Reticulocyte protein studies

A well studied example for control at the level of eIF-2 is the regulation of protein biosynthesis in erythroid cells (review Chen and London, 1995). A decrease in the heme concentration in reticulocytes leads to inhibition of globin synthesis at the level... 

Both proteins have been shown to be translated vitro In a reticulocyte lysate system as preinhibitors, 2000-3000 daltons larger than those synthesized and accumulated vivo (11). The preinhibitors may be Important In the compartmentall-zatlon of the Inhibitors as they are stored In the central vacuole, or plant lysosome, of the plant cells (12). We have now studied the time course of the Increase In translatable mRNA In leaves of wounded plants utilizing poly(A) mRNA Isolated at various times following wounding. 

Acid and alkaline phosphatases with phosphorylated intermediates are inhibited by vanadate. This has been exploited in the study of the role of alkaline phosphatase in mineralization.1069 Vanadate also inhibits the ATP-dependent degradation of proteins in reticulocytes.1070... 

Several studies have used a cell-free system prepared from rabbit reticulocytes to translate cestode messenger RNA (mRNA) in vitro. Thus, a small portion of the proteins synthesised in this heterologous system, using mRNA from T. crassiceps, were shown to be antigenic (5). The molecular weights of the translated polypeptides were generally low (13 000-22 000),... 

Schade reviewed (114) the earlier studies on the role of serum transferrin in iron transport. Various early investigators had observed that the blood serum transferrin rapidly bound iron administered either through the gastrointestinal tract or by intravenous injection. There was a rapid turnover of iron in the blood serum and the degree of saturation of the transferrin was related to the amount of iron administered. In no instances, however, was the blood serum transferrin ever saturated with iron. Jandl et al. (71) have shown that both ovotransferrin and serum transferrin can transport plasma iron into red cells and that the transport is dependent on the concentration of transferrin. Iron taken up by the blood cells could not be eluted by subsequent incubation with iron-free transferrin solutions. More recently Morgan and Laurel (99) reported that iron uptake in reticulocytes is independent of the transferrin concentration. The iron complex of serum transferrin has a higher affinity for immature red cells than does the iron-free protein (72). Both bind specifically to immature red cells and the attachment permits the cells to remove the iron. Once the iron is removed, however, the iron-free transferrin can be replaced by an iron-transferrin complex. 

The parenchymatous liver cells (hepatocytes) hold a key position in the overall metabolism of iron (57, 58, 59, 60), and since functionally intact liver mitochondria can be conveniently prepared at high yield, these mitochondria have been most extensively studied so far. The iron transporting system discussed above for liver mitochondria is present also in mitochondria from other tissues and animal species (Table III). Quantitatively, erythroid cells of the bone marrow play the most important role in the overall metabolism of iron (61), and it was therefore not unexpected to find that the energized uptake of iron by isolated reticulocyte mitochondria exceeds that of mitochondria isolated from, for example, liver, kidney, and heart (Table IV). Thus, a relationship appears to exist between the rate and extent of heme protein turnover in mitochondria isolated from different tissues and their energized iron accumulation (30). Thus, it is evident that cellular differentiation is expressed at the mitochondrial level by modulation of the activity of essential functions related to iron transport and heme biosynthesis. 

Scheele, G. (1983) Methods for the study of protein translocation across the RER membrane using the reticulocyte lysate translation system and canine pancreatic microsomal membranes. Methods Enzymol. 96, 94-111. 

Overall, much evidence suggests that a cell-mediated immune response occurs in patients with EMS that culminates in fibrosis of connective tissue in muscles, nerves, and other organs and of the skin. Possibly chemical contaminants may form protein adducts or may be incorporated into host proteins and thereby induce an immune response. A few experimental studies with EBT, one important contaminant of implicated L-tryptophan, have reported that EBT becomes incorporated into proteins of liver and blood (reticulocytes).6465 EBT has also been reported to enhance IL-5 production by splenic T lymphocytes.66... 

The protein kinase H RI (heme regulated eIF-2 kinase) was first identified in studies on the regulation of protein biosynthesis in erythroid cells. A decrease in the heme concentration in reticulocytes leads to inhibition of globin synthesis at the level of translation. This regulation mechanism ensures that only so much globin is produced as is heme available. If the level of heme drops, then HRI becomes activated. The activated HRI phosphorylates the eIF-2a subunit, which in turn shuts off protein biosynthesis (Fig. 1.48). The mechanism of regulation of HRI kinase by heme is not well understood. Heme binding sites have been identified on the N-terminus and the kinase domain of HRI. 

Immunoprecipitation studies of steroid-hormone receptor association with Hsp90 in reticulocyte lysates (Smith et al., 1995) have revealed an apparently ordered series of chaperone complexes. Early complexes contain Hsp70 and its co-chaperones, but lack Hsp90. Intermediate complexes retain Hsp70 and co-chaperones as well as Hsp90, which is coupled to Hsp70 via a common interaction with the p60/Hop scaffold protein (Chen et al., 1996b). Subsequently, late complexes can be... 

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