DNA Binding Induces a cis to trans Switch in Cre Recombinase to Enable Intasome Assembly

Abstract

Mechanistic understanding of DNA recombination in the Cre-loxP system has largely been guided by crystallographic structures of tetrameric synaptic complexes. Those studies have suggested a role for protein conformational dynamics that has not been well characterized at the atomic level. We used solution NMR to discover the link between intrinsic flexibility and function in Cre recombinase. TROSY-NMR spectra show the N-terminal and C-terminal catalytic domains (CreNTD, CreCat) to be structurally independent. Amide 15N relaxation measurements of the CreCat domain reveal fast time scale dynamics in most regions that exhibit conformational differences in active and inactive Cre protomers in crystallographic tetramers. However, the C-terminal helix N, implicated in assembly of synaptic complexes and regulation of DNA cleavage activity via trans protein-protein interactions, is unexpectedly rigid in free Cre. Chemical shift perturbations and intra- and inter-molecular paramagnetic relaxation enhancement (PRE) NMR data reveal an alternative auto-inhibitory conformation for the N region of free Cre, wherein it packs in cis over the protein DNA binding surface and active site. Moreover, binding to loxP DNA induces a conformational change that dislodge the C-terminus, resulting in a cis to trans switch that is likely to enable protein-protein interactions required for assembly of recombinogenic Cre intasomes. These findings necessitate a re-examination of the mechanisms by which this widely-utilized gene editing tool selects target sites, avoids spurious DNA cleavage activity, and controls DNA recombination efficiency. SIGNIFICANCE STATEMENTThe Cre-loxP system is a widely used gene editing tool that has enabled transformative advances in immunology, neuroscience and cardiovascular research. Still, off-target activities confound research results and present obstacles to biomedical applications. Overcoming those limitations requires understanding the steps leading to assembly of recombination complexes, intasomes. We measured the magnetic properties of nitrogen nuclei in the backbone of the enzyme to correlate its intrinsic dynamics with its function in DNA recognition and cleavage. Remarkably, we found that in the absence of DNA the C-terminus of Cre appears to block the DNA binding surface and active site of the enzyme. Binding to loxP DNA induces a conformational switch that would enable the intermolecular protein-protein interactions required for assembly of recombinogenic Cre intasomes.