Membranes intracellular

Ion Channels. The excitable cell maintains an asymmetric distribution across both the plasma membrane, defining the extracellular and intracellular environments, as well as the intracellular membranes which define the cellular organelles. This maintained a symmetric distribution of ions serves two principal objectives. It contributes to the generation and maintenance of a potential gradient and the subsequent generation of electrical currents following appropriate stimulation. Moreover, it permits the ions themselves to serve as cellular messengers to link membrane excitation and cellular... 

Adapted from Hatefi, Y, 1985. The mitochondrial electron tran.sport chain and oxidative pho.sphorylation. sy.stem. Annual Review of Biochemistry 54 1015-1069 and DePierre, J., and Ern.ster, L., 1977. Enzyme topology of intracellular membrane.s. Annual Review of Biochemistry 46 201-262. 

C1C-3, -4 and -5 form the second branch of the CLC gene family. These proteins are 80% identical, and with the exception of C1C-5, which is most highly expressed in kidney and intestine, show a broad expression pattern. C1C-3 to C1C-5 reside in intracellular membranes of the endocytotic pathway [4]. Disruption of C1C-5 leads to a defect in endocytosis in mouse... 

Mucolipin, also known as mucolipin 1 or mucolipidin (encoded by the MCOLN1 gene), is a TRP channel-related membrane protein, most probably residing in intracellular membranes. Is defective in mucolipidosis type IV disease, a developmental neurodegenerative disorder characterized by lysosomal storage disorder and abnormal endocytosis of lipids. The fimction of mucolipin is unknown. 

Aerobic respiration. Many organisms carry out aerobic respiration in which enzymes remove electrons from organic compounds and pass them through a chain of carriers including flavoproteins and cytochromes located in intracellular membranes (Fig. 3-4) until finally they are used to reduce oxygen to produce water. ATP is produced by an enzyme called ATPase, that is located in the cell membrane, and the process is driven by a proton gradient across the membrane. 

Confirmation that FMLP-induced activation involves release of intracellular calcium was obtained by loading neutrophils with CTC. Addition of 20 pH CTC to a neutrophil suspension resulted in a gradual increase in CTC fluorescence as the probe entered the cells and partitioned into intracellular membranes (Figure 9, upper panel). Addition of FMLP resulted in an abrupt decrease in fluorescence, suggesting release of calcium from intracellular membranes probed by CTC. The FMLP-induced release of calcium monitored by CTC was little affected by increased medium osmolality a similar fluorescence decrease was seen in the presence of sodium HEPES (645 mosmol/kg) or sodium sulfate (662 mosmol/kg) (Figure 9, lower panel). 

Table 46-9. Some disorders due to mutations in genes encoding proteins involved in intracellular membrane transport. ...

Olkkonen VM, Ikonen E Genetic defects of intracellular-membrane transport. N Engl J Med 2000 343 1095. 

It is well recognized that is an important regulatory element for many cellular processes, and that the major entry pathway for Ca in many cell types is via plasma membrane Ca channels. Ca channels are functional pores in membranes. They exist in plasma membranes, transverse tubule membranes and in intracellular membranes such as the sarcoplasmic and endoplasmic reticulum. Ca channels are normally closed when opened, Ca passively flows through the chan-... 

There are several major classes of Ca channels (1) receptor-operated Ca channels in plasma membranes (2) ligand-gated Ca " channels in intracellular membranes and (3) voltage-dependent Ca channels that are usually found in plasma membranes or the invaginations of the plasma membrane that are known as transverse tubule membranes. Receptor-dependent or receptor-operated Ca channels (ROCCs) are primarily opened in response to activation of their associated receptors and, by definition, exhibit a certain amount of selectivity for Ca " over other cations. Several potentially different types of ROCCs have been characterized including ATP-sensitive channels in smooth muscle [1], mitogen and IP3-sensitive... 

Both the intracellular and the plasma membranes are actively involved in the cell s vital functions. In the surface membranes of axons, processes of information transfer in the form of electrical signals (nerve impulses) lake place. Bioenergy conversion processes occur at the intracellular membranes of the mitochondria and chloroplasts. 

The pathway of the metabolic process converting the original nutrients, which are of rather complex composition, to the simple end products of COj and HjO is long and complicated and consists of a large number of intermediate steps. Many of them are associated with electron and proton (or hydrogen-atom) transfer from the reduced species of one redox system to the oxidized species of another redox system. These steps as a rule occur, not homogeneously (in the cytoplasm or intercellular solution) but at the surfaces of special protein molecules, the enzymes, which are built into the intracellular membranes. Enzymes function as specific catalysts for given steps. 

Lipids are transported between membranes. As indicated above, lipids are often biosynthesized in one intracellular membrane and must be transported to other intracellular compartments for membrane biogenesis. Because lipids are insoluble in water, special mechanisms must exist for the inter- and intracellular transport of membrane lipids. Vesicular trafficking, cytoplasmic transfer-exchange proteins and direct transfer across membrane contacts can transport lipids from one membrane to another. The best understood of such mechanisms is vesicular transport, wherein the lipid molecules are sorted into membrane vesicles that bud out from the donor membrane and travel to and then fuse with the recipient membrane. The well characterized transport of plasma cholesterol into cells via receptor-mediated endocytosis is a useful model of this type of lipid transport. [9, 20]. A brain specific transporter for cholesterol has been identified (see Chapter 5). It is believed that transport of cholesterol from the endoplasmic reticulum to other membranes and of glycolipids from the Golgi bodies to the plasma membrane is mediated by similar mechanisms. The transport of phosphoglycerides is less clearly understood. Recent evidence suggests that net phospholipid movement between subcellular membranes may occur via specialized zones of apposition, as characterized for transfer of PtdSer between mitochondria and the endoplasmic reticulum [21]. 

GENERAL MECHANISMS OF INTRACELLULAR MEMBRANE TRAFFICKING IN MAMMALIAN CELLS INCLUDE BOTH UNIVERSAL AND HIGHLY SPECIALIZED PROCESSES 139... 

Eukaryotic cells have evolved a complex, intracellular membrane organization. This organization is partially achieved by compartmentalization of cellular processes within specialized membrane-bounded organelles. Each organelle has a unique protein and lipid composition. This internal membrane system allows cells to perform two essential functions to sort and deliver fully processed membrane proteins, lipids and carbohydrates to specific intracellular compartments, the plasma membrane and the cell exterior, and to uptake macromolecules from the cell exterior (reviewed in [1,2]). Both processes are highly developed in cells of the nervous system, playing critical roles in the function and even survival of neurons and glia. 

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