosis. 17dc91bb1f