W0289

Structure of a Dinoflagellate Luciferase. L. Wayne Schultz¹, Margaret Cegielski¹, Liyun Liu² and J. Woodland Hastings², ¹Dept. of Structural Biology, Hauptman-Woodward Medical Research Institute, Buffalo, NY 14203, ²Dept. of Molecular and Cellular Biology, Harvard Univ., Cambridge, MA 02138.

The structure of a single light-emitting domain from the luciferase of the marine dinoflagellate Gonyaulax Polyedra has been solved and refined to 1.8 Å resolution. Dinoflagellates are unicellular algae responsible for much of the sparkling ocean luminescence. The Gonyaulax luciferase (LCF) is packaged in intracellular vesicles referred to as scintillons with the luciferin substrate and a luciferin binding protein, which protects the substrate from air oxidation. The LCF is a 140 kDa enzyme comprised of three contiguous, repeated luciferase domains, each expressing its own luciferase activity. This LCF has no sequence or structural similarity to that of the bacterial, coelenterate or firefly enzymes. The structure reported here is that of the C-terminal domain D3. The overall structure of the enzyme places it in the family of β-barrels with a characteristic β-clam fold at the core. The structure at pH 8.0 reveals that access to the interior of the barrel, and presumably the active site, is blocked by a helix-loop-helix that rests in contact with a small N-terminal subdomain. The pH/rate profile for the D3 domain and the full-length LCF reveal that the enzyme is most active at pH 6.3 and has little activity at pH 8.0. Preliminary molecular dynamics calculations indicate that histidine residues at the interface of the helix-loop-helix and the N-terminal subdomain become charged and initiate a large molecular motion that exposes the active site.