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

Abstract

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.