The structure of fructose-l,6-bisphosphate aldolase from Drosophila melanogaster has been determined by X-my diffraction at 2.5 .~ resolution.The insect enzyme crystallizes in space group P'2r2~2t with lattice constants a=86.60, b= 116.80, c= 151.58 ~. Molecular replacement with rabbit muscle aldolase as a search model has been employed to solve the structure. To improve the initial phases real space averaging, including phase extension from 4.0 to 2.5 ,/~, has been applied. Refinement of the atomic positions by molecular dynamics resulted in a crystallographic R-factor of 0.214. The tertiary structure resembles in most parts that of the vertebrate aldolase from rabbit muscle. Significant differences were found in surface loops and the N-and C-terminal regions of the protein. Here we present the first aldolase structure where the functionally important C-terminal arm is described completely.Crystal structure; Glycolytic enzyme; Fructose-1,6-bisphosphate aldolase; Substrate specificity; Drosophila melanogaster INTRC, DUCTIONFruetose-l,6-bisphosphate (FBP) aldolase (EC 4.1.2.13) is a glycolytic enzyme catalyzing the reversible aldol cleavage of fructose-l,6-bisphosphate to dihydroxyacetone phosphate and D-glyceraldehyde-3-phosphate. FBP aldolase from Drosophila melanogaster (strain Sevelen, wild type, pupea) forms a Schiff base between the substrate and a lysyl residue [1] and belongs therefore to the class I aldolases [2]. Contrary to class I the class II aldolases use a divalent metal ion as cofactor [3]. The vertebrate aldolases (class I) comprise three isozymes. They refer to A (muscle), B (liver) and C (brain) depending on the tissue from which they are isolated [3]. The A and C form show a higher substrate specificity for FBP over fructose-l-phosphate (F1P), while the liver isozyme has no such preference.Abbreviations: DALD, fructose-l,6-bisphosphate aldolase from Drosophila melanogaster; RMALD, rabbit muscle aldolase; HMALD, human muscle aldolase; RLALD, rabbit liver aldolase; FBP, fructose-1,6-bisphosphate; FIP, fructose-l-phosphate; r.m.s., root-meansquare.Sequence numbering: Throughout this paper the sequence numbering of FBP aldolase from rabbit muscle will be applied.

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