Structure of a Mutant Form of Translation Regulator Hfq with the Extended Loop L4

The bacterial protein Hfq is a member of a large family of Lsm proteins, which are characterized by a conserved tertiary structure and form a stable ring-shaped quaternary structure composed of several monomers. The bacterial proteins form hexamers, whereas their archaeal and eukaryotic homologues are heptamers. Until today, there is no explanation for the fact that highly homologous Lsm proteins with the same tertiary structure form multimers composed of a different number of monomers, and this issue generates a lot of discussion. The effect of the loop L4 of variable length on the quaternary structure of the Hfq protein was experimentally validated. The produced protein retains the hexameric quaternary structure, and the extended loop protrudes from the protein core in different directions. Therefore, attempts to change the quaternary structure of the protein failed, which confirms its high stability. The RNA-binding properties of the mutant protein remained virtually unchanged.