Role of the N- and C-terminal extensions on the activity of mammalian mitochondrial translational initiation factor 3

Mammalian mitochondrial translational initiation factor 3 (IF3(mt)) promotes initiation complex formation on mitochondrial 55S ribosomes in the presence of IF2(mt), fMet-tRNA and poly(A,U,G). The mature form of IF3(mt) is predicted to be 247 residues. Alignment of IF3(mt) with bacterial IF3 indicates that it has a central region with 20–30% identity to the bacterial factors. Both the N- and C-termini of IF3(mt) have extensions of ∼30 residues compared with bacterial IF3. To examine the role of the extensions on IF3(mt), deletion constructs were prepared in which the N-terminal extension, the C-terminal extension or both extensions were deleted. These truncated derivatives were slightly more active in promoting initiation complex formation than the mature form of IF3(mt). Mitochondrial 28S subunits have the ability to bind fMet-tRNA in the absence of mRNA. IF3(mt) promotes the dissociation of the fMet-tRNA bound in the absence of mRNA. This activity of IF3(mt) requires the C-terminal extension of this factor. Mitochondrial 28S subunits also bind mRNA independently of fMet-tRNA or added initiation factors. IF3(mt) has no effect on the formation of these complexes and cannot dissociate them once formed. These observations have lead to a new model for the function of IF3(mt) in mitochondrial translational initiation.