Initiation in eukaryotes

Initiation in eukaryotes
Initiation of polypeptide chain in eukaryotes is similar to that in prokaryotes, except following minor differences, (i) In eukaryotes there are more initiation factors and at least ten have been identified in red blood cells. They are named by putting a prefix 'e' to signify their eukaryotic origin. These factors are eIFl, eIF2, eIF3, eIF4A, eIF4B, eIF4C, eIF4D, eIF4F, eIF5 and eIF6 (Table 34.3). The eIF2, eIF3, eIF4A and eIF4F contain multiple polypeptide chains, but others are single polypeptides; eIF2 and eIF3 are analogous to IF2 and IF3 of prokaryotes. (ii) In eukaryotes, formylation of methionine does not take place, (iii) In eukaryotes, smaller subunit (40S) associates with initiator tRNA known as tRNA_i^met (because methionine is not formylated), without the help of mRNA, while in prokaryotes, generally the 30S-mRNA complex is first formed, which then associates with f-met-tRNA_f^met.

Following steps are involved in the initiation of eukaryotic polypeptide synthesis : (i) As shown in Figures 34.2 A and 34.5, GTP binds to eIF2, which increases its affinity for met-tRNA_i^met; met-tRNA_i^met associates with eIF2-GTP complex forming a ternary complex, i.e. met-tRNA_i^met-eIF2 -GTP. (ii) The ternary complex associates with 40S subunit, to form 43S initiation complex. (The factor eIF2 in mammals and plants like wheat has three subunits, namely α(35,000), β (38,000), γ (55,000); eIF2α binds to GTP, eIF2γ binds to met-tRNA and eIF2β may be a recycling factor (Table 34.4). (iii) The mRNA at its 5' end binds with 43S initiation complex. This reaction depends on eIF3 and the binding of mRNA is assisted by eIF4F, eIF4A, eIF4B and a high energy bond of ATP (Table 34.3; Figures 34.2, 34.5). As can be seen in Figure 34.5, the association of mRNA takes place on 5' end and not at the initiation codon AUG as in prokaryotes. (iv) After association of the 5' end of mRNA, initiation complex moves towards 3' end in search of initiation codon AUG, and then also associates with 60S subunit (you may recall that in prokaryotes, the initiation complex directly attaches to AUG codon and not to the 5' end of mRNA). Association of 60S subunit with the initiation complex requires the factor eIF5, because it helps in releasing eIF2 and eIF3 (60S can not join, if these two factors are not released). eIF2 is released as a binary complex, eIF-2-GDP (Fig. 34.2). The 40S-60S joining reaction really depends on eIF4C, and the GTP of the initiation complex is hydrolysed, when 60S subunit joins.

It should be realized that the initiation process described above is characterized in prokaryotic system by E. coli and in eukaryotic system by red blood cells, where major work has been done. Variations may by found in other systems.