Post-translational processing

Biosynthesis. The biosynthesis of neuropeptides is much more complex and involves the multistep process of transcription of specific mRNA from specific genes, formation of a high molecular weight protein product by translation, post-translational processing of the protein precursor to allow for... 

Glycosydphosphatidylinositolation The GlycoPho-sphatidyl Inositol moiety anchor of AChE consists exclusively of diacyl molecular species. Over 85% of the molecular species are composed of palmitoyl, stearoyl and oleoyl. The post-translational process of glypiation takes place in the endoplasmic reticulum, after completion of the polypeptide chain the newly synthesized protein interacts with a transamidase... 

Not necessary for protein synthesis but formed during post-translational processing of collagen. 

The gene encoding PBAN was first characterized from H. zea and B. mori [134,137,138,195]. The cDNA was found to encode the 33 amino acid PBAN plus four additional peptides with a common C-terminal FXPRL sequence motif, including that of the diapause hormone of B. mori (Fig. 6). Three additional peptides with the common C-termini and sequence homology to those of H. zea and B. mori have been deduced from cDNA isolated from pheromone glands of several other moths [194,196-200]. Studies conducted to find the post-translational processed peptides indicated that PBAN was found to a greater extent in the mandibular and maxillary clusters than in the labial cluster of neurons... 

Short-term requirements for increases in the synthesis of 5-HT can be met by post-translational processes, such as phosphorylation, that change the kinetic properties of tryptophan hydroxylase without necessitating the synthesis of more molecules of tryptophan hydroxylase. By contrast, situations requiring long-term increases in the synthesis and release of 5-HT result in the synthesis of tryptophan hydroxylase protein. For example, partial but substantial destruction (>60%) of central serotonergic neurons results in an increase in the synthesis of 5-HT in... 

FIGURE 1 8-5 Tissue-specific processing of the pro-opiomelanocortin (POMC) precursor yields a wide array of bioactive peptide products. Processing of the POMC precursor varies in various tissues. In anterior pituitary, adrenocorticotropic hormone (ACTH (1-39)) and P-1 ipo tropin (P-LPH) are the primary products of post-translational processing. Arcuate neurons produce the potent opiate P-endorphin (P-endo (1-31)) as well as ACTIK1 -13) NIT,. Intermediate pituitary produces a-melanocyte-stimulating hormone (aMSH), acetylated P endof 1 31) and P-endo(l-27). NTS, nucleus tractus solitarius. 

Post-translational processing defect Trafficking defects of lysosomal enzymes... 

The Ras proteins are synthesized as biologically inactive, cytosolic precursor proteins. They are then modified by several post-translational processing steps at the carboxyl terminal end and thereby converted into biologically active proteins localized at the plasma membrane. The cysteine of the C-terminal CAAX sequence (C is cysteine, A is generally an aliphatic amino acid, and X is methionine, serine, alanine, or glutamine) is first enzymatically S-farnesylated the AAX part is then cleaved off by a specific protease, and the free C-terminal cysteine is finally converted into a methyl ester (Scheme 1). 

Baculovirus Improved procedure Infection of insect cells High expression yields Relatively slow virus production Different post-translational processing... 

A further difference in CB between normal and malignant tissues is that in the latter, a greater proportion of CB is found on the cell membrane (K2). This preferential binding to the cell membrane in malignant tumors may be due to defects in glycosylation during post-translational processing of CB (PI). Whether CB is bound to a specific receptor, like that described earlier for uPA, is presently unclear. 

P cells of the pancreatic islets in combination with atoms of zinc, but when required to regulate blood glucose concentration, the prohormone is cleaved and functional insulin is released into the circulation along with the C-peptide. This example of post-translational processing is mediated by peptidases which are contained in the vesicles along with the proinsulin. The fusion of the secretory vesicles with the cell membrane and activation of the peptidase prior to exocytosis of the insulin are prompted by an influx of calcium ions into the P-cell in response to the appropriate stimulus. Similarly, catecholamines are synthesized and held within the cell by attachment to proteins called chromogranins. 

There are at least two answers to question (i). First, abnormal proteins can arise in cells due to spontaneous denaturation, errors in protein synthesis, errors in post-translational processing, failure of the correct folding of the protein or damage by free radicals. They are then degraded and replaced by newly synthesised proteins. Secondly, turnover helps to maintain concentrations of free amino acids both within cells and in the blood. This is important to satisfy the requirements for synthesis of essential proteins and peptides (e.g. hormones) and some small nitrogen-containing compounds that play key roles in metabolism (see Table 8.4). 

Protein processing in the endoplasmic reticulum makes mistakes. All membrane-associated proteins and proteins that are secreted by the cell are synthesised on membrane-bound ribosomes and pass into the lumen of the reticulum, where they are modified by post-translational processes, so that much biochemical manipulation of the proteins takes place. Consequentially mistakes are often made. Such abnormal proteins are exported from the lumen into the cytosol for ubiquitination and degradation in the proteasome. 

Prokaryotic expression E. coli Rapid growth with high yields (up to 50% total cell protein) Extensive range of vectors Simple, well-defined growth media Lack of post-translational processing Product may prove toxic to host Product incorrectly folded and inactive High endotoxin content... 

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