Abstracts Biological Sciences

Functional regulation of eukaryotic translation initiation factors

by Gregory P. Cosentino

Translation initiation in eukaryotes represents a major control point at which the expression of cellular genes is regulated. This process involves the activity of several polypeptide eukaryotic initiation factors (eIFs) which direct the sequential assembly of initiator methionyl-tRNA and mRNA on the ribosome. Two eukaryotic initiation factors, eIF2 and eIF4, are key targets for translational control. eIF2 is necessary to direct methionyl-tRNA to the 40S ribosomal subunit and phosphorylation of the eIF2alpha subunit leads to a dramatic repression in cellular protein synthesis. The mammalian double-stranded RNA-dependent protein kinase, PKR, is an eIF2alpha specific kinase which functions to regulate cell growth and virus replication. Previous studies implied the requirement of PKR dimerization for activation, however the potential association of PKR with itself or with other cellular proteins was ill-defined. We demonstrated that PKR dimerized in vivo and that a PKR mutant with deletion of the catalytic domain retained the ability to dimerize. In contrast, deletion of the two dsRNA-binding motifs in the N-terminal regulatory domain of PKR abolished dimerization. We also demonstrated that PKR interacts with another human dsRNA-binding protein, the TAR RNA-binding protein. Taken together, these data suggest that multimerization of these polypeptides occurs, at least in part, through the conserved dsRNA-binding motif. eIF4F is a heterotrimeric complex that interacts with the 5'-cap structure of mRNA and facilitates the binding of mRNA to ribosomes. eIF4F is Composed of eIF4E (the cap-binding subunit), eIF4A (an RNA helicase), and eIF4G (a 220 kDa polypeptide which serves as a scaffold between eIF4A and eIF4E). In mammalian cells, eIF4F function is modulated by a conserved family of 4E-binding proteins (4E-BPs). These proteins bind specifically to eIF4E and block its interaction with the eIF4G subunit of the eIF4F complex, thereby repressing cap-dependent translation