Discovery of proteinaceous N-modification in lysine biosynthesis of Thermus thermophilus

Although the latter portion of lysine biosynthesis, the conversion of alpha-aminoadipate (AAA) to lysine, in Thermus thermophilus is similar to the latter portion of arginine biosynthesis, enzymes homologous to ArgA and ArgJ are absent from the lysine pathway. Because ArgA and ArgJ are known to modify the amino group of glutamate to avoid intramolecular cyclization of intermediates, their absence suggests that the pathway includes an alternative N-modification system. We reconstituted the conversion of AAA to lysine and found that the amino group of AAA is modified by attachment to the gamma-carboxyl group of the C-terminal Glu54 of a small protein, LysW; that the side chain of AAA is converted to the lysyl side chain while still attached to LysW; and that lysine is subsequently liberated from the LysW-lysine fusion. The fact that biosynthetic enzymes recognize the acidic globular domain of LysW indicates that LysW acts as a carrier protein or protein scaffold for the biosynthetic enzymes. This study thus reveals the previously unknown function of a small protein in primary metabolism.