Although the primary structures of class 1 polypeptide
release factors (RF1 and RF2 in prokaryotes, eRF1 in eukaryotes)
are known, the molecular basis by which they function in
translational termination remains obscure. Because all
class 1 RFs promote a stop-codon-dependent and ribosome-dependent
hydrolysis of peptidyl-tRNAs, one may anticipate that this
common function relies on a common structural motif(s).
We have compared amino acid sequences of the available
class 1 RFs and found a novel, common, unique, and strictly
conserved GGQ motif that should be in a loop (coil) conformation
as deduced by programs predicting protein secondary structure.
Site-directed mutagenesis of the human eRF1 as a representative
of class 1 RFs shows that substitution of both glycyl residues
in this motif, G183 and G184, causes complete inactivation
of the protein as a release factor toward all three stop
codons, whereas two adjacent amino acid residues, G181
and R182, are functionally nonessential. Inactive human
eRF1 mutants compete in release assays with wild-type eRF1
and strongly inhibit their release activity. Mutations
of the glycyl residues in this motif do not affect another
function, the ability of eRF1 together with the ribosome
to induce GTPase activity of human eRF3, a class 2 RF.
We assume that the novel highly conserved GGQ motif is
implicated directly or indirectly in the activity of class
1 RFs in translation termination.