ABSTRACT
Tomato is a classic example of a food plant that contains a potent hemagglutinating protein in its edible part. Though the tomato lectin (also called Lycopersicum esculentum lectin or LEA) was already isolated and characterized in some detail in 1980, the exact molecular structure has not yet been elucidated. A reinvestigation of the relevant literature combined with a detailed analysis of the publicly accessible genome and transcriptome sequence data revealed that tomato fruits express a mixture of three structurally and evolutionarily related lectins: (1) a previously cloned lectin-related protein called Lycesca, (2) a previously isolated lectin-related 42 kDa chitin-binding protein and (3) the genuine lectin (LEA). Complete sequences are available for Lycesca and the 42 kDa protein but not for the lectin itself. However, a fairly accurate model could be elaborated for LEA based on partial sequences. The lectin consists of two sugar-binding modules (comprising two in tandem arrayed hevein domains) separated by a Ser/Pro-rich linker. All four hevein domains are active, rendering LEA a potent tetravalent hemagglutinating lectin. In contrast, both Lycesca and the 42 kDa protein possess a single active hevein domain and are monovalent lectins devoid of hemagglutinating activity. Recent specificity studies revealed that LEA is not a chitin-binding lectin sensu strictu because it interacts equally well with complex and high mannose N-glycans. It is predominantly but not exclusively expressed in fruits, where it accumulates at an exponential rate during the first 10 days of 166fruit development. Striking varietal differences indicate that the tomato fruit lectin content is genetically determined. At present, the physiological role of the tomato fruit lectin is poorly understood. There are no indications for acute or chronic toxicity of LEA in mammals. However, the documented biological activities of LEA and the closely related potato lectin suggest that they might affect the immune system on oral uptake. Moreover, LEA might cause latex-fruit syndrome in sensitive individuals. This raises the question whether low-lectin tomatoes are possibly safer for the consumer.
