Subcellular compartment

Eukaryote Organism whose cells have a discrete nucleus and other subcellular compartments (cf. prokaryote)... 

Protein trafficking is the transport of proteins to their correct subcellular compartments or to the extracellular space ( secretory pathway ). Endo- and exocytosis describe vesicle budding and fusion at the plasma membrane and are by most authors not included in the term protein trafficking. Protein quality control comprize all cellular mechanisms, monitoring protein folding and detecting aberrant forms. 

In eukaryotes, anabolic and catabolic pathways that interconvert common products may take place in specific subcellular compartments. For example, many of the enzymes that degrade proteins and polysaccharides reside inside organelles called lysosomes. Similarly, fatty acid biosynthesis occurs in the cytosol, whereas fatty... 

Compartmentation of pathways in separate subcellular compartments or organelles permits integration and regulation of metabolism. Not all pathways are of equal importance in all cells. Figure 15-7 depicts the subceh lular compartmentation of metabohc pathways in a hepatic parenchymal cell. 

For the sake of study, the biosynthesis of carotenoid plant pigments can be divided into parts involving enzymes and their associated activities as listed in Table 5.3.1 and further detailed in Figure 5.3.1 through Figure 5.3.4. Some of the parts have common enzymatic mechanisms and may also be in distinct subcellular compartments such as cytoplasm, endoplasmic reticulum, or plastid thylakoid. 

Okada, K. et al., Five geranylgeranyl diphosphate synthases expressed in different organs are localized into three subcellular compartments in Arabidopsis, Plant Physiol. 122, 1045, 2000. 

Poly(3HB) synthesis in various subcellular compartments could be used to study how plants adjust their metabolism and gene expression to accommodate the production of a new sink, and how carbon flux through one pathway can affect carbon flux through another. For example, one could study how modifying the flux of carbon to starch or lipid biosynthesis in the plastid affects the flux of carbon to acetyl-CoA and poly(3HB). Alternatively, one could study how plants adjust the activity of genes and proteins involved in isoprenoid and flavonoid biosynthesis to the creation of the poly(3HB) biosynthetic pathway in the cytoplasm, since these three pathways compete for the same building block, i. e., acetyl-CoA. 

Eukaryote Cell or organism with membrane-bound, structurally discrete nucleus and other well-developed subcellular compartments. Eukaryotes include all organisms except viruses, bacteria, and blue-green algae (see also prokaryote). 

The field of translation initiation has focused on the initial round ofribosomal subunit recruitment to an mRNA. Presumably, these events are mirrored in the subsequent rounds of initiation necessary for polyribosome formation. Importantly, because mRNAs are typically present in large polyribosomes (averaging 9-13 ribosomes per mRNA), the initiation events that govern ribosome recruitment to preexisting polyribosomes constitute the majority of translation initiation cycles occurring in an mRNA s lifetime. Whether or not these initiation events mimic the first round of initiation is not yet known. Since eukaryotic cells divide ribosomes between two subcellular compartments, the cytosol and ER membrane, it is also important to know if the mechanism of translation initiation on ER-bound ribosomes is similar to that occurring on soluble ribosomes and, importantly, whether ER-bound ribosomes can direcdy (re) initiate translation on bound polyribosmes. 

Taken together, the field is now well placed to design new biosensors, examine protein-protein and protein-lipid interactions, and sensitively determine protein conformation in living tissues at submicron resolution. These interactions are either impossible or extraordinarily difficult to examine in other ways, and the subcel-lular resolution of FRET-FLIM that allows detection of interactions in specific subcellular compartments may provide insight that... 

FIGURE 35-6 Examples of apoptotic and antiapoptotic mechanisms that act on or within different subcellular compartments. CBP, Ca2+ binding protein CREB, cyclic AMP response element binding protein HSP, heat shock protein IP3, inositol 1,4,5-trisphosphate ROS, reactive oxygen species. 

Enriched subcellular compartments can be analyzed by MS/MS to determine their constituent proteins. One advantage of analysis of different cellular fractions is pre-analytical simplification that offers rewarding yields in dealing with the proteins identified in large-scale MS experiments. One of the major initiatives of the Human Proteome Organization (HUPO) is the comprehensive characterization of the complete subproteome of each cell type. 

The intracellular environment of eukaryote cells can be subdivided into many regions, including the organelles, nucleus, cytoplasm and the cell periphery. Thus solutes must be delivered to the right intracellular compartment at the correct time to efficiently serve cellular biochemistry. Uncharged solutes such as glucose presumably diffuse across the cell, and the traditional view held until recently was that the major electrolytes, such as Na+,K+,CF and Mg2+, also move around the cell by simple diffusion to eventually arrive at the relevant subcellular compartment by chance. 

While our data using this technique are still preliminary, we have observed that 25 yU/ml insulin inhibits the rate of calcium efflux from renal slices (28). This effect of insulin was gradually reduced at the higher concentrations of insulin. The effects of insulin on calcium exchange appear to be localized in the mitochondrial compartment. Further work is needed to determine whether insulin affects specific enzyme systems which are known to play a role in renal calcium transport, and which cellular or subcellular compartments are involved. This would substantially increase our understanding of the regulation of urinary calcium excretion, and of ways in which excessive loss of calcium by this route might be avoided. 

M. Stitt, R. M. Lilley, R. Gerhardt, and H. W Heldt, Metabolite levels in specific cells and subcellular compartments of plant leaves. Methods Enzymol. 174, 221 335 (1989). 

The El protein is highly conserved through evolution. For example, human and yeast Els are 53% identical. El contains a nuclear localization signal and is found in the nucleus in addition to other subcellular compart-... 

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