Evolution of Translation Initiation Factor 2 Extensions Links Initiation to Bacterial Stress Response.

Terminal extensions are recurrent features in protein evolution, often linked to environmental adaptation and novel regulatory or interaction functions. Here, we combine comparative genomics, structural modeling, and functional assays to elucidate the evolutionary diversification and functional significance of one of the key proteins of all cells, translation initiation factor 2 (IF2) terminal extensions across the tree of life. Specifically, we reconstruct the first comprehensive evolutionary map of IF2 across life, analyzing ~800 homologs and classify seven distinct structural architectures of IF2 based on extension regions. These extensions are enriched for intrinsically disordered and phase-separation-promoting residues, suggesting roles beyond the conserved catalytic core. Further, functional characterization of IF2 with varying N-terminal lengths show that loss of the N-terminal extension slows bacterial growth specifically under temperature and pH stress. Appending C-terminal extensions from different organisms to the