Modular organization of a Cdc6-like protein from the crenarchaeon Sulfolobus solfataricus

In the present paper, we report that a Cdc6 (cell-division control)-like factor from the hyperthermophilic crenarchaeon Sulfolobus solfataricus (referred to as SsoCdc6-2) has a modular organization of its biological functions. A reliable model of the SsoCdc6-2 three-dimensional structure was built up, based on the significant sequence identity with the Pyrobaculum aerophylum Cdc6 (PaeCdc6), whose crystallographic structure is known. This allowed us to design two truncated forms of SsoCdc6-2: the ΔC (residues 1-297, molecular mass 35 kDa) and the ΔN (residues 298-400, molecular mass 11 kDa) proteins. The ΔC protein contains the nucleotide-binding Rossmann fold and the Sensor-2 motif (Domains I and II in the PaeCdc6 structure), and retains the ability to bind and hydrolyse ATP. On the other hand, the ΔN protein contains the C-terminal WH (winged helix)-fold (Domain III), and is able to bind DNA molecules and to inhibit the DNA helicase activity of the SsoMCM (mini-chromosome maintenance) complex, although with lesser efficiency with respect to the full-sized SsoCdc6-2. These results provide direct biochemical evidence that the Cdc6 WH-domain is responsible for DNA-binding and inhibition of MCM DNA helicase activity.