Location Within the Cell

Protein phosphorylation-dephosphorylation is a highly versatile and selective process. Not all proteins are subject to phosphorylation, and of the many hydroxyl groups on a protein s surface, only one or a small subset are targeted. While the most common enzyme function affected is the protein s catalytic efficiency, phosphorylation can also alter the affinity for substrates, location within the cell, or responsiveness to regulation by allosteric ligands. Phosphorylation can increase an enzyme s catalytic efficiency, converting it to its active form in one protein, while phosphorylation of another converts it into an intrinsically inefficient, or inactive, form (Table 9—1). 

There are a number of possible locations within the cell wall for the pectin further away from cellulose. If there are covalent links between pectins and xyloglucans (16), then pectic chain segments close to these links would appear in the region sharing the same mean mobility characteristics as cellulose. The majority of the pectic molecule, diverging from the microfibrils would appear in the region with greater mean mobility. 

At present, the two-step theory is still accepted, but it leaves out the question of receptor location within the cell so as to be able to cover all members of a family. The receptor, in the absence of hormone, is found associated with other proteins (hsp90, p59, and perhaps others) and very weakly bound to cell structures (nuclear or cytoplasmatic). The arrival of hormones transforms the receptor, freeing it from other proteins, giving it a greater affinity for nuclear structures, and causing it to achieve an active state as a transcription factor (Beato et al. 1996 Beato 1989). The difference is that the receptors not... 

Receptors are proteins or glycoproteins found either on the surface of the target cell or located within the cell interior. The surface receptors engage peptide hormones which, being hydrophilic, do not traverse the fatty plasma membrane intracellular receptors combine specifically with particular steroids or tri-iodothyronine, T3. 

Depending on the desired antigen to be identified and its location within the cell, the cells could be subjected to a detergent incubation (5). A pretreatment of DMSO, Triton X-lOO in PBS can be employed for better immunoglobulin penetration. It is important to completely dissolve the Triton X-100 in the PBS before adding the DMSO. 

Fluorescence excitation with a laser microbeam allows for a smaller region to be illuminated. Monitoring fluorescence with a sensitive photomultiplier tube also permits the use of lower intensities of irradiation for shorter periods of time. Therefore, unwanted photobleaching can be significantly reduced. If the spot size is adjusted to illuminate an entire cell, information analogous to spectrofluorometry or flow cytometry can be obtained on an individual cell basis with a high degree of temporal resolution. If the spot size is smaller than the cell, similar information can be obtained from a particular location within the cell. 

As an example, one may consider that a random process occurs where a protein may bind at a given location within the cell as often as y = 1, 2, 3,. .., times per second, but on average the protein binds once per second. If y is described by a Poisson probability function, estimate the probabihty that the protein will not bind at the site during a one-second interval. In this case, the time period is one second and the mean binding steps is A equal to 1. Therefore,... 

The challenge is to get the genetic construct into the appropriate cell, and have it settle down in the cell to be copied faithfully every time the cell divides. Most importantly, it must be placed where it would direct the production of the correct protein so that the protein would be produced in the right amount and in the proper location within the cell. So far, gene therapy has not had much success. 

Receptors for steroid hormones are located within the cell and bind to specific DNA sequences when they are activated by the appropriate hormones. The binding of these... 

Oncogene products assume specific locations within the cell. Usually they are associated with one of two locations The plasma membrane or the nucleus. This specificity is consistent with the hypothesis that oncogene products are associated with normal cellular metabolism relating to regulation of cell proliferation it seems likely that many of the elements regulating cell proliferation would be found at the membrane and nucleus of the cell. 

Data of release rates for seven enzymes from baker s yeast with a high-pressure homogenizer revealed that differences in release rates agreed with reported locations in the cell but are not sufficient to fractionate the enzymes release rates did not seem to depend much on operating pressure, temperature, or initial cell concentration (Follows, 1971). Both the dependence of release rates on the location within the cell and the description of release by a first-order law has been confirmed in both yeast and E. coli with several disruption techniques such as sonica-tion, high-pressure homogenization, and hydrodynamic cavitation (Balasundaram, 2001). 

Hence, steroids exert their primary effects by acting on receptors located within the cell. As discussed in Chapter 4, it is apparent that these substances exert some of their effects by binding to a second set of receptors located on the cell surface.1318 Studies continue to define the exact role of steroidal surface receptors and their contribution to the effects of each type of steroid... 

Modern life forms are based on DNA which is located within the cell nucleus and only self-replicates with the aid of enzymes that are synthesised based on instructions coded into the DNA and mediated by RNA. There is thus a chicken and egg paradox - which came first the DNA or the enzymes ... 

Cellular transformation by DNA tumour viruses in most cases has been shown to be the result of protein-protein interaction. Proteins encoded by the DNA tumour viruses, termed tumour antigens (T antigens), can interact with cellular proteins. This interaction effectively sequesters the cellular proteins away from their normal functional locations within the cell. The predominant types of protein sequestered by viral T antigens have been shown to be of the tumour-suppressor type. It is the loss of their normal suppressor functions that results in cellular transformation. 

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