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Introduction
SegC (also known as SSO0033) is a recently characterized protein from Sulfolobus/Saccharolobus species that is involved in archaeal chromosome segregation. Although its sequence has no strong similarity to previously known proteins, structural studies revealed an unexpected fold reminiscent of the ThDP-binding superfamily. Functional assays demonstrated that SegC binds DNA, participates in SegAB complex interactions, and displays NTPase activity. These features suggest a role in remodeling or stabilizing the archaeal segregation machinery.
Structural highlights
The crystal structure of SegC has been solved in multiple forms, including PDB entries 8WQ8, 8WQN, and 8YK9. The 8WQ8 structure provides a clear view of the protein’s tertiary architecture, consisting of a compact α/β fold with a positively charged surface implicated in DNA binding. The core fold resembles enzymes of the ThDP-dependent family, despite low sequence identity, suggesting an evolutionary relationship.
A major functional feature is a broad positively charged surface enriched in lysine and arginine residues. Electrostatic mapping shows this region as a prominent blue patch, which is proposed to mediate DNA interaction.
Another important structural feature is a putative NTP-binding pocket, composed of residues such as Lys38, Asp86, Lys90, Glu122, and Thr124. This pocket supports the biochemical observation that SegC possesses NTPase activity.
SegC also forms a dimeric or oligomeric assembly in the biological assembly models, and this quaternary structure is likely relevant to its function. The interface involves hydrophobic and charged interactions between residues such as Ile55, Leu58, Arg60, and Phe112.
Finally, structural comparison with ThDP-dependent enzymes reveals a conserved arrangement of the central β-sheet and surrounding helices, even though SegC lacks ThDP-binding residues. This highlights an evolutionary fold re-use.
Function
SegC binds nonspecific DNA and interacts with other segregation proteins, possibly acting as a regulator or structural scaffold. Its discovery adds a new component to the archaeal segregation system, which is fundamentally different from bacterial ParABS and eukaryotic cohesin/condensin systems.
Mutagenesis studies identify key DNA-binding residues (Arg72, Lys101, Arg142) and NTP-related residues (Asp86, Glu122). Mapping these onto the structure helps visualize the functional importance of each region.
