Effects on Protein Synthesis

Drought also has a profound effect on protein synthesis. In many plant tissues, a reduced water potential causes a reduction of total protein synthesis and a rapid dissociation of polyribosomes. The latter has been shown not to be the consequence of increase in ribonuclease activity (Hsiao, 1973 Dhindsa Bewley, 1976). For a specific protein, Jacobsen, Hanson Chandler (1986) have shown in barley leaves that water stress enhances the synthesis of one of the a-amylase isozymes. Using a cDNA probe they found that water-stressed leaves contained much more a-amylase mRNA than unstressed plants. 

Biochemical and genetic experiments in yeast have revealed that the b poly(A) tail and its binding protein, Pablp, are required for efficient initiation of protein synthesis. Further studies showed that the poly(A) tail stimulates recruitment of the 40S ribosomal subunit to the mRNA through a complex set of interactions. Pablp, bound to the poly(A) tail, interacts with eIF-4G, which in turn binds to eIF-4E that is bound to the cap structure. It is possible that a circular structure is formed and that this helps direct the 40S ribosomal subunit to the b end of the mRNA. This helps explain how the cap and poly(A) tail structures have a synergistic effect on protein synthesis. It appears that a similar mechanism is at work in mammalian cells. 

Figure 12.17 Sites at which insulin stimulates protein synthesis in a muscle. The sites are indicated by 0. Insulin has its anabolic effect on protein synthesis in muscle by affecb ng six processes or reactions (i) it inhibits protein degradab on in the muscle (ii) it sb mulates amino acid transport from the blood into the muscle (iii) it stimulates the inib ab on-reacb on of the pathway for protein synthesis, i.e. formab on of the complex (tRNA-amino acid-mRNA-ribosomal RNA) (iv) it increases the rate of mRNA synthesis, and therefore the number of mRNA molecules (v) it stimulates ribosomal RNA synthesis (vi) it sb mulates elongabon of the pepbde (see Chapter 20).

It is also believed that the triphosphorylated form of fluorinated analogs of pyrimidine can be included in RNA, and can have an effect on protein synthesis. 

Effects on protein synthesis In most tissues, insulin stimulates the entry of amino acids into cells, and protein synthesis. 

Glucocorticoids Steroids (cortisol, corticosterone) Many diverse effects on protein synthesis and inflammation... 

Emetine and cephaeline are both potent inhibitors of protein synthesis, inhibiting at the translocation stage. They display antitumour and antiviral as well as antiamoebic activity, but are too toxic for therapeutic use. In recent studies, O-methylpsychotrine has displayed fairly low effects on protein synthesis, but a quite potent ability to curb viral replication through inhibition of HIV-reverse transcriptase. This may give it potential in the treatment of AIDS. 

A selective effect on protein synthesis influencing the relative amounts of enzymes, ion carriers, and structural proteins produced. 

In addition to these various actions on rather general metabolic functions, GH also has some very specific effects. It has a general stimulatory effect on protein synthesis but also stimulates synthesis of some proteins selectively. Predominant among these is somatomedin C/insulin-like growth factor I (Sm-C/IGF-I), production of which is stimulated markedly in many tissues, especially the liver [13,16]. 

Mutations in tRNALeu n UR) may be expected to have an important effect on protein synthesis in mitochondria. The MELAS-associated human mitochondrial tRNALeu <[ UR) mutation causes aminoacylation deficiency and a concomitant defect in translation initiation (Borner et al., 2000). The expected result would be a deficit in general mitochondrial protein synthesis with resulting deficiencies of multiple OXPHOS components. Patients with MELAS disorder have been found to have a marked decrease in the activity of respiratory Complex I with a secondary reduction in the activity of Complex IV... 

Some other recent work indicates an apparent qualitative effect on protein synthesis. Ho (31) and Jacobsen et al. (32) detected new peptides when aleurone layers were treated with ABA. These peptides turned over rapidly, and the quantities formed were reduced by inhibitors of transcription and translation. Little else is known currently about the peptides. 

Kilberg MS, Pan YX, Chen H. 2005. Nutritional control of gene expression How mammalian cells respond to amino add limitation. Ann Rev Nutr 25 59-85. (Excellent review ofthe responses to lAA limitation in terms of the effects on protein synthesis that follow AA sensing)... 

Brassinosteroids (BSs) represent a new group of plant hormones that possess a broad spectrum of physiological activities (1,2). A most Intriguing property of BSs Is their capacity to Increase stress resistance In plants, but the mechanism of such an antistress activity still remains unknown (1). As cell stress resistance Is usually associated with stress protein synthesis (3 4) our aim was to study the BS effect on protein synthesis and ultrastructure of wheat leaf cells at normal temperature and under heat shock conditions. We have also studied the Influence of BSs on mesophyll cell ultrastructure under saline stress. 

KULAEVA ET AL. Effect on Protein Synthesis and Cell Ultrastructure... 

EB exerted similar effects on protein synthesis in wheat leaves as HBR (Figure 5). EB was less active in the stimulation of polypeptide synthesis in wheat leaf segments at 23°C than HBR and the number of lsoforms induced (or activated) by EB was fewer (data are not presented). Nevertheless EB was as efficient as HBR in the alteration of protein set synthesized in leaves at 42°C (Figure 5 B). 

In addition to being an indispensable amino acid that is present in proteins of animals and humans and that becomes incorporated into proteins during protein synthesis, tryptophan itself has been found to have a regulatory effect on protein synthesis. Tryptophan can stimulate hepatic protein synthesis. Although the mechanism for this regulatory action appears to be complex, it is apparent that this action involves a specific nuclear envelope receptor to which it binds, followed by enhanced nucleocytoplasmic translocation of mRNA, and subsequent increased cytoplasmic protein synthesis. 

Lectin from Adenia volkensii the Kilyambiti plant, an African succulent. Comparable to ricin and abrin same mechanism and effects on protein synthesis. 

Effect on protein synthesis, proliferation, differentiation, glucose transport, apoptosis... 

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