Mutations in the nucleotide binding and hydrolysis domains of Helicobacter pylori MutS2 lead to altered biochemical activities and inactivation of its in vivo function

Plant Organellar MSH1 Is a Displacement Loop-Specific Endonuclease

MutS HOMOLOG 1 (MSH1) is an organellar-targeted protein that obstructs ectopic recombination and the accumulation of mutations in plant organellar genomes. MSH1 also modulates the epigenetic status of nuclear DNA, and its absence induces a variety of phenotypic responses. MSH1 is a member of the MutS family of DNA mismatch repair proteins but harbors an additional GIY-YIG nuclease domain that distinguishes it from the rest of this family. How MSH1 hampers recombination and promotes fidelity in organellar DNA inheritance is unknown. Here, we elucidate its enzymatic activities by recombinantly expressing and purifying full-length MSH1 from Arabidopsis thaliana (AtMSH1). AtMSH1 is a metalloenzyme that shows a strong binding affinity for displacement loops (D-loops). The DNA-binding abilities of AtMSH1 reside in its MutS domain and not in its GIY-YIG domain, which is the ancillary nickase of AtMSH1. In the presence of divalent metal ions, AtMSH1 selectively executes multiple incisions at D-loops, but not other DNA structures including Holliday junctions or dsDNA, regardless of the presence or absence of mismatches. The selectivity of AtMSH1 to dismantle D-loops supports the role of this enzyme in preventing recombination between short repeats. Our results suggest that plant organelles have evolved novel DNA repair routes centered around the anti-recombinogenic activity of MSH1.

B. subtilis MutS2 splits stalled ribosomes into subunits without mRNA cleavage

Stalled ribosomes are rescued by pathways that recycle the ribosome and target the nascent polypeptide for degradation. In E. coli, these pathways are triggered by ribosome collisions through the recruitment of SmrB, a nuclease that cleaves the mRNA. In B. subtilis, the related protein MutS2 was recently implicated in ribosome rescue. Here we show that MutS2 is recruited to collisions by its SMR and KOW domains, and we reveal the interaction of these domains with collided ribosomes by cryo-EM. Using a combination of in vivo and in vitro approaches, we show that MutS2 uses its ABC ATPase activity to split ribosomes, targeting the nascent peptide for degradation through the ribosome quality control pathway. However, unlike SmrB, which cleaves mRNA in E. coli, we see no evidence that MutS2 mediates mRNA cleavage or promotes ribosome rescue by tmRNA. These findings clarify the biochemical and cellular roles of MutS2 in ribosome rescue in B. subtilis and raise questions about how these pathways function differently in diverse bacteria.

Archaeal MutS5 tightly binds to Holliday junction similarly to eukaryotic MutSγ

Archaeal DNA recombination mechanism and the related proteins are similar to those in eukaryotes. However, no functional homolog of eukaryotic MutSγ, which recognizes Holliday junction to promote homologous recombination, has been identified in archaea. Hence, the whole molecular mechanism of archaeal homologous recombination has not yet been revealed. In this study, to identify the archaeal functional homolog of MutSγ, we focused on a functionally uncharacterized MutS homolog, MutS5, from a hyperthermophilic archaeon Pyrococcus horikoshii (phMutS5). Archaeal MutS5 has a Walker ATPase motif-containing amino acid sequence that shows similarity to the ATPase domain of MutSγ. It is known that the ATPase domain of MutS homologs is also a dimerization domain. Chemical cross-linking revealed that purified phMutS5 has an ability to dimerize in solution. phMutS5 bound to Holliday junction with a higher affinity than to other branched and linear DNAs, which resembles the DNA-binding specificities of MutSγ and bacterial MutS2, a Holliday junction-resolving MutS homolog. However, phMutS5 has no nuclease activity against branched DNA unlike MutS2. The ATPase activity of phMutS5 was significantly stimulated by the presence of Holliday junction similarly to MutSγ. Furthermore, site-directed mutagenesis revealed that the ATPase activity is dependent on the Walker ATPase motif of the protein. These results suggest that archaeal MutS5 should stabilize the Holliday junction and play a role in homologous recombination, which is analogous to the function of eukaryotic MutSγ.

Molecular basis for the functions of a bacterial MutS2 in DNA repair and recombination

Bacterial MutS2 proteins, consisting of functional domains for ATPase, DNA-binding, and nuclease activities, play roles in DNA recombination and repair. Here we observe a mechanism for generating MutS2 expression diversity in the human pathogen Helicobacter pylori, and identify a unique MutS2 domain responsible for specific DNA-binding. H. pylori strains differ in mutS2 expression due to variations in the DNA upstream sequence containing short sequence repeats. Based on Western blots, mutS2 in some strains appears to be co-translated with the upstream gene, but in other strains (e.g. UA948) such translational coupling does not occur. Accordingly, strain UA948 had phenotypes similar to its ΔmutS2 derivative, whereas expression of MutS2 at a separate locus in UA948 (the genetically complemented strain) displayed a lower mutation rate and lower transformation frequency than did ΔmutS2. A series of truncated HpMutS2 proteins were purified and tested for their specific abilities to bind 8-oxoG-containing DNA (GO:C) and Holiday Junction structures (HJ). The specific DNA binding domain was localized to an area adjacent to the Smr nuclease domain, and it encompasses 30-amino-acid-residues containing a "KPPKNKFKPPK" motif. Gel shift assays and competition assays supported that a truncated version of HpMutS2-C12 (∼12kDa protein containing the specific DNA-binding domain) has much greater capacity to bind to HJ or GO:C DNA than to normal double stranded DNA. By studying the in vivo roles of the separate domains of HpMutS2, we observed that the truncated versions were unable to complement the ΔmutS2 strain, suggesting the requirement for coordinated function of all the domains in vivo.