Crystal Structure of the Conserved Hypothetical Cytosolic Protein Xcc0516 from Xanthomonas campestris Reveals a Novel Quaternary Structure Assembled by Five Four-Helix Bundles

Institute of Biochemistry, National Chung Hsing University
○Nei-Li Chan Li-Ying Lin Chung-Lun Ching Ko-Hsin Chin Shan-Ho Chou

The 128-residue Xcc0516 (SwissProt accession number: Q8PD29) of phytopathogen Xanthomonas campestris was annotated as a conserved hypothetical cytosolic protein with a molecular weight of 14.6 kDa and a calculated isoelectric point of 9.7. A BLAST database search clearly showed that Xcc0516 belongs to the structurally and functionally uncharacterized Ribosomal_S23p protein family (Pfam domain PF05635). In order to obtain functional insights from the structural viewpoint as well as provide structural coverage for this protein family, Xcc0516 was selected as a target for structural determination. And the crystal structure of Xcc0516 has been determined at 2.5 Å resolution by the MAD method. While it turns out that the Xcc0516 monomer folds into an antiparallel four-helix bundle, interestingly, five protomers associate around a 5-fold axis to form a pentameric toroid-shaped structure with a tapered central channel. To our knowledge, this is the first example of a petameric toroid formed by non-transmembrane four-helix bundle domain. In addition, with the formation of a 10-helix inner ring that faces the central channel, the architecture of Xcc0516 homopentamer contrasts sharply with that of the pentameric membrane-spanning acetylcholine receptor. Therefore, the pentameric structure we report here represents a new quaternary assembly formed by four-helix bundles.