Eukaryotic cells compartmentation

Nicchitta, C. V., Lerner, R. S., Stephens, S. B., Dodd, R. D., and Pyhtila, B. (2005). Pathways for compartmentalizing protein synthesis in eukaryotic cells The templatepartitioning model. Biochem. Cell Biol. 83, 687—695. 

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. 

Substrate availability to the cell is affected by the supply of raw materials from the environment. The plasma membranes of cells incorporate special and often specific transport proteins (translocases) or pores that permit the entry of substrates into the cell interior. Furthermore pathways in eukaryotic cells are often compartmentalized within cytoplasmic organelles by intracellular membranes. Thus we find particular pathways associated with the mitochondria, the lysosomes, the peroxisomes, the endoplasmic reticulum for example. Substrate utilization is limited therefore by its localization at the site of need within the cell and a particular substrate will be effectively concentrated within a particular organelle. The existence of membrane transport mechanisms is crucial in substrate delivery to, and availability at, the site of use. 

To evaluate the actual protein levels ofpeptide synthetases with respect to translation, posttranslational processing, stability, and localization. These aspects are especially related to the compartmentation of eukaryotic cells. So far, evidence for a vacuolar attachment of ACV synthetase in P. chrysogenum has been obtained [95], but it has not been fully substantiated in further studies [96], The synthetase is now considered to be cytoplasmatic. 

The best known functions of the ER require a high membrane surface and/or a separate, specific microenvironment within the organelle. Although many enzymes hosted by the ER use its membranous structure only as a scaffold, others are compartmentalized within the ER i.e., their active site is localized in the lumen. The activity of these enzymes usually is dependent on the special composition of the luminal compartment. The enzymes often receive their substrates and cofactors from or release their products to the cytosol therefore, the transport of these compounds across the ER membrane is indispensable. This article focuses on this latter group of the ER enzymes, the functioning of which makes the ER a separate metabolic compartment of the eukaryotic cell. 

Accumulating evidence clearly points at involvement of the cell cytoskeleton in the compartmentalization of the membrane, in particular, the fine cytoskeleton filaments formed by actin in most eukaryotic cells or spectrin in mammalian red blood cells. However, single-particle tracking experiments show the same patterns of hop-diffusion for lipid molecules located in the extracellular leaflet of the plasma membrane. How can the membrane skeleton, which is located only on the cytoplasmic surface of the membrane, suppress the motion of lipids on the extracellular side ... 

Some of the most intriguing differences between the met-allomes of different cell types occur in subcellular organelles or vesicles. Eukaryotic cells in particular have carefully compartmentalized essential transition metals for specific biological purposes. The mitochondria and the chloroplast both contain high levels of metalloproteins relative to the cytoplasm and may have distinct metal quotas. Mammahan cerebrocortical neurons possess zinc-filled vesicles with labile zinc pools that approach... 

The compartmentalization of eukaryotic cells makes possible many processes that must be separated from the remainder of the cellular environment to function properly. Specific proteins are found in peroxisomes, others in mitochondria, and still others in the nucleus. How do proteins end up in the proper compartment Even for bacteria, some targeting of proteins is required some proteins are secreted from the cell, whereas others remain in the cytosol. 

Compartmentation. The metabolic patterns of eukaryotic cells are markedly affected by the presence of compartments (Figure 30 3). The fates of certain molecules depend on whether they are in the cytosol or in mitochondria, and so their flow across the inner mitochondrial membrane is often regulated. For example, fatty acids are transported into mitochondria for degradation only when energy is required, whereas fatty acids in the cytosol are esterified or exported. 

The smallest free-living microorganisms are the prokaryotes, comprising bacteria and archaea (see Chapter 2). Prokaryote is a term used to define cells that lack a true nuclear membrane they contrast with eukaryotic cells (e.g. plants, animals and fungi) that possess a nuclear membrane and internal compartmentalization. Indeed, a major feature of eukaryotic cells, absent from prokaryotic cells, is the presence in the cytoplasm of membrane-enclosed organelles. These and other criteria differ-... 

FIG. 3.1 Main types of subcellular organization of carbohydrate catabolism in eukaryotes. (A) No compartmentation (Giardia sp., Entamoeba sp.) (B) cytosolic/hydrogenosomal compartmentation (Trichomonas vaginalis, other trichomonads) (C) cytosolic/mitochondrial compartmentation (most eukaryotic cells). 1, hydrogenosome 2, mitochondrion. 

Figure 3.3 Depiction of a eukaryotic (animal) cell. All eukaryotic cells have evolved with significant compartmentalization resulting in spacio-temporal separation of transcription and translation. DNA is transcribed to mRNA in the nucleus before being shuttled out of the nucleus for translation in the cytosol where ribosomes are located (Reproduced from Voet Voet, 1995 [Wiley] Fig. 1-5).

The enzyme studies described above are also compatible with a number of experiments in which incorporation of ribonucleotides into DNA has been shown to be more efficient than incorporation of deoxyribonucleotides Functional compartmentation of DNA precursors is also observed in the utilization of deoxyuridine or thymidine for DNA which in E. coli or in eukaryotic cells occurs without prior equilibration with the free dTTP pool On the other hand, in thymocytes purine deoxyribonucleosides are converted to nucleotides but are not utilized for DNA replication however here they allosterically inhibit ribonucleotide reduction . All these data agree with the existence of two different deoxyribonucleotide pools, one associated with the replitase complex and another independent pool of free deoxyribonucleotides. 

In plant cells, as in other eukaryotic cells, the endomembrane system is composed of multiple organelles with distinct morphology and functions. The compartmentalized endomembrane system ensures proper processing and trafficking of macromolecules to the sites of function. A normally functioning endomembrane trafficking machinery is essential for plant growth and development [1]. 

Most E.are strictly intracellular, but some are excreted into the body fluids or the medium of unicellular organisms. Proteolytic E. axe secreted in the form of inactive precursors which are only activated after they are safely out of the cell those which are not secreted are sequestered into a special organelle, the ly-sosome. Some E. are specific for particular organs or tissues, and their presence in the blood can be used as a diagnostic test for damage to the tissue of origin. Within the eukaryotic cell there is a fair amount of compartmentation of the E. (and reactions) within or-... 

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