Interactions of C‐ and N‐domains of mammalian mitochondrial translational initiation factor 3 with mammalian mitochondrial 28S subunits and their role in initiation complex formation

Abstract

Bacterial initiation factor 3 (IF3) is organized into two domains (N‐domain and C‐domains) separated by a linker region. The C‐domain harbors the basic functions of IF3 with the N‐domain enhancing the binding of this factor to the small subunit. Mitochondrial IF3 (IF3mt) is also composed of N‐ and C‐domains. However, the N‐domain is preceded by an extension and the C‐domain is followed by an extension. To investigate the roles of these extended domains in IF3mt, constructs of the N‐ and C‐domains of IF3mt with and without the connecting linker were prepared and their activities tested. The Kd's for the binding of full length IF3mt and its C‐domain with and without the linker to mitochondrial 28S subunits are 30, 60 and 67 nM, respectively indicating that much of the ribosome binding interactions are mediated by the C‐domain. The N‐domain binds to 28S subunits with a 10‐fold lower affinity. Removal of the N‐domain drastically reduces the activity of IF3mt in the dissociation of mitochondrial 55S ribosomes although the C‐domain itself retains some activity. This residual activity depends significantly on the linker region. The N‐domain alone has no effect on the dissociation of ribosomes. The C‐domain with the linker has very similar activity as full length IF3mt in initiation complex formation. Surprisingly, the N‐domain with the linker significantly inhibits initiation complex formation. (Supported by NIH Grant GM32734)