The molecular structure of insecticyanin from the tobacco hornworm Manduca sexta L. at 2.6 A resolution.

Insecticyanin, a blue biliprotein isolated from the tobacco hornworm Manduca sexta L., is involved in insect camouflage. Its three‐dimensional structure has now been solved to 2.6 A resolution using the techniques of multiple isomorphous replacement, non‐crystallographic symmetry averaging about a local 2‐fold rotation axis and solvent flattening. All 189 amino acids have been fitted to the electron density map. The map clearly shows that insecticyanin is a tetramer with one of its molecular 2‐fold axes coincident to a crystallographic dyad. The individual subunits have overall dimensions of 44 A X 37 A X 40 A and consist primarily of an eight‐stranded anti‐parallel beta‐barrel flanked on one side by a 4.5‐turn alpha‐helix. Interestingly the overall three‐dimensional fold of the insecticyanin subunit shows remarkable similarity to the structural motifs of bovine beta‐lactoglobulin and the human serum retinol‐binding protein. The electron density attributable to the chromophore is unambiguous and shows that it is indeed the gamma‐isomer of biliverdin. The biliverdin lies towards the open end of the beta‐barrel with its two propionate side chains pointing towards the solvent and it adopts a rather folded conformation, much like a heme.