Lachrymation caused while cutting onions is due to the well known volatile lachrymatory factor (LF) propanthial S-oxide released by cutting (injuring) the onion bulb. The lachrymatory factor irritates the corneal nerve endings causing tears in the eyes. Recently, genetically modified "tear-free" onions were developed by a collaborative effort between House Foods Corporation (Japan) and Crop & Food Research (New Zealand). In these onions, the lachrymatory factor synthase, responsible for LF synthesis is shut down using RNAi. These onions retain the flavor of the alliums but do not cause lachrymation while being processed.
The lachrymatory factor synthase is a member of the Birch allergen-like START domains. Members of this family are greatly expanded in plants, and in an early study on the unification of various START domains, we noticed over 55 copies of this family in Arabidopsis. Other experimentally characterized members of this family include the cytokinin-specific binding protein from mung bean, the birch allergen which also has ribonuclease activity, the stress induced protein PR10, and the major latex proteins. The wide spectrum of ligand binding and enzymatic activities of these proteins suggest that the plant-specific expansion corresponds to adaptations to binding or modifying various small molecules. In this study, we had predicted a critical role for certain residues in the upper rim of the helix-grip structure of the Birch allergen-like START domains for enzymatic activity.
The START superfamily in turn comprises a wide range of ligand binding and enzymatic domains, such as the lipid-binding classical START domains, the polyketide cyclases and aromatases that are greatly expanded in actinomycetes and involved in secondary metabolite synthesis, and the Birch allergen-like START domains that have both ligand binding domains and enzymes. Many of these proteins are poorly characterized. The START domain is a rare case of adaptation of a ligand binding protein fold for both enzymatic and non-enzymatic activity.
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