Biochemical and crystallization analysis of the CENP-SX–DNA complex

The CENP-SX (MHF) complex is a conserved histone-fold protein complex that is involved in chromosome segregation and DNA repair. It can bind to DNA on its own as well as in complex with other proteins such as CENP-TW and FANCM to recognize specific substrates. CENP-SX binds nonspecifically to dsDNA, similar to other histone-fold proteins. Several low-resolution structures of CENP-SX in complex with DNA are known, but a high-resolution structure is still lacking. The DNA-binding properties of CENP-SX and FANCM–CENP-SX complexes with various lengths of dsDNA were compared and the band-shift patterns and migration positions were found to differ. To confirm the DNA-binding properties in detail, CENP-SX–DNA and FANCM–CENP-SX–DNA complexes were crystallized. Analysis of the crystals revealed that they all contained the CENP-SX–DNA complex, irrespective of the complex that was used in crystallization. Detailed diffraction data analyses revealed that there were two types of crystal with different space groups, P2(1) and C2, where the volume of the P2(1) asymmetric unit is twice as large as that of the C2 asymmetric unit. Analysis of the self-rotation function revealed the presence of twofold and fourfold symmetry in both crystals. This suggests that there may be multiple molecules of CENP-SX and DNA within the asymmetric unit with respective symmetry. Structure determination of the present crystals should reveal details of the DNA-binding properties of CENP-SX.