Ceil membranes proteins

Just how fast can proteins move in a biological membrane Many membrane proteins can move laterally across a membrane at a rate of a few microns per minute. On the other hand, some integral membrane proteins are much more restricted in their lateral movement, with diffusion rates of about 10 nm/sec or even slower. These latter proteins are often found to be anchored to the cytoskeleton (Chapter 17), a complex latticelike structure that maintains the cell s shape and assists in the controlled movement of various substances through the ceil. 

The amino acids of a protein control its location in the cell. Some proteins are water soluble, whereas others are bound to the ceil membrane (plasma membrane), the mitochondrial membrane, and the membranes of the endoplasmic reticulum and nucleus. The association of a protein with a membrane is maintained by a stretch of lipophilic amino acids. Insertion of this stretch into the membrane occurs as the protein is synthesized. Water-soluble proteins are formed on ribosomes that "float" free in the cytoplasm. Membrane-bound proteins are formed on ribosomes that associate with the endoplasmic reticulum (ER). As the amino acids are polymerized in the vicinity of the F,R, a stretch of lipophilic acids becomes inserted into the membrane of the FR. This anchoring of the protein is maintained when it is shuttled from its location in the ER to its desired location in the plasma membrane. 

Mothes, W., et al. 1997. Molecular mechanism of membrane protein integration into the endoplasmic reticulum. Ceil 89 523-533. 

Fig. 10. Structure of the glycosytinositolphosphate structure used by nature to anchor proteins at the C-terminus to ceil membranes.

Cells need a certain amount of eneigy for maintenance. The maintenance enogy is, for instance, needed for maintaining the proton motive force which is, among other purposes, used for maintaining the ion gradients across the ceil membrane. Furdiermore, eneigy is needed for die turnover of proteins and mRNA, for rqiair and for movement (if mobile). 

Rgure 16 Attempts to record absorbance spectra in a very turbid ceil suspension. A shows the absoiute spectrum (a scan of a cuvette with the buffer taken as the baseline) of reduced membranes of the bacterium Zymomonas mobilis, acquired with a single-beam instrument (Beckman DU 650). B shows AA acquired with a duai-waveiength instrument (SDB-4) with 500 nm as the reference wavelength. C is the reduced minus as prepared difference spectrum obtained with the same duakwaveiength instrument. The concentration of membrane protein was 10 mg mr sodium dithionite was used as the reductant. 

Lipids also undergo rapid lateral motion in membranes. A typical phospholipid can diffuse laterally in a membrane at a linear rate of several microns per second. At that rate, a phospholipid could travel from one end of a bacterial ceil to the other in less than a second or traverse a typical animal ceil in a few minutes. On the other hand, transverse movement of lipids (or proteins) from one face of the bilayer to the other is much slower (and much less likely). For example, it can take as long as several days for half the phospholipids in a bilayer vesicle to flip from one side of the bilayer to the other. 

There are receptors (TfRs) on the surfaces of many cells for transferrin, it binds to these receptors and is internalized by receptor-mediated endocytosis (compare the fate of LDL Chapter 25). The acid pH inside the lysosome causes the iron to dissociate from the protein. The dissociated iron leaves the endosome via DMTl to enter the cytoplasm. Unlike the protein component of LDL, apoTf is not degraded within the lysosome. Instead, it remains associated with its receptor, returns to the plasma membrane, dissociates from its receptor, reenters the plasma, picks up more iron, and again delivers the iron to needy ceils. 

The major functions of the red blood ceil are relatively simple, consisting of dehvering oxygen to the tissues and of helping in the disposal of carbon dioxide and protons formed by tissue metabolism. Thus, it has a much simpler structure than most human cells, being essentially composed of a membrane surrounding a solution of hemoglobin (this protein forms about 95% of the intracellular protein of the red cell). There are no... 

Induction of Calbindin-D In response to calcitriol administration, there is an increase in mRNA synthesis and then in the synthesis of calbindin-D in intestinal mucosal cells, which is correlated with the later and more sustained increase in calcium absorption. In vitamin D-deficient animals, there is no detectable calbindin in the intestinal mucosa, whereas in animals adequately provided with vitamin D, it may account for 1 % to 3% of soluble protein in the cytosol of the colunmar epithelial ceils. Although the rapid response to calcitriol is an increase in the permeability of the brush border membrane to calcium, the induction of calbindin permits intracellular accumulation and transport of calcium. The rapid increase in net calcium transport in tissue from vitamin D-replete animals is presumably dependent on the calbindin that is already present in deficient animals, there can be no increase in calcium transport until sufficient calbindin has accumulated to permit intracellular accumulation, despite the increased permeability of the brush border. 

A stirred cell equipped with a 0.22iuni membrane filter was charged with 30 mL of latex, the dispersion of microsphere. The specific surfrice area was adjusted to 0.19 m per ImL and the ionic strength was calibrated to 0.01. At the constant stirrer speed, buffer solution was introduced into the stirred ceil until steady state flux was attained. Protein solutions were introduced with step of pulse injection. The permeate flux was measured continuously with an electronic balance (Precision plus, Ohaus Co., USA) by a data acquisition system. The electronic balance was connected to a PC through a RS 232C interfece. The surface charge density of microspheres was varied as 0.45, S.94, 9.14 and 10.25, and the stirrer speed was varied as 300,400 and 600rpm. 

The ceils of mammals and other animals contain the following structures. A plasma membrane (FM), which is the outer border of the cell, has a structure similar to the bilayer sheet shown in Figure 1.12. The PM contains phospholipids and many membrane-bound (embedded) proteins used to facilitate the transport of nutrients and minerals into and out of the cell, The outside of the FM of some cells is coated with polysaccharides for protection. The outside of the PM of other cells bears proteins that control which cells are chosen neighbors. Generally, the material bound to the outside of the PM is synthesized by the cel itself rather than derived from other cells. The cytoplasm is the fluid contained and bounded by the plasma membrane. This fluid has a gel-like consistency because it contains a high concentration of proteins. Most of the biochemical reactions that occur within the cell take place in the cytoplasm. The remainder take place within various organelles. 

The major sites of regulation of Na balance are the principal cells of the renal tubule. The proteins involved in regulating salt balance include Na,K ATPase, the sodium channel, and the potassium channel. These proteins arc membrane-bound proteins. They are used for transporting Na and K across the membranes of the tubule cell. Sodium rcabsorption involves the transport of Na appearing in the lumen of the renal tubule into the tubule ceil and on through the cell to the interstitial space. The Na appearing in the interstitial space can then pass into the capillaries to enter the bloodstream. 

The cell consists of a ceil wall and an ouier membrane that enclo.ses cytoplasm containing a nuclear region and ribosomes. The cel wall protects the cell from external influences. The cell membrane provides for selective transport of materials into and out of the cell. Other substances can attach to the cell membrane to carry out important cell functions. The cytoplasm contains the ribo-some.s that contain ribonucleic acid (RNA), which are important in the synthesis of proteins. The nuclear region contains deoxyribonucleic acid... 

A FIGURE 16-2 Electron micrograph of ribosomes attached to the rough ER in a pancreatic acinar cell. Most of the proteins synthesized by this type of ceil are to be secreted and are formed on membrane-attached ribosomes. A few membrane-unattached (free) ribosomes are evident presumably, these are synthesizing cytosoiic or other nonsecretory proteins. [Courtesy of G. Paiade.l... 

Figure 10. Recovery of proteins into a dialysis bag. Upon terminaticn of the IPG run, the gel strip containing the protein of interest is cut along the contours, chopped to pieces and loaded on top of a stacking gel in a preparative disc electrophoresis apparatias (here the glass tube has an inner diameter of 1.5 cm). After zone electrophoresis (usually 30-45 min at 4 C and 250 V), the protein is collected into the chamber having as a floor the dialysis membrane and as a ceiling the 5%T stacking gel, in a free liquid phase (20% sucrose in 100 mM Tris-acetate, pH 8.5) (Reproduced with permission from Ref. 48. copyright 1986, from Elsevier).

As with other essentiai nutrients there are homeostatic mechanisms which maintain a constant tissue concentration of zinc in spite of fiuctuations in dietary suppiy. The total dietary intake is 10-15 mg per day. The bioavailability of zinc from different foodstuffs varies. Some 40% of zinc is absorbed from the average diet. Inside the intestinai mucosai ceil zinc enters a metabolic pooi in equiiibrium with zinc-thionein. The synthesis of this metal binding protein is induced by various metals, and it appears to reguiate their intracellular transport. Zinc leaves the intestinal mucosal cell across the plasma membrane and is taken up by albumin in the portal circulation. The liver extracts zinc with a high... 

Figure 7.7-2. Liposome-cell interactions. Drug-loaded liposomes can specifically (A) or nonspecificaiiy (B) adsorb onto the ceil surface. Liposomes can also fuse with the cell membrane (C) and release their contents into the cell cytoplasm, or they can be destabilized by certain cell membrane components when adsorbed on the surface (D) so that the released drug can enter the cell via micro-pinocytosis. Liposome can undergo the direct or transfer-protein mediated exchange of lipid components with the cell membrane (E) or be taken up by specific or nonspecific endocytosis (F). In the case of endocytosis, a liposome can be delivered by the endosome into the lysosome (G), or en route to the lysosome, the liposome can provoke endosome destabilization (H), which results in drug liberation into the cytoplasm. (With permission from Ref. 29.)...

These lipid bilayers are not very permeable towards a variety of molecules. Nevertheless, for cell metabolism and growth to occur, molecules such as sugars and amino acids must enter the ceil. Specific transport of this type is accomplished by proteins which are incorporated within the bilayer membrane. The protein serves as a carrier and the tjrpe of transport can be defined as carrier-mediated transport. The cell membrane consists of two main components the lipid bilayer which is the backbone, whereas the proteins take care of the specific transport functions. Some of the proteins are located on the outside of the lipid bilayer (the extrinsic proteins), whereas other proteins (the intrinsic proteins), completely penetrate through the lipid bilayer. The intrinsic proteins especially... 

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