Cytoplasmic reticulum

Figure 2. Scanning electron micrograph of a mesophyll cell of a dormant cotyledon of Buffalo gourd (Cucurbita foetidissima). Tissue was fixed in aqueous glutaraldehyde, dehydrated with ethanol and critically point dried. Note cell wall (W) and intracellular components including protein bodies (P) and emptied spherosomes that appear as a cytoplasmic reticulum.

Fig. 4.27. Expression of GPP labeled transcription factor in the salivary gland cells of Drosophila [62]. The generation of transcription factor molecules can be seen in the cytoplasmic reticulum as well as the distribution on the polytenic chromosomes in the nucleus. Image taken in a confocal laser scan microscope from C. Zeiss, modified for APD Imaging [6f]...

The membranes ofplant cells are remarkably like those of animals. The plasma membrane of plants seems not to offer so great a barrier as that of the mitochondria, or of the cytoplasmic reticulum, or of the vacuole (a feature absent in animals). Most, if not all, movement into plant roots is physical diffusion unaided by biochemical processes. The plasma membrane of roots seems to have a special structure with unusual selectivity. Maleic hydrazide causes chromosome breakages in plant cells, but not in mammalian cells. As mammalian and plant chromosomes have the same chemical composition, a difference in permeability is indicated (Barnes etaL, 1957). 

A large group of even- and odd-numbered a-hydroxy fatty acids with 20 to 26 C-atoms occurs in brain cerebrosides (see chapter BII, 2a). These acids are formed by a direct a-hydroxylation in the cytoplasmic reticulum. The loss of one C-atom of the a-hydroxy acids leads to the odd-numbered fatty acids (Fulco et al. 1961). 

In mammals the introduction of new double bonds into mono- and polyunsaturated fatty acids exclusively occurs in the carboxyl end and is never directed toward the terminal methyl-group. Therefore no transition of fatty acids belonging to the linoleic acid family into those of the linolenic acid type has been observed. This has been shown by means of terminally labeled synthetic polyunsaturated fatty acids (Stoffel 1961, Klenk 1964). The complete enzyme system for polyunsaturated fatty acid synthesis is arranged on the cytoplasmic membranes. In view of the importance of polyunsaturated fatty acids for the structure of glycero-phospholipids, it is interesting to mention the acyl-transferases catalyzing the acylation of the j8-position of lysolecithin, lysophosphatidic acid and L-a-glycero-phosphate. These and other enzymes of phospholipid biosynthesis are located in the cytoplasmic reticulum, which therefore appears to be the main site of lipid synthesis of the cell. 

Cellular component involved Chromatin Nucleoplasm and cytoplasm Cytoplasm (reticulum) Total cell... 

Although lanosterol may appear similar to cholesterol in structure, another 20 steps are required to convert lanosterol to cholesterol (Figure 25.35). The enzymes responsible for this are all associated with the endoplasmic reticulum. The primary pathway involves 7-dehydroeholesterol as the penultimate intermediate. An alternative pathway, also composed of many steps, produces the intermediate desmosterol. Reduction of the double bond at C-24 yields cholesterol. Cholesterol esters—a principal form of circulating cholesterol—are synthesized by acyl-CoA cholesterol acyltransferases (ACAT) on the cytoplasmic face of the endoplasmic reticulum. 

The Ca2+-AIPase transports Ca2+ ions into endoplasmic reticulum or out of the cell from the cytoplasm, using the energy of ATP hydrolysis. 

COPII vesicles are transport intermediates from the endoplasmic reticulum. The process is driven by recruitment of the soluble proteins that form the coat structure called COPII from the cytoplasm to the membrane. 

Intracellular Ca2+-levels are controlled by release into, and removal from, the cytoplasm (Fig. 1). Ca2+-pumps in the plasma membrane and endoplasmic reticulum (ER the Ca2+-store in a cell) keep cytoplasmic Ca2+-levels low (about 0.1 pmol/L in resting cells) and generate a 10,000-fold concentration gradient across membranes (because extracellular Ca2+ is in the millimolar range). Upon stimulation, Ca2+ enters the cytosol of the cell via Ca2+-channels (plasma membrane) or via Ca2+-channels in the ER, leading to the activation of a great variety of Ca2+-dependent processes in the cell. 

The UPS also plays a major role in protein quality control. In a process known as endoplasmic associated degradation (ERAD), misfolded proteins, which are formed in the endoplasmatic reticulum, are translocated back to the cytoplasm and degraded by the proteasome. 

Like other cells, a neuron has a nucleus with genetic DNA, although nerve cells cannot divide (replicate) after maturity, and a prominent nucleolus for ribosome synthesis. There are also mitochondria for energy supply as well as a smooth and a rough endoplasmic reticulum for lipid and protein synthesis, and a Golgi apparatus. These are all in a fluid cytosol (cytoplasm), containing enzymes for cell metabolism and NT synthesis and which is surrounded by a phospholipid plasma membrane, impermeable to ions and water-soluble substances. In order to cross the membrane, substances either have to be very lipid soluble or transported by special carrier proteins. It is also the site for NT receptors and the various ion channels important in the control of neuronal excitability. 

The reaction of choline with mitochondrial bound acetylcoenzyme A is catalysed by the cytoplasmic enzyme choline acetyltransferase (ChAT) (see Fig. 6.1). ChAT itelf is synthesised in the rough endoplasmic reticulum of the cell body and transported to the axon terminal. Although the precise location of the synthesis of ACh is uncertain most of that formed is stored in vesicles. It appears that while ChAT is not saturated with either acetyl-CoA or choline its synthesising activity is limited by the actual availability of choline, i.e. its uptake into the nerve terminal. No inhibitors of ChAT itself have been developed but the rate of synthesis of ACh can, however, be inhibited by drugs like hemicholinium or triethylcholine, which compete for choline uptake into the nerve. 

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. 

Mutations in another region, the second cytoplasmic loop between M2 and M3 in Ca-ATPase of sarcoplasmic reticulum (Thr ->Ala, Gly -t Ala, and Glu Gln) also result in a complete loss of Ca-transport and Ca-ATPase activity associated with a dramatic reduction in the rate of phosphoenzyme turnover [96]. These mutations do not affect the affinity of the enzyme for Pj and therefore resemble the Pro mutants [123] in that they affect only the E1P-E2P conformational change and not the affinities for ATP, Ca or Pj. 

Most living cells, including muscle, maintain the cytoplasmic Ca concentration at submicromolar levels, against steep gradients of [Ca ], both at the cell surface and across the endoplasmic reticulum membrane [17]. In the musele cell two membrane systems are primarily involved in this function the sarcoplasmic reticulum and the surface membrane. 

The Ca transport ATPase of sarcoplasmic reticulum is an intrinsic membrane protein of 110 kDa [8-11] that controls the distribution of intracellular Ca by ATP-dependent translocation of Ca " ions from the cytoplasm into the lumen of the sarcoplasmic reticulum [12-16],... 

As in the Ca -ATPase of sarcoplasmic reticulum, the predicted number of membrane spanning sequences in PMCAl and PMCA2 is even, with both N- and C-terminus located on the cytoplasmic side, but their actual number is uncertain and may be 10 or less [30]. 

The cytoplasmic domain of the Ca -ATPase of rabbit sarcoplasmic reticulum is very similar to the structure derived from Fourier-Bessel reconstructions of the Ca -ATPase tubules of scallop sarcoplasmic reticulum [176]. 

The A20 antibody did not bind significantly to native SR vesicles, but solubilization of the membrane with C Eg or permeabilization of the vesicles by EGTA exposed its epitope and increased the binding more than 20-fold [139], By contrast, the A52 antibody reacted freely with the native sarcoplasmic reticulum, while the A25 antibody did not react either in the native or in the C Eg solubilized or permeabilized preparations, and required denaturation of Ca " -ATPase for reaction, Clarke et al, [139] concluded that the epitope for A52 is freely exposed on the cytoplasmic surface, while the epitope for A20 was assigned to the luminal surface, where it became accessible to cytoplasmic antibodies only after solubilization or permeabilization of the membrane. The epitope for A25 is assumed to be on the cytoplasmic surface in a folded structure and becomes accessible only after denaturation. 

Essentially identical conclusions arose from the studies of Matthews et al. [138], An anti-peptide antibody directed against the cytoplasmically exposed C-terminal region of the Ca " -ATPase (985-994) reacted freely in native sarcoplasmic reticulum, in agreement with earlier observations [137], while the antibody directed against the putative luminal loop (877-888) reacted strongly only after solubilization of sarcoplasmic reticulum with Ci2Eg, Purified ATPase preparations reacted freely with both antibodies under both conditions. A 30-kDa protease-resistant fragment obtained... 

Of the 20 residues that react with A-ethylmaleimide in the non-reduced denatured Ca -ATPase at least 15 are available for reaction with various SH reagents in the native enzyme [75,239,310]. These residues are all exposed on the cytoplasmic surface. After reaction of these SH groups with Hg-phenyl azoferritin, tightly packed ferritin particles can be seen by electron microscopy only on the outer surface of the sarcoplasmic reticulum vesicles [143,311-314]. Even after the vesicles were ruptured by sonication, aging, or exposure to distilled water, alkaline solutions or oleate, the asymmetric localization of the ferritin particles on the outer surface was preserved [311,313,314]. 

The Ca transport and Ca -stimulated ATPase activity of sarcoplasmic reticulum is inhibited by 10-30nmol dicyclohexylcarbodiimide per mg protein in a Ca free medium [372]. A23187 enhanced the sensitivity of the enzyme to DCCD, while Ca or Sr at micromolar concentrations prevented the inhibition. Since Ca -loaded vesicles retained their sensitivity to DCCD in a Ca -free medium, the reactivity of the enzyme with DCCD is controlled by the occupancy of the high-affinity Ca sites on the cytoplasmic surface of the membrane. 

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