Control of Eukaryotic Translation

Prokaryotes do not seem to make extensive use of control at the translational level, whereas eukaryotes use translational control much more widely. In part, translational control in eukaryotes occurs at the mRNA level. It may involve the sequestering of specific mRNAs by combining with specific mRNA-binding proteins and/or rapid degradation of mRNA so that they do not persist in inappropriate phases of the cell cycle. Other translational controls include the phosphorylation of factors involved in translation, as listed below:

> 1. Globin synthesis control - Synthesis of globin in reticulocyte cells is pointless unless there is sufficient heme present. Reticulocyte cells contain a protein kinase called heme-controlled inhibitor (HCI) (Figure 28.37). In the presence of adequate heme levels, the kinase is inactive, but if heme levels fall, however, HCI becomes activated and specifically phosphorylates the initiation factor eIF2, causing the complex between eIF2 and eIF2B to become unusually stable. The result is that all of the eIF2 is tied up and can no longer be recycled for new initiation. Hence, protein synthesis is halted, and no more globin is made until heme supplies are again adequate.

> 2. Interferon Action - Anti-viral agents called interferons are cellular glycoproteins produced in response to virus infections. In addition to stimulating mRNA degradation, interferons induce the synthesis of a protein kinase that phosphorylates eIF2 (like HCI above) and destabilizes the eIF2-eIF2B complex, inhibiting protein synthesis in the process.

Phosphorylation of initiation factors appears to be a general method for translational control in eukaryotes.

See also: Eukaryotic vs Prokaryotic Translation

INTERNET LINKS:

> 1. Globin Gene Server

> 2. Evolution of Interferon Therapy