Regulation by Modification of Initiation Factors

The de novo synthesis of proteins can be varied in response to external stimuli, such as hormones or heat stress. The regulation of protein biosynthesis ocems primarily via phosphorylation of translation initiation factors. The regulatory points in eucaryotes are, above all, the translation factors eIF-2 and elF-4. 

The fimction of eIF-2 is illustrated schematically in Fig. 1.55. eIF-2 belongs to the superfamily of regulatory GTPases (see ch. 5). elF-2 fulfills the task of bringing the methionyl-initiator-tRNA to the 40S subimit of the ribosome. The active eIF-2 GTP form binds the methionyl-initiator-tRNA, associates with the cap structure of the mRNA, then commences to scan along the mRNA. Once an AUG codon is encoimte-red, the boimd GTP is hydrolyzed to GDP, resulting in the dissociation of the 

The transition of the inactive eIF2 GDP into the active GTP form is subject to regulation by phosphorylation of the a-subimit of eIF-2. Phosphorylation of the a-subunit of elF-2 increases its affinity for the nucleotide exchange factor elF-2B, without inducing nucleotide exchange. eIF-2 is foimd in excess of eIF-2B in the cell, so that phos-phorylated elF-2 binds the entire reservoir of eIF-2B. As a consequence, no further eIF-2B is available for nucleotide exchange, and protein biosynthesis is halted. 

The translation of certain mRNAs, including that encoding the transcription factor GCN4 of yeast, is stimulated rather than inhibited according to the above mechanism. In this case there exists a complex interaction between several initiation sites. 

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 

Four different protein kinases have been identified that specifically phosphorylate eIF2 on Ser 51 (review Dever, 2002). The eIF2a kinases comprise the families of HRI, PKR, PERK and GCN2 kinases. 

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. 

F g- 1.48 Control of elF-2 by phosphorylation. Phosphorylated elF-2 GDP binds strongly to the elF-2B complex without necleotide exchange occurring. Initiation of protein biosynthesis is not possible in this case. At least four different protein kinases control the phosphorylation state of elF-2 GDP. 

In reticulocytes, elF-2 is subject to phosphorylation by the heme-regulated elF-2-kinase (HRI) which is regulated via the heme concentration. Another protein kinase that can phosphorylate and regulate elF-2 is the RNA-dependent elF2a-kinase (PKR). The latter is induced by interferons and activated by double stranded RNA. Stress influences activate the protein kinases PRPK and GCN2 allowing for inhibition of protein synthesis via elF-2 phosphorylation too. 

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