Conformational selection in the Cre CBD domain: Insights into intasome assembly

During site-specific recombination tyrosine recombinases promote scission, insertion, and recombination of specific DNA sequences. The Cre/loxP system from P1 bacteriophage promotes viral DNA circularization after infection and equitable plasmid assortment during cell division. It acts through the formation of a tetramer (intasome) in which Cre monomers are bound in a head-to-tail fashion to a half loxP site, alternating between active and inactive conformations. Despite knowledge of the recombination mechanism and strand exchange order, information regarding target selection and intasome assembly is still missing. We used molecular dynamics simulations (MD) to assess the conformational flexibility of the CBD domain of Cre recombinase. The starting seeds for MD were structures with PDB ID 1q3u (an intasome) and 7rhy (a Cre monomer bound to a loxP half-site); these structures vary in the position of helix A. We simulated four replicas for each system for 5 microseconds in an explicit solvent model (TIP3P) with the software GROMACS using CHARMM36m at 300K and 1 atm. Van der Waals interactions were calculated using a 1.2 nm cutoff and PME for electrostatics, with a 4 fs timestep. Our results show that helix A can switch from an intasome-like to a monomer-like conformation, passing through distinct intermediate and unfolded states. These results suggest that the position of helix A is selected concomitantly with establishing monomer-monomer interactions during intasome assembly. Also, the flexibility of helix A could help to interpret NMR data in which the signals for this helix are missing.We acknowledge the computer resources provided by the LNS del Sureste de México through grant 202101023N, Laboratorio de Supercómputo y Visualización en Paralelo, and LANCAD through grants 99-2021 and 49-2022, and CONACyT (graduate school scholarship 858905, grant INF-2014-02-231504, grant A1-S-11842).