Structure and DNA-binding activity of the Pyrococcus furiosus SMC protein hinge domain.

Structural Maintenance of Chromosomes (SMC) proteins are essential for a wide range of processes including chromosome structure and dynamics, gene regulation, and DNA repair. While bacteria and archaea have one SMC protein that forms a homodimer, eukaryotes possess three distinct SMC complexes, consisting of heterodimeric pairs of six different SMC proteins. SMC holocomplexes additionally contain several specific regulatory subunits. The bacterial SMC complex is required for chromosome condensation and segregation. In eukaryotes, this function is carried out by the condensin (SMC2-SMC4) complex. SMC proteins consist of N-terminal and C-terminal domains that fold back onto each other to create an ATPase "head" domain, connected to a central "hinge" domain via a long coiled-coil region. The hinge domain mediates dimerization of SMC proteins and binds DNA. This activity implicates a direct involvement of the hinge domain in the action of SMC proteins on DNA. We studied the SMC hinge domain from the thermophilic archaeon Pyrococcus furiosus. Its crystal structure shows that the SMC hinge domain fold is largely conserved between archaea and bacteria as well as eukarya. Like the eukaryotic condensin hinge domain, the P. furiosus SMC hinge domain preferentially binds single-stranded DNA (ssDNA), but its affinity for DNA is weaker than that of its eukaryotic counterpart, and point mutations reveal that its DNA-binding surface is more confined. The ssDNA-binding activity of its hinge domain might play a role in the DNA-loading process of the prokaryotic SMC complex during replication.