The three-dimensional (3D) structure of fructan biosynthetic enzymes is still unknown. Here, we have explored folding similarities between reported microbial and plant enzymes that catalyze transfructosylation reactions. A sequence-structure compatibility search using TOPITS, SDP, 3D-PSSM, and SAM-T98 programs identified a β-propeller fold with scores above the confidence threshold that indicate a structurally conserved catalytic domain in fructosyltransferases (FTFs) of diverse origin and substrate specificity. The predicted fold appeared related to that of neuraminidase and sialidase, of glycoside hydrolase families 33 and 34, respectively. The most reliable structural model was obtained using the crystal structure of neuraminidase (Protein Data Bank file: 5nn9) as template, and it is consistent with the location of previously identified functional residues of bacterial levansucrases (Batista et al., 1999; Song & Jacques, 1999). The sequence–sequence analysis presented here reinforces the recent inclusion of fungal and plant FTFs into glycoside hydrolase family 32, and suggests a modified sequence pattern {H-x(2)-[PTV]-x(4)-[LIVMA]-[NSCAYG]-[DE]-P-[NDSC]-[GA]} for this family.

Fructans (polyfructosylsucrose) are synthesized by a number of plants and micro-organisms. Plant fructans are localized in the vacuole and have a low degree of polymerization (DP), whereas the fructans synthesized by micro-organisms are usually much bigger. There is an increasing interest in fructans for both food and non-food applications. In order to accumulate fructans of high DP in the plant vacuole, the levansucrase protein of Bacillus subtilis was fused to the vacuolar targeting sequence of sporamin and expressed in plants. Transgenic tobacco plants in which this fusion gene is expressed accumulate fructans to levels up to 21% of the d. wt. They showed a reduced translocation of carbohydrates, bleaching of the leaves, stunted growth and increased levels of hexoses and starch. The levansucrase protein was not translocated to the plant vacuole, but retained in the endomembrane system, even though the same targeting signal was able to translocate the E. coli GUS protein to the plant vacuole.