Study of the DnaB:DciA interplay reveals insights into the primary mode of loading of the bacterial replicative helicase

Replicative helicases are essential proteins that unwind DNA in front of replication forks. Their loading depends on accessory proteins and in bacteria, DnaC and DnaI are well characterized loaders. However, most bacteria do not express either of these two proteins. Instead, they are proposed to rely on DciA, an ancestral protein unrelated to DnaC/I. While the DciA structure from Vibrio cholerae shares no homology with DnaC, it reveals similarities with DnaA and DnaX, two proteins involved during replication initiation. As other bacterial replicative helicases, VcDnaB adopts a toroid-shaped homo-hexameric structure, but with a slightly open dynamic conformation in the free state. We show that VcDnaB can load itself on DNA in vitro and that VcDciA stimulates this function, resulting in an increased DNA unwinding. VcDciA interacts with VcDnaB with a 3/6 stoichiometry and we show that a determinant residue, which discriminates DciA- and DnaC/I-helicases, is critical in vivo. Our work is the first step toward the understanding of the ancestral mode of loading of bacterial replicative helicases on DNA. It sheds light on the strategy employed by phage helicase loaders to hijack bacterial replicative helicases and may explain the recurrent domestication of dnaC/I through evolution in bacteria.

Dynamics of a type 2 secretion system pseudopilus unraveled by complementary approaches

Secretion pili, bacterial fibers responsible for transporting proteins to the extracellular milieu in some secretion systems, are very strong structures but at the same time highly flexible. Their flexibility and helical symmetry make structure determination at atomic resolution a challenging task. We have previously used an integrative structural biology approach including liquid-state NMR, cryo-electron microscopy (cryo-EM), and modeling to determine the pseudo-atomic resolution structure of the type 2 secretion system pseudopilus in a mutant form, where we employed NMR to determine the high resolution structure of the pilin (the monomer building block of the pilus). In this work, we determine the pseudo-atomic structure of the wild type pilus, and compare the dynamics of wild type and mutant pili by normal mode analysis. We present a detailed NMR analysis of the dynamics of the pilin in isolation, and compare dynamics and solvent accessibility of isolated and assembled pilins by Hydrogen/Deuterium eXchange Mass Spectrometry (HDX-MS). These complementary approaches provide a comprehensive view of internal and overall dynamics of pili, crucial for their function.