Translation initiation factor IF3: two domains, five functions, one mechanism?

Initiation factor IF3 contains two domains separated by a flexible linker. While the isolated N-domain displayed neither affinity for ribosomes nor a detectable function, the isolated C-domain, added in amounts compensating f or its reduced affinity for 30S subunits, performed all activities of intac t IF3, namely: (i) dissociation of 70S ribosomes; (fi) shift of 30S-bound m RNA from 'stand-by' to 'P-decoding' site; (iii) dissociation of 30S-poly(U) -NacPhe-tRNA pseudo-initiation complexes; (iv) dissociation of fMet-tRNA fr om initiation complexes containing mRNA with the non-canonical initiation t riplet AUU (AUUmRNA); (v) stimulation of mRNA translation regardless of its start codon and inhibition of AUUmRNA translation at high IF3C/ribosome ra tios. These results indicate that while IF3 performs all its functions thro ugh a C-domain-30S interaction, the N-domain function is to provide additio nal binding energy so that its fluctuating interaction with the 30S subunit can modulate the thermodynamic stability of the 30S-IF3 complex and IF3 re cycling. The localization of IF3C far away from the decoding site and antic odon stem-loop of P-site-bound tRNA indicates that the IF3 fidelity functio n does not entail its direct contact with these structures.